Civil Service - Electrical Engineering Main Paper I & II - 1992 - 2007

PPAAPPEERR -- II Time Allowed: Three Hours Maximum Marks: 300

Candidates should attempt Question 1 and 5 which are compulsory, and any THREE of the remaining question selecting at least ONE question from each Section. All question carry equal marks.

SSEECCTTIIOONN AA

1. Answer any three of the following:

(a) In the circuit show in Fig. Q.1. (a), the switch S is opened for a long time and is closed at t = 0 with the capacitor initially uncharged. Calculate the output voltage V0 at t = 0, at t = 25 m sec and at t = . Also sketch the corresponding transient.

(b) For the circuit show in Fig. Q. 1(b), find out the magnitude of the supply voltage V. The resistor in series with the battery absorbs a power of 1 watt.

Fig. Q.l (b)

(c) Consider a thick hemispherical shell of inner radius and outer radius b, show in cross-section in Fig. Q.1. (c). If the shell is uniformly magnetizen along the axis of symmetry (z-axis), show that a small compass needle placed at the origin P will swing freely.

(d) Prove by considering the axial point in a disk that the electric potential undergoes no sudden change from one side to the other of a charge layer

2. (a) R making use of matrix algebra, calculate the voltages at nodes N1 and N2 Ajar the network show in Fig. Q2. (a).

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EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

(b) Using Laplace transforms find the current i1 (t) that enters the network of Fig. Q2.(b) when the switch is closed at t=0 with zero initial current in the inductor. Numerical values of the circuit elements areas shown.

(c) Synthesize the following driving point impedance in Foster-I and Cauer-I froms:

2 4

1 3 5

s sz s

s s s

3. (a) The core (radius a) of a coaxial cylindrical cable is surround by an insulating sheath of conductivity 1 and outer radius b, and a second layer of conductivity 2 extending to the outer metallic conductor of radius c. Find the resistance per unit length of the cable between the core and the outer conductor.

(b) Plane monochromatic waves are propagated parallel to the axis in both the positive and negative directions. At the origin, the field strengths are given by

Ex = A cos (t); Ey = 0

Hx = 0 ; Hy = Bcos(t)

Calculate the mean intensity of the radiation in each of the two directions in terms of A, B and the constants of the medium.

(c) A parallel plate capacitor is made with a composite dielectric. A sheet of dielectric of permittivity 1, thickness d1 is placed on top of a second dielectric sheet (permittivity 2 thickness d2). The combination is placed between parallel conducting plates which are separated by the distance (d1+d2). What is the capacitance per unit plate area of the capacitor?

4. (a) Give an account of the construction of a cathode-ray oscilloscope and explain the functions of various constituents. The horizontal deflection sensitivity of a CRO is 0.02 mm/V. Wan unknown voltage is applied to the horizontal plates, the spot shifts 4.0 mm horizontally. Find the value of the unknown voltage.

(b) As shown in Fig.Q. 4. (b), an uncompensated electro-dynamic wattmeter has its voltage coil connected directly across the load. Show that the reading of the instrument is approximately (P1+P2 P3), where P1 is the power dissipation in the voltage coil circuit, P2 is that in the load and P3 is an error term due to the presence of inductance in the voltage coil circuit.

(a) A voltmeter has a sensitivity of 100/V and three scales; 50 V, 150 V and 300 V. When

connected in the circuit of Fig. Q. 4(c), the voltmeter reads 4.65 V on its lowest (0–50V) scale. Calculate Rz .


(a) A waveform produced by an electronic circuit is to be attenuated with minimum waveform distortion (maximum bandwidth). Using the high impedance attenuation circuit shown in Fig. Q. 5(a), draw in any additional components required to fulfill the above requirements and determine dies values of the required components.

(b) A silicon p-n junction diode is fabricated from a 0. 1 -m p-material and 0.02-m n-material. Calculate the value of potential barrier for the unbiased diode at 3000 K. For silicon, n=0.14m2/V- sec. p = 0.048 m2/V-sec, and ni=1.4x1016/m3.

(Electronic charge e = 1.6x10–19C; Boltzmann’s constant K = 1.38x10–23 JK–1)

(c) Using NAND gates, implement the equation:

K = (AB + CD)(F+G)

(d) A 100 kW, 250 V shunt generator has a field circuit resistance, including a rheostat of 60 and a total armature circuit resistance of 0.065. Calculate (i) field circuit current, (ii) full-load line current flowing to the load, (iii) armature circuit current and (iv) full-load generated voltage that is required to deliver rated voltage to the load, Ignore armature reaction.

6. (a) A transistor is connected in common-emitter (CE) configuration in which collector supply is 8 V and voltage drop across resistance R0(=800) connected in the collector circuit is 0.5V. If = 0.96, determine (i) collector-emitter voltage and (ii) base-current.

(b) A transistor in common-emitter configuration employs a 4 k load and Vcc = 13 V. Estimate the maximum peak to peak value of the input signal for linear operation if = 100. Given: Vknee = 1V and a change of 1 V in VBE causes a change of 5 mA in collector current.

(c) In a CE transistor amplifier, when the signal changes by 0.02 V. the base current changes by 10 A and collector current changes by 1 mA. If the collector resistance RC = 5k and load resistor RL = 10, find

(i) current gain (ii) input impedance, (iii) ac. load resistance, (iv) voltage gain and (v) power gain.

7. (a) The primary of a transformer consists of 1000 turns of wire and produces a maximum flux of 0.03 Wb alternating at 50Hz in the iron core. The secondary winding has 40 turns. Calculate

the r.p.m. values of primary and secondary e.m.f’s on the assumption that the flux change us sinusoidal.

(b) A d.c. series motor supplied from 230 V source draws an armature current of 16 A and drives a load at 1600 r.p.m. The resistance of armature and field is 1. The torque required to drive the load varies as N1.8, where N is the speed. What should be applied voltage to the motor if the speed is to be raised to 1750 r.p.m.? Assume that the field is unsaturated.

(c) Calculate the suitable auto-transformation ratio for starling a 3-phase induction motor with line current not exceeding three times the full-load current. The short-circuit current with rated applied voltage is 5 times the full-load current and full-load slip is 5%. Find also the starting torque in terms of the full-load torque, when the auto-transformer is employed.

8. (a) Find the percentage voltage drop for a transformer with a percentage resistance of 2% and a percentage reactance of 4%, of rating 550 kVA, when it is delivering 450 kVa and 0.6 p.f. lagging.

(b) A 3300 V, 24 pole, 50 Hz 3-phase star-connected induction motor has a slip-ring rotor resistance of 0.016 and stand-still reactance of 0.265 per phase. Calculate (i) speed at maximum torque and (ii) ratio of full-load torque to maximum torque if fill-load torque is obtained at 247 r.p.m.

(c) A 3-phase synchronous motor, when working on constant volt-age mains at constant load, takes a current which varies with the excitation. Explain this result. How is the power factor affected?

PPAAPPEERR -- IIII Time Allowed: Three Hours Maximum marks: 300

Candidates should attempt FIVE questions only selecting at last TWO questions including the compulsory question from each of Section A and B or Section A and C. The Number of marks carried by each question is indicated at the end of the question.

SSEECCTTIIOONN AA

1. Select any three of the following statements, read them carefully and identify the correct and incorrect ones. Justify your answer using not more than 200 words in each case:

(20x3=60)

(a) In order to increase the margin of stability for type-2 or higher order system, phase lag compensation is desirable. (b) For a second order system and a tolerance band of 2% the setting time is given by 3/ where is the damping factor. (c) A semiconverter system provides better performance to d.c. series motors as compared to the full converter system. (d) An invertor in the rotor circuit of a wound-rotor induction motor can serve as a means of a speed control.

2. (a) Explain what you mean by relative stability of a closed-loop system and how it is assessed.

(15)

(b) A turntable with a moment of inertia of 10kg rn1 is used with a proportional error controller in a unity negative feedback system. The controller develops a torque of 60 Nm per radian of misalignment. The viscous friction is such that the damping factor is 0.3.

(i) Draw a signal flow graph of the system and determine 0(s)/1(s) and e(s)/(s) Where 1(s), o(s) and e(s) are the Laplace transforms of input, output and error signals.

(ii) Determine the steady state tracking error for a constant velocity input of 0.03 rad/sec.

(25)

(c) The open-loop transfer function of a unity feedback control system is given by

( )1 0.1 1 0.05

KG s

s s s

(i) Determine the value of K so that the gain margin of the system is 20 dB. (ii) Determine the value of K so that the phase margin of the system is 600.

(20)

3. (a) What are generalised error coefficients? Discuss their advantages. Explain various classical error constants and discuss their disadvantages.

Determine the position, velocity and acceleration error coefficients for a feedback control system with unity feedback, and the following open-loop transfer function.

2 2

1 2 1 4

2 10

K s sG s

s s s

CC..SS..EE..--((mmaaiinnss))--11999922


(30)

(b) Discuss the disadvantages of harmonics and suggest methods for reducing a.c. harmonics in a converter circuit.

(20)

(c) Explain the working of time delay circuit which may be used to apply power to an industrial load at a predetermined time after the initiating singal.

(10)

4. (a) Describe with a neat diagram a step up chopper and discuss its merits and demerits.

(15)

(b) Describe with a neat diagram a static version of the WordLonard system of speed control of a d.c. drive.

(20)

(c) A single phase full converter is used to control the speed of a 220 volt, 1500 r.p.m. separately excited d.c. motor. The armature resistance is 0.3 ohm and the rated armature current is 20 A. The a.c. supply voltage is 250 volt. The motor voltage constant is Ka = 0.03 volt/ rafdian/min. For a firng angle = 30° and rated motor current, determine (i) the speed of the motor and (ii) motor torque, assuming continuous and ripple free armature current. Derive any formula used.

(25)

SSEECCTTIIOONN BB

5. Comment with justification on the correctness of any three of the following statements, restricting your answer to within 200 words for each:

(20x3=60)

(a) With V/f speed control of 3-phase induction motor the maximum torque and slip at which maximum torque occurs, respectively, increases and decreased with increase in frequency.

(b) The higher the SCR of a machine, the heavier is the rotor of the machine.

(c) For economic operation, the generator with highest positive incremental transmission loss will operate at the lowest positive incremental cost of production.

(d) For an existing transmission line the string efficiency is 70%. Now if d.c. voltage is supplied for the same setup, the string efficiency will be 90%,

6. (a) Explain the principle of operation and applications of an induction generator.

(10)

(b) Describe with neat sketches the construction and principle of operation of a fast breeder reactor.

(15)

(c) A 3-phase 50 Hz induction motor has equivalent stator and rotor resistance of 0.05 ohm each and reactances of 0.3 ohm each. The motor is to be operated at one half of its rated frequency. Determine (i) the maximum torque at the reduced value of supply as the a fraction of its normal value (ii) the starting torque at the reduced value of supply as a fraction of its normal value.

(20)

(d) A synchronous motor is connected to an infinite bus and draws 1.0 p.u. current at 0.9 p.f.

leading. Its synchronous reactance is 1.0 p.u. and resistance is negligible. Determine E, Q, and P. If with the same excitation the loading on the motor is increased by 10%, determine the new values of E,Q,I, , P and cos .

(15)

7. (a) Explain with the help of Flowchart the procedure for solution of Coordination Equations.

(10)

(b) Explain with neat diagrams the process of current chopping in circuit breakers and suggest a suitable method to minimise its effect on power systems.

(10)

(c) Explain the carrier system of protection in power systems. With a block diagram and neat sketches discuss how the phase comparison scheme can be used for protecting a feeder fed from (i) one end and (ii) both the ends.

(d) Differentiate between a current transformer and a power transform. Discuss various errors associated with a current transformer and the methods to minimise these errors.

(15)

8. (a) Explain clearly ‘Pinch effect’ and ‘Skin effect’. Compare the performance of Ajax Wyatt and coreless induction furnaces. State the applications of the two types of furnaces.

(15)

(b) What is regenserative braking? Explain the operation of (i) d.c. shunt motor (ii) d.c. series motor and (iii) induction motor when subjected to regenerative braking.

(15)

(c) A motor works on 2 mm load cycle constituted as follows: (i) 0 to 15 sec-load rising from 0 to 1050kW; (ii) 15 to 85 sec-constant load of 600 kW; (iii) 85 to 95 sec-regenerative braking with power returned falling uniformly from 200 to 0 kW; (iv) 95 to 20 sec-motor is at rest. Determine the continuous rating of the motor, assuming the rating to depend upon (1) r.m.s. value of loading and (2) average value of loading.

(20)

(d) The tractive effort exerted by a locomotive while hauling a train on level track at 50 kmph is 35000 newtons. If the locomotive has to haul the same train on a gradient and the tractive effort required is 55000 newtons, determine the power required by the locomotive when the motors used art (i) d.c. series motors (ii) induction motors.

(10)

SSEECCTTIIOONN CC

Question No.9 is compulsory

9. Comment with justification on the correctness of any three of the following statements restricting your answer to within 200 words for each:

20x3=60

(a) For transmission line load matching over a range of frequencies, it is best to use a balun.

(b) The standard reference antenna for the directive gain is the isotropic antenna.

(c) A disadvantage of microstrip compared with stripline is that microstrip is more likely to radiate.

(d) A parametric amplifier is used for low noise high frequency amplification.

10. (a) Explain the basic theory of reactance modulator. Also explain and compare the performance of some of the reactance modulators for generation of frequency modulation.

(15)

(b) Define signal to noise ratio, noise figure and equivalent noise temperature of a receiver. Under what conditions is equivalent noise temperature a more useful quantity than noise figure?

(15)

(c) Define and describe pulse-position modulation. Explain with waveforms how it is derived from pulse width modulation. Compare the types of modulation.

(15)

(d) Differentiate between telex and telegraphy. Describe briefly the system and machines used in telegraphy.

(15)

11. (a) Explain with the help of neat diagrams the principle or operation of the two-Cavity Klystron amplifier.

(20)

(b) Describe with a neat diagram the principle of operation and application of Gunn-effect diode.

(15)

(c) Explain with a neat diagram the construction principle of operation and application of Surface Acoustic Wave devices.

(15)

(d) Discuss and compare various methods of tuning a cavity resonator.

(10)

12. (a) Derive the impedance-inversion property of a quarter wave transformer and explain some of its applications.

(15)

(b) Explain how you would measure coupling factor and directivity for a two-hole directional coupler.

(10)

(c) Explain how you would measure the common mode rejection ratio of a differential amplifier.

(10)

(d) Show the simulation of the following equation on analogue computer after using suitable time scaling:

(25)

2

22 30 200 0

d x dxx

dt dt

x(0) = 5

x(0) = 9.


Candidates should attempt Question 1 and 5 which are compulsory, and any THREE of the remaining question selecting at least ONE question from each Section.

Assume suitable data if considered necessary and indicate the same clearly. All question carry equal marks.

SSEECCTTIIOONN AA


(a) Consider a circuit shown in Fig. Q. 1(a). The capacitor, initially charged to 10 V, is suddenly discharged. Obtain an expression for the resultant transient current.

(b) In the circuit shown in Fig. Q. 1(b), determine the potential difference between the points A and B in the steady state.

(c) Fig. Q. 1(c) shows two identical parallel plate capacitors connected to a battery with a switch S closed. The switch is now opened and the free space between the plates of the capacitors is filled with a dielectric of dielectric constant K = 2. Find the ratio of the total electrostatic energy stored in both capacitors before and after the introduction of the dielectric.

(d) A square wave of amplitude 1 V and period 2 sec is connected in series with a resistance of 1 and an inductance of 1 H at t = 0. Find the current in the circuit.

2. (a) State and prove Thevenin’s theorem. Determine Thevenin’s equivalent circuit which may be used to represent the network shown in Fig. Q. 2(a) at the terminals CD.



(b) In the network shown in Fig. Q.2 (b), the switch is closed at t=0. At t=t>0, it is found that

i(t)=1.0A and dv t

10 / secdt

V . Find C.

(c) A circuit consists of two parallel branches, one having a coil of inductance 0.5 H and resistance 20 and the other, capacitance C in series with resistance It. Determine C and R so that the combined impedance shall be independent of frequency and have zero phase angle.

3. (a) A uniform plane wave is described by the phasor H = 2e–j 0.1 Z ây, A/m. If the velocity of the wave is 2 x 108 m/s and the relative permeability is 1.8, determine the frequency, relative permittivity, wavelength and electric intensity.

(b) A point charge q is situated at a distance d from a grounded conducting plane of infinite extent. Obtain the total charge induced on the plane by direct integration of the surface charge density.

(c) Two parallel conducting plates are separated by the distance d and maintained at the potential difference U. A dielectric slab, of dielectric constant K and of uniform thickness t<d, is

inserted between the plates. Determine the field vectors E

and D

in the dielectric and also in the vacuum between dielectric and one plate. Neglect edge effects due to the finite size of the plates.

4. (a) A car battery has an open-circuit p.d. of 13.4 V and would supply 2000 A when short-circuited. By drawing equivalent circuits in terms of (i) a voltage source and (ii) a current source, determine the current and power in each case in an external load of 0.1 .

(b) What is the function of time-based voltage in a cathode-ray oscilloscope? A cathode-ray oscilloscope has an accelerating anode voltage of 5 kV and length of plates 4 cm. If the maximum error due to the time taken for the electrons to pass the plates is to be limited to 3%, what is the maximum frequency of sine waves that can be displayed?

(c) Explain the construction and working of an electrodynamic type wattmeter. Give an account of its merits and limitations.

SECTION B 5. Answer any three of the following:

(a) In an inverting voltage amplifier circuit using an operational amplifier, the operational amplifier gain is 10,000, the input series resistance Rf is 100 and the feedback resistance Rf is 500. If the input voltage is 1 V, find the exact output voltage and the percentage error that would be committed by considering an infinite gain for the operational amplifier.

(b) Design a logic circuit whose output is given by the Boolean expression: A+B .AB .

(c) Discuss the advantages of a junction field-effect transistor over a conventional bi-polar transistor. Calculate the value of drain load resistance, RL required in the circuit shown in Fig. Q.5(c) at audio frequencies to obtain a voltage gain of 60. The PET used has gm = 5 x 10–3 S and rds = 100 k.

(d) What are the functions of a commutator in a d.c. machine? Explain the various methods of obtain effective commutation.

6. (a) In the circuit shown in Fig. Q. 6(a), there are two identical transistors T1 and T2 with current gain . Another transistor T3 has current gain 3. Establish a relation between I2 and I0.

(b) A transistor used in common-emitter (CE) configuration has the following set of h-parameters when the d.c. operating point is Vce = 10 V and IC = 1 mA.

hie = 2000 ; hoe = 10–4 S

hre = 10–3; hfe = 50

If the a.c. load line seen by the transistor is

rL = 600 , determine input impedance, current gain and voltage gain.

(c) In a single-supply bistable multivibrator, emitter biasing resistance RE = 100, RL = 1,000 .

R1 = 100 k, R2 = 50 k, and Vcc = 18 V. Find (i) the output voltages E1 and E2 and (ii) the collector and base currents for each transistor. Make necessary assumptions and state them.

7. (a) Explain how a 2-winding transformer can be converted into an autotransformer. Two single-phase transformers T1 and T2 rated at 250 kVA each are operated in parallel on both sides. Percentage impedances for T1 and T2 are (1 + j6) and (1.2 + j4.8) respectively. Calculate the load shared by each when the total load is 500 kVA at 0.8 p.f. lagging.

(b) Explain the principle of operation of a polyphase induction motor. A 4-pole, 50 Hz polyphase induction motor has a full-load speed of 1440 r.p.m. Estimate the full-load slip.

(c) Two transformers connected in open delta supply a 400 WA balanced load operating at 0.866 p.f. lagging. The load voltage is 440 V. Calculate the values of kVA and kW supplied by each transformer.

8. (a) A 4-pole, 50 Hz, 3-phase induction motor has motor resistance and standstill rotor reactance of 0.04 and 0.16 per phase respectively. Calculate the value of the external rotor resistance per phase to be inserted to obtain 70% of maximum torque at starting.

(b) Two identical 30 kW, 500 V, 1200 r.p.m. d.c. series motors A and 13 are connected in series

across a 500 V d.c. supply. They drive a common load through reduction gearing of a : 1 and b : 1 respectively. When the load shaft has a speed of 120 r.p.m., motor A has 225 V across it and runs at 1080 r.p.m. Find for this condition the speed of the second motor, the current taken from the mains and total load torque on the shaft. Neglect all losses and saturation effects.

(c) Explain the principle of operation of a synchronous motor. Discuss the various methods of starting the motor. What is a synchronous capacitor?

Some useful data:

Mass of electron, M = 9.1 x 10–31 kg

Electronic charge, e = 1.6 x 10–19 C

Velocity of light, c = 3 x 108, m/s

Boltzmann constant, k = 1.38 x 10–23 J/K

Permittivity of free space, 0 = 8.85 x 10–12 F/m

Permeability of free space, 0 = 4 x 10–7 H/m

PPAAPPEERR -- IIII Time Allowed: Three Hours Maximum Marks: 300

Candidates should attempt five questions only selecting at least two Questions including the compulsory question from each of Section A and B or Section A and C.

Assume suitable data if considered necessary and indicate the same clearly. The number of marks carried by each question is indicated at the end of the question.

SSEECCTTIIOONN AA Question 1 is compulsory.


(20 x 3 = 60)

(a) The performance of a contort system is affected by the characteristics of the measuring element.

(b) In a system which uses a gear train to couple a load to the motor, the equivalent moment of inertia referred to the motor shaft and that referred to the load shaft is independent of gear ratio.

(c) In the operation of a three-phase full-bridge rectifier both in the inverter-mode and converter-mode, the direction of current and polarity of voltage must remain same at different points of the circuit.

(d) In spite of rapid technological advancement, electrical engineers have not been able to apply solid-state devices in the variables-speed motor drives.

2. (a) Enumerate the major qualities of a good adjustable-speed drive system.

(10)

(b) A half-wave rectifier circuit using thyristors feeds a d.c. series motor from a three-phase source.

(10x2=20)

(i) Draw the circuit diagram and the waveforms showing motor voltage and armature currents for firing (delay) angles of 0* and 90*.

(ii) Explain how to find the firing (delay) angle for a particular operating condition of the motor, assuming continuous conduction, when the a.c. source voltage is given. All the drops on d.c. side ace to be considered.

(c) Describe the variable-voltage variable-frequency method of speed control of a.c. motors.

(20)

(d) Explain what is meant by the turn off time of a thyristor and the factors on which it depends

(10)

3. You are given the following components of a position control System:

Two potentiometers including one for yielding the error signal,

One armature controlled d.c. motor,



One amplifier,

Gear train having gear ratio of 1:10.

The field of the d.c. motor is excited from a constant voltage source while the error signal is amplified and applied to the armature circuit.

Assume appropriate numerical values for the system constants and answer (a) to (e).

(a) Draw a diagram showing the scheme of connection to form a positional servomechanism with the above components, indicating the load on the motor.

(10)

(b) Write down the equations describing the operation of (i) the error detector and (ii) the amplifier.

(10)

(c) Obtain the transfer function relating armature voltage and angular displacement of the motor shaft.

(10)

(d) Draw the block diagram of the system (including the simplified one showing a single block for the forward transfer function).

(15)

(e) Obtain the transfer function relating the angular displacement of the output shaft and angular displacement of the reference input shaft.

(15)

4. (a) Explain why a.c. drive systems are becoming increasingly popular.

(15)

(b) In connection with d.c. motor drives explain what is a chopper and how it controls the speed of the drive.

(15)

(c) Give the simplified circuit diagram for a chopper controlled drive scheme using a d.c. series motor and sketch the voltage and current waveforms. Define “pulse ratio’ and “pulse frequency”. Explain how the limits of current pulsation are determined.

(30)

SSEECCTTIIOONN BB Question 5 is compulsory.

5. Comment with justification on the correctness or otherwise of any three of the following statements, restricting your answer to within 200 words for each:

(20x3=60)

(a) Induction motors, whether three-phase or single-phase, must always run at a slip.

(b) The excitation e.m.f. of a loaded synchronous motor working at lagging power factor must be greater than the terminal voltage.

(c) Protection techniques have failed to devise means for protection of alternators against inter-turn faults.

(d) The limitations of the concept of ABCD constants in transmission line calculation is that it

may be applied only to the line itself.

6. (a) What are the specific fields of application of two-phase induction motors? State the special requirements for such uses and discuss the design considerations making the motor suitable for those uses.

(15)

(b) Explain why starters are to be used for starting three-phase induction motors. Mention the factors to be considered before selecting a method for starting a three-phase squirrel-cage induction motor. Compare the available starting torque in the case of auto-transformer starting with that in the case of stator-reactor starting for the same value of line current in both.

(15)

(c) Based on the concept of sudden symmetrical short-circuit stator-current wave of a three-phase synchronous generator obtain the expression for the transient reactance (i) in terms of steady-state reactance, magnetizing reactance and field reactance, as well as (ii) in terms of armature leakage reactance, magnetizing reactance and field reactance.

(15)

(d) Draw the transformer type equivalent circuit for a synchronous machine and write down the expression for short circuit tim constant.

(15)

7. (a) Show that with induction heating, the heat generated in the charge in case of magnetic material is more than that in case of non-magnetic material. Enumerate the advantages of induction heating. State why its use is restricted.

(20)

(b) A high-voltage alternator supplies a group of four synchronous motors through a transformer bank connected in star with solidly grounded neutral on the alternator side and in delta on the low- voltage side. Each motor is grounded through a reactance (current limiting). The alternator neutral is solidly grounded. A single line-to-ground fault occurs on the low-tension side of the transformer bank.

(i) Draw a single-line diagram of the system showing the point of fault.

(ii) Specify completely the sequence network showing the proper connections when the reactances computed on the selected base are given as:

Alternator : Sub-transient reactance = 15%

Zero-sequence reactance = 6%

Transformer Bank: Leakage reactance = 12%

Motor group : Sub-transient reactance = 20%

Zero-sequence reactance = 9%

Current limiting reactance per each motor = 12%

(iii) Neglecting the pre-fault current determine the sub-transient fault current if the p.u. voltage behind the sub-transient reactance for the alternator is 1 + j 0. Make appropriate assumptions regarding negative sequence reactances of the machines.

(25)

(c) Name a d.c. machine in which the troublesome armature reaction has been put to advantage. Draw a schematic representation of the machine. Describe its constructional features with necessary sketches and draw curves to show the nature of its external characteristics.

(15)

8. (a) Explain the functions of:

(i) Pumped Storage Plant (ii) Lightning Arrestor (iii) Phase Advancer (iv) Peterson Coil

(20)

(b) Draw the speed-time curve for the following traction service on level track without any free-running period, and estimate the maximum power input to the traction motors:

The train having a dead weight of 400 tonnes accelerates for 30 seconds attaining a maximum speed of 70 km per hour. It takes 20 secs to bring it at dead stop, the brake being applied at the speed of 55 km ph when the coasting ends. Before starting again it is allowed to deck for 20 secs. Assume the train resistance to be 40 Nw/tonne and allow 10% of dead weight for rotational inertia.

Given: efficiency of the transmission gear is 90% and efficiency of motors is 85%.

(20)

(c) Give brief answers to the following:

(i) What is the data used in load-flow studies? (ii) What is meant by a Swing Bus? (iii) How is an A-C Calculating Board set up? (iv) What information is obtained from load-flow studies?

(20)

SSEECCTTIIOONN CC

Question 9 is compulsory.

9. Select any three of the following statements and indicate with justification whether they are correct or wrong. Your answer must not exceed 200 worth for each statement:

(20x3=60)

(a) There will be no harmonics problem in an amplifier if its input can be made harmonics free.

(b) The characteristics of all Antennas, whether of resonant type or of non-resonant type, are practically same.

(c) In designing choppers for d.c. amplifiers it is necessary to ensure proper damping.

(d) Fillers are to be used in a Visual Modulator, a component of TV transmitter.

10. (a) Give a brief account of the sampling theorem.

(10)

(b) Define the term ‘noise figure’ as applied to an amplifier and explain how it is related with the available power gain.

(10)

(c) Describe the Noise-generator Method for the measurement of Noise Figure. Explain why this method is most desirable.

(20)

(d) In connection with a two-conductor transmission line defines ‘characteristic (surge) impedance’ and ‘propagation constant’. Express the values of the characteristic impedance for co-axial lines, and for parallel-wire lines in terms of line dimensions, as well as for a usual radio-frequency line in terms of line parameters.

(20)

11. (a) What is a Radar? Draw a block diagram showing the components on which radar systems are

based. Explain function of radar as a navigational aid.

(25)

(b) A standing wave indicator used with a square law detector the wave guides having internal dimensions of width 3.75 cm and depth 1.25 cm (the electrical field being parallel to depth) shows adjacent minima at a distance of 2.5 cm horn each other.

Assuming that the guide walls are perfectly conducting calculate (i) the free space wavelength of the singal in the wave guide and (ii) the reflection co-efficient and normalized load impedance when the load impedance is 75 + j 0 ohm and the guide impedance is 5.53 + j 23.7 ohm.

(25)

(c) “The magnitude of the radiation resistance of a transmitting antenna depends upon the antenna configuration.” –Discuss.

(10)

12. (a) A pentode operating as an amplifier has a grid leak resistance of 150 k. A signal source of e.m.f. 125 v and internal resistance 100 k is connected across the input. The signal to noise power-ratio is found as 25 db at the input of the amplifier. Calculate the value of the equivalent input noise voltage.

(20)

(b) Explain how the use of an RF amplifier improves the performance of a super heterodyne receiver.

(15)

(c) Explain what is meant by ‘Excess or flicker noise’ in a transistor.

(10)

(d) Explain how automatic frequency control is achieved in a receiving system.

(15)


Candidates should attempt Question 1 and 5 which are compulsory, and any THREE of the remaining question selecting at least ONE question from each Section. Assume suitable data if considered necessary and indicate the same clearly. All question carry equal marks.

SSEECCTTIIOONN AA


(a) In the d.c. circuit shown in Fig 1 (a), it is desired that V0 = R0 (I1 + 2 I2) where R0 is some resistance, 0 < < 1 and I1 and I2 are two current sources. Find the required values of R1 and R2 in terms of R0 and .

(b) In the ac. circuit shown in Fig 1 (b), find the power delivered by each source. The figure gives the ratios of the ideal transformers, the values of the resistances and the phases of the two voltage sources. Points A and A’ in Fig 1(b) are in phase. The points B and B’ are also in the same phase.

Fig. 1(b)

(c) Give brief explanations in support of the following statements:

(i) The potential function V (x, y, z) gets uniquely fixed in the interior of a charge-free region of an electrostatic field, if V is specified on all the boundaries of the region.

(ii) An infinitely long lossless transmission line draws input current and power from a source just like a resistor, even though the line itself is lossfree.

(d) Give brief explanations in support of the following statements :

(i) The dual-slope, integration method of A to D conversion is the preferred choice for a digital voltmeter.

(ii) Wherever feasible, it is an advantage to use two identical transducers in push-pull fashion for the measurement of a non-electrical quantity.

CC..SS..EE..--((MMaaiinn))--11999944


2. (a) In the two circuits shown in Fig 2 (a), N is the same 2-port network containing linear

resistors. From the terminal conditions given in the first figure, deduce the current I in the second figure. If N is symmetrical, find also the power dissipated in it in the second case.

(b) The switch S is opened at t = 0 in the circuit of Fig 2 (b) after steady state has previously

been reached with S closed. Deduce the values of cdv

dt and Ldv

dt at t = 0+ i.e., immediately

after opening of the switch.

(c) A linear 1-port network with zero initial stored energy draws a current of 5 + 6t – e–2t amperes when connected to a voltage source of 4 V d.c. at t = 0, What would be the rms value of current and the power taken by the same network under steady state when excited by a sinusoidal voltage source of 8 V rms value and radian frequency of rad/sec. ?

3. (a) Four infinitely long parallel cylindrical conductors of radius r oriented in the z-coordinate direction, have their axes passing through x-y plane at the points indicated n the table below and carry currents as specified (current in the positive z-direction being considered positive). Calculate the magnetic field at the origin and discuss its nature. r << .

(b) A capacitor is formed by two parallel circular plates of A m2 area with a separation of d m. The dielectric employed is lossy, with a conductivity and dielectric constant r. The capacitor is charged to a potential of V0 and isolated from the source at t = 0. Find expressions for the conduction and displacement current densities in the dielectric for t 0. What will be the magnetic field in the dielectric during this period ?

(c) A Plane sinusoidal electromagnetic wave travels in free space in the z-direction. Given that H = 0.8 cos ( t – z) i + 0.8 sin (t – z) j .

where i and j are unit vectors in x and y directions, find the corresponding electric field

vector E and the Poynting vector. What type of polarization does this wave have ?

4. (a) Fig 4 (a) shows a limit bridge which is employed to test if a given resistor Rx has a value within ± 5% of its nominal value of 1000 W. In the test, the signs of the deflection of the centre-zero galvanometer G are observed for the two positions of the switch S. Based on the information given, explain the working of the scheme and deduce the required values of R1 and R2.

(b) A periodic voltage having the waveform shown in Fig 4 (b) is applied to the following types of voltmeters. Assuming the meters to function ideally, deduce the reading in each case. If the connections to the meter terminals are interchanged, state for each case if there would be a change in the reading.

(i) Permanent magnet moving coil type meter with centre zero

(ii) Full wave rectifier type a.c. voltmeter

(iii) Moving iron voltmeter

(iv) Shunt-diode type rectifier-amplifier ac. voltmeter

(c) Describe the essential constructional features and properties of metallic strain gauges. Indicate how strain gauges may be used to easure the axial force transmitted by a rod of square cross section.

SECTION B


(a) Give brief explanation in support of the following statements:

(i) The ‘NAND’ gate is one of the mast widely used logic gates.

(ii) Oscillators requiring high frequency stability employ crystals.

(b) A 100 V/400 V, 2 kVA single phase transformer has 5% impedance. If it is connected as a 500 V1400 V autotransformer, deduce from fundamentals its kVA rating and percentage impedance as an autotransformer.

(c) Find an expression for the maximum steady state power output of a d.c. shunt motor with an armature resistance Ra, if the applied voltage V’ and the shunt field current ace kept constant. Neglect rotational losses and assume that the theoretical maximum power output will not overload the motor.

(d) Give brief explanations of the following statements

(i) Semiconductor devices made of silicon can be operated at higher temperatures than those made of germanium.

(ii) From a knowledge of the variation of dielectric constant at different temperatures, for static field conditions, the contribution of the orientational polarizability to the total static polarizability of a dielectric can be deduced.

6. (a) Find the common mode rejection ratio of the simple differential amplifier shown in Fig 6(a).

Assume that the JFET’s are identical and have gm = 3 mS and rd = 40 k. Take RD = 10 k and RS = 20 k.

(b) Assume that the OP-AMPS in the circuit of Fig 6 (b) are ideal, show that 2

2

d vi M

dt

.Compute the value of M in terms of the component values in the circuit and justify the name “frequency dependent negative resistor’’ given to this circuit.

(c) Draw an astable multivibrator circuit employing two BJT’s in the common emitter configuration and derives an expression for the frequency of its oscillation, taking the mark/space ratio of the waveform to be 1.

7. (a) Sketch on the same pair of axes, the torque-speed characteristics of a typical 3-phase induction motor under the following operating conditions. Neglect stator impedance. Mark the torque and speed values of salient points on the characteristics. Explain your answer briefly.

(i) Normal operation

(ii) Rotor resistance doubled, other quantities remaining the same as under normal operation

(iii) Applied voltage and frequency halved, all other quantities remaining the same as under normal operation.

(b) A 220 V d.c. shunt motor has an armature resistance of 0.5 and field circuit resistance of 220 . It takes a line current of 41 A when delivering full load torque. If it is desired to raise the speed by 60 percent with the load torque remaining constant, find the additional resistance to be inserted in the field circuit. Assume linear magnetisation characteristic.

(c) A 3-phase synchronous motor has 80% synchronous reactance and negligible resistance, When connected to busbars at rated voltage and the excitation adjusted for an e.m.f. of 120%, the machine draws an input kVA of 100%. Find the mechanical power delivered by the motor, neglecting all losses. If for the same load on the motor, the excitation is reduced to generate an e.m.f. of 100%, what would be the input kVA ?

8. (a) A 3-phase turbo-alternator with a synchronous reactance of 10 per phase and negligible

armature resistance is connected to 11 kV constant-voltage, constant-frequency busbars and supplies 100 A at unity p.f. to the system. If the turbine power is kept constant and the excitation of the alternator is increased by 25%, what would be the new current and p.f. ? Make appropriate assumptions.

(b) A 100 kVA 11 kV/ 400 V, 3-phase transformer has its maximum possible efficiency of 98%, when it delivers 80 kVA at unity p.f. and rated voltage. The maximum possible voltage regulation 61 the transformer is 4%. Find the efficiency and voltage regulation of the transformer for rated kVA output at 0.8 p.f. lagging.

(c) A rectangular metal strip carries a current of 50 A in the x-direction and is subjected to a magnetic field of 1 tesla in the z-direction. The thickness of the strip in the z-direction is 0.5 mm. What would be the direction of tile Hall voltage? If the latter has a magnitude of 125 V, find the number of conduction electrons per m3 in the metal.


Candidates should attempt FIVE questions only selecting at last TWO questions including the compulsory question from each of Section A and B or Section A and C.

Assume suitable data if considered necessary and indicate the same clearly. The Number of marks carried by each question is indicated at the end of the question.

SSEECCTTIIOONN AA Questions No. 1 is compulsory.


(2x3=60)

(a) If mathematical and functional relationships of all the system components are known a block diagram can be constructed as a reference for analytical or the computer solution of the system.

(b) Knowing the transfer function of a linear time invariant system, it is possible to obtain the state diagram and dynamic equations of the system

(c) The closed loop stability of a system is determined by the properties of the Nyquist plot of the open-loop transfer function of the system.

(d) A cycloconverter is a unit for obtaining variable-voltage and variable-frequency power directly from, fixed frequency supply, but has not gained popularity because of certain disadvantages.

2. (a) Fig 1 shows the circuit of a field controlled d.c. motor. The equivalent moment of inertia of the motor is 3 in kg-m2 the equivalent friction coefficient of the motor is f in Nm/rad/sec. ‘The motor torque per unit of field current is Km in Nm/A. Derive the transfer function of the

motor, f

s

E s

. Find

f

s

E s

, if J = 0.5, f = 0.25 , Km = 25, Rc = 100 , Lf = 15 H and ia = 10

A(constant).

(20)

(b) Fig 2 shows a non-unity feedback control system.

CC..SS..EE..--((MMaaiinn))--11999944


Find the steady state error due to (i) step input and (ii) ramp input.

(20)

(c) Give a schematic diagram and explain the operation of a synchro-error detector.

(20)

3. (a) Give the significance of state transition matrix. A linear-time-invariant system is described by

X= AX + BU

Where

1 0 1

,1 1 0

A B

and u(t) is a unit step function at t = 0 and X(0) = [1 0]T. Find X (t).

(20)

(b) A system has

2 4

KG s H s

s s s

What value of K makes the system unstable? Find the intersection of the root-loci with the imaginary axis.

(20)

(c) Explain with the help of a block diagram the principle of state feedback control. Discuss pole-placement design of a system through state variable feedback.

(20)

4. (a) What is a chopper ? Give various configurations of chopper for operation in different quadrants. Explain the operation of each configuration.

(20)

(b) A 3-phase fully controlled bridge convener with 415 V supply, 0.04 resistance/ phase and 0.25 reactance per phase is operating in the inverting mode at a firing advance angle of 35°. Calculate the mean generator voltage when the current is level at 80 A. The thyristor voltage drop is 1.5 V.

(20)

(c) Draw and explain vacuum tube oscillator and SCR inverter circuits for converting electrical energy at low frequency to energy at high frequency suitable for induction heating. Why are vacuum tube oscillators less efficient as compared to SCR?

(20)

SSEECCTTIIOONN BB Question 5 is compulsory.

5. Comment with justification on the correctness or otherwise of any three of the following statements, restricting your answer to within 200 words for each

(20x3=60)

(a) Oscillations take place in a synchronous machine if there is a sudden change in load, which can be minimized by damper windings.

(b) A cross-field machine may behave as a constant voltage or a constant current generator depending upon the degree of compensation.

(c) Transient stability of a synchronous generator may be improved by using auto-reclose circuit breakers.

(d) Ward-Leonard method of speed control of a d.c. motor, provides a smooth control in both the directions.

6. (a) Distinguish between singly-excited and doubly-excited magnetic system.

In a doubly-excited rotary machine, the inductance coefficients are

L11 = (1.1 + 0.4 cos 2)

L22 = (0.03 + 0.005 cos 2)

L12 = 0.2 cos

The exciting currents are i1 - 8 A and i2 = 50 A. Obtain the torque/angular displacement relation. Derive the expression used, if any.

(20)

(b) Discuss speed control of wound rotor induction motor by injecting voltage info its rotor circuit. Draw and explain the complete phasor diagram of the motor with injected e.m.f.

(20)

(c) Give the constructional features, principle of operation and applications of a variable reluctance stepper motor.

(20)

7. (a) Draw and explain a 3-phase bridge rectifier circuit for converting a.c. into d.c. for HVDC transmission.

A bridge connected rectifier is fed from 220 kV/110 kV transformer with primary connected to 220 kV. Determine the d.c. output voltage when the commutation angle is 20° and the delay angle is

(i) 00, (ii) 30°, and (iii) 45°

(20)

(b) Explain what you understand by positive, negative and zero sequence voltage or current.

A generator is subjected to a sustained fault with phases B and C short circuited and connected to neutral. Show that the zero-sequence impedance may be determined by

ao

n

VZ

I , where Va is the voltage from the unfaulted conductor to neutral and Ia is the neutral

current.

(20)

(c) Differentiate between type tests and routine tests. Briefly explain the different tests to be

carried out to prove the ability of a circuit breaker.

(20)

8. (a) Explain the effect of change of (i) excitation, and (ii) prime-mover input of a synchronous generator connected to an infinite bus.

A turbo-alternator having a reactance of 10 ohms has an armature current of 200 A at unity power factor, when running on 11000 V, constant frequency bus-bars, If the prime-mover input is not changed but the e.m.f. is raised by 20%, find the value of armature current and power factor.

(20)

(b) What do you understand by steady state and transient state characteristics of an electric drive?

Derive expressions for acceleration time of an electric drive under conditions of (i) constant motor anti load torque, (ii) constant load torque and variable motor torque.

(20)

(c) Discuss the characteristics and limitations of different motors suitable for electric traction.

(20)

SSEECCTTIIOONN CC

Question 9 is compulsory.

9. Select any three of the following statements and with justification whether they are correct or incorrect; your answer must not exceed 200 words for each statement:

(20x3=60)

(a) Broadcast systems do not employ SSB and DSB-SC signals though these signals offer bandwidth and power advantages.

(b) Gunn diode amplifiers are not used nearly as much as Gunn oscillators.

(c) Each cavity resonator has an infinite number of resonant frequencies,

(d) PCM is more noise-resistant than other forms of pulse modulation.

10. (a) Distinguish between amplitude modulation, frequency modulation and phase modulation. Give the advantages of frequency modulation over both amplitude and phase modulation.

(15)

(b) The antenna current of an AM transmitter is 12 A when only carrier is sent but it increases to 13.5 A when carrier is modulated by a single sine wave. Find the percentage modulation. Also determine the antenna current when the depth of modulation is 0.8. Derive the relation used.

(15)

(c) Explain the concept of the noise triangle. What is pre-emphasis? Why is it used?

(15)

(d) What is a directional coupler? Define directivity and directional coupling and give their significance.

(15)

11. (a) Describe with the help of a block diagram, the arrangement of a typical high power pulsed radar set. Give the function of each block.

(20)

(b) Explain ISDN and LAN. What is X-25 interface protocol? Explain its function and use.

(20)

(c) An aerial array consists of 12 vertical aerials in a straight line spaced half a wave-length apart and equally energised in phase. Deduce the angular width of the forward beam in the horizontal plane.

(20)

12. (a) A TE11 mode is propagating through a circular wave-guide. The guide contains an air dielectric. Given that X’11 = 1.841 and cut-off frequency fc is 1.758 x 109 Hz, determine:

(i) the diameter of the guide

(ii) the wave-length in the guide for an operating frequency of 3 GHz

(iii) wave impedance

(b) Give the major differences between TWT and Klystron. Explain, with the help of a schematic and simplified diagram, the operation of a helix travelling wave tube.

(20)

(c) Describe an optical communication system with the help of a suitable block diagram. Discuss the advantages of optical communication using optical fibres.

(20)


Instructions: Candidates should attempt Questions 1 and 5 which are compulsory, and any three of the

remaining question selecting at least one question from each Section. Some useful data is given at the end of the question paper. Assume any other data if considered

necessary and indicate the same clearly. All questions carry equal marks. Parts of the same question must be answered together and must not be interposed between

answers to other questions. _______________________________________________________________________________________

SSEECCTTIIOONN AA

1. Answer any three of the following

(a) Consider the electric fluid due to an infinitely long uniform line charge. Show that the divergence of this field is zero.

(b) Fig. Q. 1(b) shows the details of a portion of a d.c. network. Evaluate the voltage V.

(c) In the a.c. circuit of Fig. Q.1(c), the rms values of different voltages under steady state are:

VS = 16V; VR = 10V; V1 = 10V; V2 = 7V.

Find the value of RL

(d) Justify the following statements:

(i) It is advisable to measure a small quantity directly and not to deduce it as the difference of two individually measured large values.

(ii) The plain Wheatstone bridge method is not suitable measurement of low-valued resistors.

2. (a) Find the rms value of current through the switch S when it is closed in the 3-phase circuit shown in Fig. Q. 2(a). Hence find the rms value of voltage across it when S is opened.

CC..SS..EE..--((MMaaiinn))--11999955


1 2Z 40 j0 , Z j80 ohms ohms

(b) The two figures show a lineal reciprocal 2-port network N under two different terminal conditions. The switches are closed at t = 0 in both the circuits with zero initial conditions in N. Given that the currents in (i) for t > 0 are :

i1 = 0.5 (e–t + e–3t) amps;

i2 = 0.1 (3e–3t – e–t) amps, find v1(t) for t > 0 in (ii).

(c) A digital filter is characterised by the following recursive relation, where x(n) and y(n) are the input and output samples at the nth sampling instant. The sampling frequency is 100 Hz.

y(n) = 0.8y (n – 1) – 0.64 y (n – 2) + x (n)

–x(n – 1) + x(n – 2)

Find the poles and zeros of the discrete-time transfer function of the filter. Hence deduce and sketch the magnitude response characteristic of the filter from f = 0 to f = . Mark the values at f = 0, 50 and 100 Hz.

3. (a) Consider an electric displacement vector

2

0 0

ˆ ˆ15exp /r z

r zD i i c m

x x

in cylindrical coordinates r, , z. Calculate the outward flux crossing a right circular cylinder of radius 2x0, its axis coinciding with the z-axis and with the two plant faces at z = 0 and z = 5x0, where x0 = 1 cm.

(b) Derive an expression for the inductance per unit length of a coaxial cable with radii a and b. Also calculate the value when a = 0.5 cm and b = 2.0 cm.

(c) A 2 m long current carrying conductor is aligned in y-direction, its x-and z-coordinates being 5 m each. The current is 8 A in the negative y-direction. The conductor is subjected to a

uniform magnetic field and the resulting force on the conductor is 4 x 10–3 ˆ ˆx zi i N . Find

the magnetic flux density (of the applied field).

4. (a) A 20 k standard resistor Rs and a test resistor Rx are connected in series across a d.c. voltage source of negligible internal resistance. The voltages across Rs, Rx and the source are measured in turn by an accurate 0 – 120 V d.c. voltmeter. The readings obtained are 75 V, 21 V and 100 V respectively. Find the sensitivity of the d.c. voltmeter.

(b) Give the block diagram of a typical modem digital counter and show how it is used to measure the frequency of a given lest signal. What other categories of measurement can be performed using a digital counter ?

(c) A thermo-electric sensor with a time constant of 50 ms is used to record the temperature of a furnace. Assuming that the furnace temperature varies sinusoidally between the two limits 300°C and 500°C with a frequency of 2 Hz, determine the limits of the temperature as recorded from the sensor. Indicate a method of reducing the resultant error.

SECTION B

5. Answer any three of the following :

(a) Explain how the input current changes in a three phase synchronous motor with constant load when the field current is varied. Draw phasor diagrams.

(b) In the circuit of Fig. Q 5(b), the diodes are ideal. Determine and sketch v0(t) for 0 < t < 8 ms when v1(t) is a square waveform of ± 1 V amplitude, 8 ms period with a positive going zero crossing at t = 0, the input v2(t) 2 sin t/3 volt with t in ms and v3(t) is a 3 V amplitude rectangular pulse existing over 1 ms < t < 3 ms.

(c) Calculate the conductivity of silicon when it is doped by an n-type impurity to the extent of 1 in 106. The atomic density of silicon is 5 x 1022 atoms/cm3, the intrinsic carrier density is 1.48 x 1010 pair/cm3 and the mobilities are 500 cm2/V-s and 1300 cm2/V-s. Compare the conductivity with that of intrinsic silicon.

(d) The circuit shown in Fig. Q. 5(d) is using master-slave flip-flops. VCC is high and can be taken to be 1. Find B0 and B1 for the first seven clock cycles after these are initially cleared. Give precise explanations for your answers.

6. (a) Two & series motors with different airgaps, but otherwise identical, run at 700 and 750 rpm respectively, when taking 50 A at 500 V. The resistance of each motor including the series field is 0.36 . If the two motors are mechanically coupled and connected in series to 500 V supply taking a current of 50 A, determine the speed and the voltage across each machine.

(b) Two single phase transformers, T1 and T2 are operated in parallel on both sides. The transformer, T1 is rated at 100 kVA having a percentage impedance of (1 + j5), The transformer, T2 is rated at 200 kVA having a percentage impedance of (1.5 + j4). Calculate the load shared by each, when the total load is 250 kVA at 0.8 power factor tagging. Also calculate the total load at the same power factor that the two transformers in parallel can supply such that neither of the transformers gets overloaded.

(c) Explain the following in relation to a plain single phase induction motor:

(i) It fails to start when connected to ac supply.

(ii) If it is started in either direction, it continues to rotate in the same direction.

7. (a) A 3-phase, 6600 V, 100 kVA, synchronous generator delivers rated current at 0.8 pf lagging, the load voltage being 6 kV. The resistance and synchronous reactance per phase are 1 and 30 respectively. Calculate the load voltage when the load current remains same at rated value, but the power factor changes to 0.8 leading. The field current is the same as before.

(b) The rotor of a 3-phase, 6-pole, 50 Hz, slip-ring induction motor has resistance of 0.2 per phase and runs at 960 rpm on hill load. Calculate the resistance to be inserted in the rotor circuit so that the speed reduces to 800 rpm, with torque (i) remaining constant, and (ii) varying as square of speed. Neglect stator impedance. Make assumptions as needed.

(c) A 400 V, 15 kW, dc shunt motor takes 42 A and runs at 1200 rpm. The resistances of armature and field are 0.8 and 200 respectively. Neglect armature reaction. (i) Find the field current required to reduce the speed to 1000 rpm with load torque at half of previous value. (ii) Find also the initial armature current, when the field current is changed to that found in (i), with the speed not changing instantaneously.

8. (a) Find the CB h-parameters of a transistor in terms of its r-parameters and .

(b) A silicon transistor has a -independent bias using an emitter resistor potential divider combination. The emitter circuit resistance is 300 , the collector circuit resistance is 500 , collector supply voltage is 15 V and = 100. Find the potential divider resistances so that maximum symmetrical swing is permitted at the collector. Take VCE, sat = 0.

(c) Describe a successive approximation type of an A to D converter and give its merits. Find the conversion time and the resolution of an 8-bit ADC of this type when the clock period is 1.25s.

Some useful data:

Electronic charge : e = 1.6 x 10–19 C

Permeability of free space : o = 4 x 10–7 H/m

Permittivity of free space : o = 8.85 x 10–12 F/m


Instructions:

Candidates should attempt five questions in all selecting at least two questions including the compulsory question from each of Section A and B or Section A and C.

Assume suitable data if considered necessary and indicate the same clearly.

The number of marks carried by each question is indicated at the end of the question.

Parts of the same question must be answered together and must not be interposed between answers to other questions.

.

SSEECCTTIIOONN AA 1. Select any three of the following statements, read them carefully and identify the correct and

incorrect ones. Justify your answer using not more than 200 words in each case.

(20x3=60)

(a) Real parts of the Eigen values of a stable system are always positive.

(b) Knowing the step response of a system the response to a ramp input cannot be deduced.

(c) Use of a free wheeling diode improves the quality of speed-torque characteristics of a thyristor controlled d.c. motor.

(d) State variable method of analysis can be applied both to linear and non-linear systems.

2. (a) Sketch and explain the frequency-response characteristics of a second-order control system. What is the effect of damping ratio on the amplitude and phase of the steady state output ?

(20)

(b) The forward path transfer function of a position control system with velocity feedback is given by

K

G ss s p

Determine the sensitivity of the transfer function of the closed-loop system to changes in K, p and a for transfer function of the feedback path (i) H(s) = 1, and (ii) H(s) = (1 + s). The nominal values of the parameters are K = 12, p = 3 and = 0.14.

(20)

(c) A linear feedback control system has an open-loop transfer function

2

2 1

sA

s s

where and are adjustable parameters. Find the relation between and so that the system is stable with unity feedback.

CC..SS..EE..--((MMaaiinn))--11999955


3. (a) What are Tachogenerators ? Discuss with block diagram, their application to dosed-loop

control systems.

(20)

(b) The block diagram of an electronic pacemaker is given in Fig 1, where K = 400.

(i) Calculate the output c(t) for a unit step input.

(ii) Calculate the integral of the square of the error between input and the output.

(iii) Determine the steady-state error for a unit ramp input.

(iv) Determine the value of K for which the steady-state error to a unit ramp will be 0.02.

(20)

(c) A linear control system is described by the following equations in state-space domain:

1 1

2 2

2 0 0

1 1 1

x xu t

x x

1

2

1 2 1x

y u tx

Find the transfer function of the system.

(20)

4. (a) What are the important ratings that specify an SCR? Compare the performance characteristics of a rectifier circuit using ordinary diodes with that of one using SCR’s.

(20)

(b) A chopper circuit as shown in Fig 2 is inserted between a battery, Vdc = 150 V and a load resistance RL = 10 . The mm-off time for the main thyristor Th1 is 110 s and the maximum permissible current through it is 30A.Calculate the values of the commutating components L and C.

(20)

(c) A slab of material 2.5 cm thick and 200 sq. cm in area, having relative permittivity 5 and 0.05 is to be heated using dielectric heating. The power required is 200 watts at a frequency of 30 MHz. Determine the voltage required and the current that will flow through the material. If the voltage were to be limited to 600 V, what should be the frequency for the same power requirement?

(20)

SSEECCTTIIOONN BB Question no. 5 is compulsory.

5. Comment with justification on the correctness or otherwise of any three of the following statements, restricting your answer to within 200 words for each:

(20x3=60)

(a) Lightning arrestor should be located as close to the transformer being protected as is physically possible.

(b) Tap changing transformers are used in power systems primarily to control reactive volt-amperes.

(c) The main purpose for which the damper bars are located in the rotor faces of a synchronous motor is to prevent hunting of the machines.

(d) Economic operation of a power system requires that the incremental fuel cost of each plant multiplied by its penalty factor is the same for all plants of the system.

6. (a) Explain the concept of self GMD and mutual GMD in respect of 3-phase transmission lines. Indicate how these concepts are used in estimating the inductance and capacitance of 3-phase overhead lines.

(15)

(b) Discuss briefly the various methods adopted for improving the transient stability of a power system.

(15)

(c) A 3-phase, 50 Hz, transmission line is 400 km long. The voltage at the sending-end is 220 kV. The line parameters are:

R = 0.125 /km, X = 0.4 /km, and Y = 2.8 x 10–6 S/km. Find, using approximate expressions for A, B, C, D constants of long lines, the following:

(i) The sending-end current and receiving end voltage when there is no-load on the line.

(ii) The minimum permissible line length if the receiving no-load voltage is not to exceed 235 kV.

(30)

(iii) For part (i), the maximum permissible line frequency, if the no-load voltage is not to exceed 250 kV.

7. (a) Explain the various reactances associated with a salient-pole synchronous machine when it is subjected to sudden short circuit at its armature terminals. How can these reactances be determined from the short circuit oscillograms of the machine?

(15)

(b) Briefly explain the various controls in a.c. drives. Why are such drives becoming increasingly more pou1ar as compared to d.c. drives ?

(15)

(c) A 4-pole, 3-phase, 400 V, 50 Hz, induction motor has the following per phase parameters for its circuit model (referred to the stator side):

r1 = 1.2 x1 = 1.16

r2

’ = 0.4 x2’ = 1.16

Find the slip and corresponding torque when the machine develops

(i) maximum torque, and (ii) maximum power. The machine is Y-connected.

(30)

8. (a) Discuss various types of electric traction used in India. Explain the characteristics of the motor used in any one such system.

(15)

(b) Explain clearly with a flowchart, the computational procedure for load flow solution using Gauss-Seidel method when the power system contains all types of buses.

(15)

(c) A 40 MVA, 3-phase, 345/34.5 kV transformer is star/delta connected. Select standard CT ratios on the two sides of the transformer for percentage differential protection of the transformer. Would an auto-transformer be required? If so, what should be its current ratio?

(30)

SSEECCTTIIOONN CC

Question no. 9 is compulsory.

9. Select any three of the following statement and indicate with justification whether they are correct or incorrect. Your answer must not exceed 200 worth for each statement.

(20 x 3=60)

(a) In PM, the frequency deviation is proportional to instantaneous amplitude of modulating voltage but not to the modulating frequency.

(b) An ideal directional coupler must have infinite directivity.

(c) Telecommunication carriers can increase the utilization of their existing cable ducts significantly if fibre-optic cables are substituted for copper.

(d) Digital processing is used widely because it provides economic and rapid manipulation of data.

10. (a) Explain what you understand by shot noise, signal-to-noise ratio and noise figure. When might the noise-figure be a more suitable piece of information than the equivalent noise resistance?

The first stage of a 2-stage amplifier has a voltage gain of 10, a 500 input resistor, a 1500 equivalent noise resistance and a 30 k ohm output resistance. For the 2nd stage these values are 20, 80 k, 10 k and 1 M respectively. Calculate the equivalent input-noise résistance of the two-stage amplifier.

(20)

(b) Give the differences between FM and AM receivers. How can the overall limiting performance of an FM receiver be improved? Explain the operation of AGC in addition to a limiter.

(20)

(c) Explain, with the help of block diagram, the principle of operation of an ISB transmitter.

(20)

11. (a) What is resonator? Define and obtain expression for the Q-factor of a parallel plate resonator.

The parallel plates of a resonator are 1.5 cm apart and have free space in between them. Find Q of the resonator for fundamental frequency of oscillations. For copper = 5.8 x 107 S/m,

0 = 4 x 10–7 H/m.

(20)

(b) What is MESFET? Give its characteristics. What are intrinsic and extrinsic parameters of a MESFET? What is the effect of increasing the channel doping to a value as high as possible?

(20)

(c) Discuss the advantages and list applications of parametric amplifiers. Draw the circuit diagram of a TW parametric amplifier and explain briefly how it works.

(20)

12. (a) Compare and contrast the performance and advantages of co-axial cable and microwave links as broad band ‘continental’ transmission media. Draw and explain block diagram of a microwave link carrier chain.

(20)

(b) What is a Modem? Draw and explain a Communication system using Modems. What is RS-232 interface? Explain its use for data transmission.

(20)

(c) With the help of a block diagram, explain a Radio-telemetery transmitter using frequency-division multiplex with TDM for sub-communication.

(20)


INSTRUCTION Candidates should attempt Question 1 and 5 which are compulsory, and any three of the remaining

questions selecting at least one question from each Section. Some useful data is given at the end of the question paper. Assume, any other data if considered

necessary and indicate the same clearly. All questions carry equal marks. Parts of the same question must be answered together and must not be interposed between answers

to other questions. .

SSEECCTTIIOONN AA


(a) The h-parameters of a 2-port network used in the circuit of Fig. 1a are h11 = 100 , h12 = 0.0025, h21 = 20 and h22 = 1 mS. Find the ratio V2/V1.

(20)

(b) A hollow cylindrical conductor of length L and radius R and carrying a surface current of density Jb = Jz iz A/m is placed in a B-field given by

ˆ ˆr

aB i bi T

r

Find the magnitude and direction of torque on the conductor.

(20)

(c) Using Thevenin’s theorem, determine the current in the 2 resistor in the circuit of Fig. 1c.

(20)

(d) (i) Give the schematic of two synchro pairs, one pair being useful for remote control and the other pair functioning as a transducer for converting angular position into an electrical signal.

CC..SS..EE..--((MMaaiinn))--11999966


(10)

(ii) Give a typical set up for measuring the stain of a cantilever beam, with temperature compensation.

(10)

2. (a) A series R-L circuit with R = 50 and L = 0.2 H has a sinusoidal voltage v(t) = 75 sin (500 t + 0.785) applied at t = 0, t being taken in sec. Find the complete current for t > 0. What is the current at t = 0 + ? Why?

(20)

(b) Show that an n-mesh, passive, three-terminal network may be replaced by a delta-connection of three impedances. Here n can be arbitrarily large.

(20)

(c) A 300 kW, 0.65 lagging power factor load is shunted by a capacitor so that the power factor improves to 0.90 lagging. Find the kVAr that the capacitor must furnish and the reduction in apparent power.

(20)

3. (a) An aeroplane flies over the surface of the ocean and sends a 1 MHz plane wave vertically downwards with a field intensity of 1000 V/m just at the ocean surface. If a submarine requires a 10 V/m field, how deep can it be submerged and still be contacted by the aeroplane. The ocean water constants are = 4 S/m, r = 1 and r = 81.

(20)

(b) A 2 C charge is located at (0, 3, 0) m and a 4 C charge at (4, 0, 0)m. Find the electric field at (0, 0, 5) m due to both these charges.

(20)

(c) The phase velocity vp of a wave of frequency f and wavelength is given by vp = c(0/)2 where c and 0 are constants. Derive an expression for the group velocity vg in terms of vp and vp.

(20)

4. (a) Using an RTD with 0.00251/°C coefficient and R (25°C) as 1000 , design a Wheatstone bridge with an OP-AMP arrangement to provide a 0 to 10 V signal for a temperature span of 25°C to 125°C.

(20)

(b) (i) Define the terms ‘Dynamic error’ and ‘Resolution’ of an instrument. Explain how these can be determined in the laboratory.

(10)

(ii) Give two examples each of active and passive transducers. Suggest suitable signal conditioning circuits for the same.

(10)

(c) For the R-2R type of digital-to-analog converter shown in Fig. 4c, find the output voltage when S4 is closed and S3, S2 and S1 are grounded.

(20)


(a) In the circuit of Fig. 5a, the diode is ideal, R = 1.2 k and I = 5 mA. Draw the v – i characteristics of the circuit (i) with S open and –10 volt <v < 15 volt and (ii) with S closed and 0 <v < 15 volt What happens when v <0 with S closed ?

(20)

(b) Show that the NMOS circuit of Fig. 5b is a NAND gate for positive logic. Write down the various combinations of inputs and output in a tabular form and clearly state the meanings of any symbols or parameters used therein.

(20)

(c) Describe different methods for starting of synchronous motors.

(20)

(d) (i) Explain the following terms as referred to an operational amplifier:

(a) Input offset voltage and

(b) Input offset current

(10)

(ii) The input vi (t) in the circuit of Fig. 5d is vi (t) = 1.5 sin 2 103 t volt with t in sec, Sketch v0 (t) and label important points.

(10)

6. (a) Two single-phase transformers T1 and T2, rated at 300 kVA and 400 WA respectively, when operated in parallel share a 500 WA, 0.8 lagging power factor as follows:

T1 : 192 + j 117 kVA

T2 : 210 + j 184 kVA

Find the regulation of T2 for rated kVA output at 0.8 lagging power factor when the percentage impedance of T1 is 1.5 + j 6.

Also find the total load that the two transformers can share without any of these getting overloaded at 0.9 lagging p.f.

(20)

(b) Two shunt generators rated at 150 kW and 250 kW when running at rated speeds have rated open circuit voltages of 245 V and 243 V respectively. Their external characteristics are linear. The regulation of the 150 kW machine is 5% and that of the 250 kW machine is 4%, both in terms of their full Load terminal voltages, If these machines are connected in parallel, how much load will be carried by each machine when supplying a connected load of 360 kW? What will be the common terminal voltage?

(20)

(c) (i) Name different types of single phase induction motors. ‘Draw a comparison of their performance features.

(5)

(ii) ‘The range of stable operation of a three-phase induction motor can be extended by connecting a larger resistance in the rotor.’ Justify the statement.

(5)

(iii) Show that the reactive power drawn by a three- phase induction motor is maximum at standstill and decreases as the slip increases, the motor being operated at constant voltage.

(10)

7. (a) A 6-pole, 400 V, 50 Hz, 3-phase induction motor has a rotor resistance such that the maximum torque occurs at a sup of 0.2. An additional resistance of 0.5 has to be inserted to obtain 75% of the maximum torque at starting. Find the rotor resistance and reactance.

(20)

(b) A 6600 V, 3-phase star-connected alternator has a resistance of 0.4 and a synchronous reactance of 6.0 per phase. If the regulation at fill load 0.8 power factors leading is 7.5%, find the kVA rating of the alternator.

(20)

(c) (i) What do you understand by the terms ‘short-circuit ratio and ‘synchronizing torque’ as applied to a synchronous generator ?

(10)

(ii) A 1500 kVA, 4–pole, 3-phase, 50 Hz non-salient pole synchronous motor of

negligible armature resistance having a synchronous reactance of 45 is operating on an infinite bus with its voltage at 6.6 kV 50 Hz. Find the synchronising torque per mechanical degree of rotor displacement (a) at no load and (b) when the motor is operating at full load with 0.85 leading power factor.

8. (a) Find the relation between vo and vs for the circuit of Fig. 8a and show that, with proper adjustments, the voltage v0 can be made proportional to the logarithm of the input voltage vs.

(20)

(b) Find the mid-frequency gain of the amplifier shown in Fig. 8b. Neglect the loading effect of R1 and R2. The transistor parameters with usual notations are gm = 0.015 S, rb’e = 1 k, rbb’ = 90 , Cb’e = 20 pF and Cb’e = 3 pF while RE = 1 k.

(c) Design a combinational logic circuit to generate the 2’s complement of a 2-bit number. You may use two flail adders and other gates as required but keeping the number of other gates to a minimum.

(20)

Some useful data:

Electronic charge : e = 1.6 x 10–19 C

Permeability of free space : o = 4 x 10–7 H/m

Permittivity of free space : o = 8.85 x 10–12 F/m

PPAAPPEERR -- IIII Time Allowed: 3 Hours Maximum marks: 300

SSEECCTTIIOONN AA Question no. 1 is compulsory.

1. Select any three of the following statements, read them carefully and identify the correct and incorrect ones. Justify your answer using not more than 200 words in each case.

(20 x 1 = 60)

(a) State equations are first order differential equations that can be applied to portray only the linear time-invariant systems.

(b) By adding a pole to the closed loop transfer function, the maximum overshoot increases and the effect is opposite to that of adding a pole to the open loop transfer function.

(c) For a closed-loop system to be asymptotically stable, there is no restriction on the location of the poles and zeros of the loop transfer function F(s), but the poles of the closed loop transfer function must all be located in the left half of S-plane.

(d) A vacuum tube oscillator has small power outputs high at frequencies and is less efficient compared to SCR.

2. (a) Describe a simple position control system.

A certain position control system is driven by 100 : 1 gear by a motor whose torque characteristics is of the form (10 Vm – 0.3 N) 10–4 N-m, where N is the speed in R.P.M. and is the voltage supplied by the amplifier. If the synchros produce 1 V/ degree error and if the steady state error should be 5o for a speed input of 20 RPM, calculate the required amplifier gain. Viscous friction at the motor shaft is 60 x 10–6 Nm-sec.

(b) Distinguish between open-loop and closed-loop system. Convert the system of Fig. 1 into a unity feedback system.

(20)

(c) Obtain the state equations in (i) the controller form, and (ii) and observer form for the transfer function G(s) given by

3 2

3 5 7

4 5 18 29 35

s sG s

s s s s

CC..SS..EE..--((MMaaiinn))--11999966


(20)

3. (a) What do you mean by Root-locus of a system ? Give properties of the root-locus.

A unity feedback system has a forward path transfer function

4 5

KG s

s s s

Sketch the locus of the poles of the transfer function of the closed-loop system as ‘K’ varies from zero to infinity, explaining all the steps involved.

(20)

(b) What is a servomotor ? Give the characteristics of a 2-phase servomotor and obtain its transfer function.

A 50 Hz, 2-phase, ac. servomotor has the following parameters:

Starting torque = 0.186 N-m

Rotor inertia = 1 x 10–5 kg m2

Supply voltage = 120 V

No-load angular velocity = 304 rad/sec

Assuming straight line torque-angular speed characteristics of the motor and zero viscous friction derive the transfer function.

(20)

(c) A system is described by

X = AX+BU

Y = CX

where

0 1 –8 2

A = 1 0 –5; B = 1

0 2 –6 0

and C = [1 2 1]

Determine whether the system is stable.

(20)

4. (a) Describe with a neat sketch a step up chopper and discuss its merits and demerits.

(20)

(b) Explain the principle of electric welding. Describe with the help of a neat diagram the principle of operation of a timer circuit used for electric welding.

(20)

(c) Fig. 2 shows a circuit used for self-commutation by resonance.

(i) With the thyristor in blocking state, what is steady state capacitor voltage and load current ?

(ii) Derive an expression for the load current, thyristor current and Vc (t) after a thyristor is triggered at t = 0.

(iii) Calculate the time when the thyristor would stop conducting.

(20)

SSEECCTTIIOONN BB


5. Comment with justification on the correctness or otherwise of any three of the following statements, restricting your answer to within 200 worth for each.

(20x3=60)

(a) Single phase induction motors with auxiliary winding neither develop starting torque nor the running torque.

(b) For EHV transmission lines, it is dangerous to use bundle conductors from the viewpoint of sub-synchronous resonance.

(c) Sudden loss of load may cause severe disturbance in the operation of a power system.

(d) Self-excitation of an induction generator requires the machine to be run at super synchronous speed.

6. (a) Derive an expression for the charge (complex) value per meter length of conductor ‘A’ of untransposed 3-phase line shown in Fig. 3. The applied voltage is balanced 3-phase, 50 Hz. Take the voltage of phase ‘A’ as reference phasor. All conductors have the same radii. Also find the charging current of phase A’. Neglect the effect of ground.

(b) Discuss briefly different methods used for Load-Flow studies. Give their merits and demerits.

(20)

(c) A single-phase, 50 Hz generator supplies an inductive load of 5 MW at a p.f. of 0.707 lagging by means of an overhead line 20 km long. The line R and L are 0.0195 /km and 0.63 mH/km. The voltage at the receiving end is required to be kept constant at 10 kV. Find

(i) the sending end voltage regulation of the line,

(ii) the value of the capacitors to be placed in parallel with the load such that the regulation is reduced to 50% of that obtained in part (i),

(iii) transmission efficiencies in both the cases.

(20)

7. (a) Discuss the behaviour of a 3-phase induction motor on single-phasing. Compare its characteristics, rating and efficiency with that under balanced 3-phase supply.

(20)

(b) Give the constructional features, principle of operation and applications of a variable reluctance stepper motor.

(20)

(c) A salient-pole synchronous motor has xd = 0.85 p.u and xq = 0.55 pu. It is connected to bus-bars of 1-0 p.u. voltage, while-its excitation is adjusted to 1.2 p.u. Calculate the maximum power output that the motor can supply without loss of synchronism. Compute the minimum p.u. excitation that is necessary for the machine to stay in synchronism while supplying the full-load torque.

(20)

8. (a) Explain the principle and applications of (i) induction heating, and (ii) dielectric heating. Discuss the range of frequencies used in these systems of heating.

(20)

(b) Discuss the electrical, mechanical and economic characteristics to be considered in selecting a motor for a given drive. What are the special features required for selecting electrical equipment for mines with fire hazard ?

(20)

(c) An 11 kV, 1-MVA, Y-connected generator has a reactance of 4.2 ohms/phase. The generator neutral is grounded through a resistance of 22 ohms. Determine what percent of the generator winding remains unprotected by the percentage differential relaying. Assume for simplicity that the percentage differential relay operates when out-of-balance current exceeds 25% of the full-load current.

(20)

SSEECCTTIIOONN CC


9. Select any three of the following statements and indicate with justification whether they are correct or incorrect. Your answer must not exceed 200 words for each statement.

(20x3=60)

(a) Modem communication system is concerned with development of transmission and not with the sorting, processing and storing of information.

(b) AM is more likely to be affected by noise than is FM.

(c) In comparison with wave-guides and co-axial lines, strip line has reduced bulk, lower bandwidth, higher power-handling capability and components made of it are readily adjustable.

(d) The conical scanning method of tracking an acquired target is an improvement over lobe switching.

10. (a) (i) What are the three main systems of SSB generation ? Give the salient characteristics of each system.

(ii) Prove that the balanced modulator produces an output consisting of side bands only,

with the carriers removed.

(20)

(b) What is an RF amplifier ? Give its advantages. What do you understand by image frequency and its rejection ratio?

In a broadcast super-heterodyne receiver having no RF amplifier, the loaded Q of the antenna coupling circuit (at the input to the mixer) is 100. If the intermediate frequency is 555 KHz, calculate

(i) the image frequency and its rejection ratio at 1000 KHz, and

(ii) the image frequency and its rejection ratio at 25 MHz.

(20)

(c) Give the properties of paraboloid reflectors. Explain why an antenna using a paraboloid reflector is likely to be a highly directive receiving antenna.

Describe the Cassegrain method of feeding a paraboloid reflector. Give some shortcomings and difficulties associated with the method.

(20)

11. (a) What is the importance of impedance matching in RF transmission lines ? Explain single stub-matching and double stub-matching.

A loss-less line has a characteristic impedance of 75 ohms and is terminated in a load of 300 ohms. The line is energised by a generator which has an open circuit voltage of 20 V (r.m.s) and output impedance of 75 ohms. The line is assumed to be 2¼ wavelengths long. Determine :

(i) the input impedance

(ii) the magnitude of instantaneous load voltage, and

(iii) the instantaneous power delivered to the load. 20

(b) (i) What is meant by saying that colour TV must be compatible ?

(ii) Explain what is meant by the Y, I and Q signals in colour TV, and why they are generated. With the help of circuit diagram of a simplex matrix, explain how the I, Q and Y signals are generated in a colour TV transmitter.

(20)

(c) Give the advantages and disadvantages of submarine cables and communication satellites for intercontinental telephone and television. Show how the two media may be complementary.

Explain what is done to ensure that intercontinental telephone calls are not misrouted.

(20)

12. (a) What do you understand by S-parameters of a 2-port network ? Why are these parameters preferred to H, Y and Z parameters for operation in microwave range of frequencies ? Show that the S-matrix of a directional coupler may be reduced to the form

0 0

0 0

0 0

0 0

p jq

p jqS

jq p

jq p

(20)

(b) With the help of a schematic diagram, explain a four-cavity klystron amplifier.

A four-cavity klystron amplifier has the following parameters:

Beam voltage : V0 = 20 kV

Beam current : I0 = 2 A

Operating frequency : f = 10 GHz

d.c. charge density : 0 = 10–6 C/m2

RF charge density : = 10–8 C/m2

Velocity perturbation : v = 105 m/s

Determine

(i) The d.c. electron velocity

(ii) The d.c. phase constant

(iii) The plasma frequency

(iv) The reduced plasma frequency for R = 0.5

(v) The d.c. beam current density

(vi) The instantaneous beam current density

(20)

(c) Explain what you understand by:

(i) MMIC

(ii) MOSFET

(iii) NMOS Growth

(iv) CMOS Development

(v) RAM Device

(vi) SRAM Cell

(vii) DRAM Cell

(20)


Candidates should attempt Questions 1 and 5 which are compulsory, and any three of the remaining questions selecting at least one question from each Section.

Some useful data is given at the end of the question paper. Assume any other data if considered necessary and indicate the same clearly

All questions carry equal marks.

Marks assigned to each part of the question is indicated at the end of the respective part.

SSEECCTTIIOONN AA


(a) Fig. 1 (a) shows a network of resistors each os R ohms. Find the resistance between the terminals P and Q.

(20)

(b) A charge Q is distributed over two concentric hollow spheres of radii r and R (R > r) such that their surface densities are equal. Find the potential at the common centre of spheres.

(20)

(c) Two sinusoidal signals of equal amplitude and frequencies fv and ih (Hz) are applied simultaneously to vertical and horizontal deflecting plates respectively of a CRO. Sketch the Lissajous patterns that appear on the CRT screen, when.

(i) fV : fh = 3 : 1

(ii) fv : fh = 1 : 2; and

(iii) fv = fh = 1kHz with a phase difference of 90° between the two signals.

If now, the control grid electrode is subjected to an additional sinusoidal signal of 10 kHz and of such amplitude that the grid is driven into cut-off state for part of its signal cycle what will be the change in the pattern displayed for part (iii)?

(20)

CC..SS..EE..--((MMaaiinn))--11999977


(d) For the voltage-series-feedback amplifier circuit shown in Fig. 1(d), the open-loop gain A of

the OP-AMP varies from 15000 to 20000 as the temperature changes from 25°C to 35°C Calculate the closed-loop gain at both the extreme temperatures, assuming R1 = 470 and Rf = 47 k. Calculate the percentage change in open-loop and closed-loop gains and explain the results. Assume the source impedance to be zero.

(20)

2. (a) A 10 kV is applied across an air-filled capacitor formed by two parallel plates with a separation of 1.2 cm. A glass sheet of 2mm thickness and of 6.5 relative permittivity is inserted in the air dielectric and kept on one plate. Find the capacitance per unit area of the parallel plates along with the glass, the electric field intensities in air glass dielectric, and the charge density on the plates.

(20)

(b) Calculate the power fed to the 15 resistor by using Thevenin’s equivalent circuit for the portion to the left of points A and B in the circuit of Fig. 2 (b).

(20)

(c) In the circuit of Fig. 2 (c), when the switch is closed t = 0, it is noticed that i (0+) = 10 mA and vab (t) = 0, t 0 sec. Assuming zero initial current for inductor and zero initial charge for capacitor, find the values of and R1 and L.

(20)

3. (a) Define the attenuation coefficient and phase coefficient in the context of a transmission line. A correctly terminated transmission line has Z0 = 600 , = 1.2 dB/m and = 18°/m (with usual notations). The line is 5 m long and is fed from a 600 source of e.m.f. 3V. Find the magnitude of the received current at the output and its phase relative to the input voltage.

(20)

(b) The Fig. 3(b) shows the Wien’s bridge for measurement of frequency. Drive suitable expressions under balanced bridge condition. What should then be the relation between R and R1 and between C and C1 so that the excitation frequency is expressed as 1/(2 RC)?

(20)

(c) The output of an LVDT is connected to a 10 V voltmeter through an amplifier with a gain of 500. An output of 2.5 mV appears on the terminals of LVDT, when the core moves through a distance of 0.5 mm. Calculate the sensitivity of LVDT and that of the whole set-up. The voltmeter scale has 100 divisions and the scale can be read to 1/5 of a division. Calculate the resolution of the instrument in mm.

(20)

4. (a) The JFET used in the circuit of Fig. 4 (a) has the following data :

RD = 2 k, RS = 8 k VDD = 15V, VG = 10 V and VSS = –8 V.

(20)

Assuming IDSS = 5 A and VPO = –3 V for the JFET calculate VGS and VO.

(20)

(b) For full-wave rectification, what circuit configuration would be preferred and why?

A bridge rectifier fed through a transformer from a 230 V, 50 Hz supply is used to deliver power to a transistor radio at 9V d-c. Assuming ideal dides and load resistance of 90, specify the turns-ratio and current rating of transformer. Also find the d-c and a-c (rms) values of Load current and Peak-Inverse-Voltage.

(20)

(c) The mid-band equivalent circuit of a common emitter transistor amplifier is shown in Fig. 4(c). The h-parameters of the transistor used are

hie = 1500 , hre = 10–3 , hfe = 100,

hoe = 50 x 10–6

Find the mid-band gain 2

/S bei i where 2Si where is the current in 5 k resister.

(20)


(a) Minimize the following logic functions using Karnaugh maps and realize them using NAND gates.

(i) f(A,B,C,D)=

m (2, 3, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,)

(ii) f(A, B, C, D) =

M (0,1,2,3,4,5,6,7,8,9,10,12,13)

(20)

(b) It is desired to add two one-bit binary words and produce the result as Sum and Carry. Prepare a truth table, Karnaugh map and Boolean expressions to design a Half-Adder using minimum number of NAND gates.

(20)

(c) What do you understand by the forbidden gap in energy bands?

Electrons are excited across the forbidden energy gap E0 in Germanium by bombarding of photons. Calculate the maximum wavelength for a photon which will produce electron-hole pairs in Germanium, if E0 = 0.71 eV.

(20)

(d) A 10kW, 1440rpm d-c shunt generator has resistance Loss of 400 watts in its field winding under normal operating conditions. The time-constant of the field winding is 0.2 second. Calculate the energy stored in the magnetic field produced by the field winding.

(20)

6. (a) Draw the circuit diagram of a NOR gate using MOS transistors (n-channel) and verify the NOR operation with. The description of the functioning of this gate.

(15)

(b) Name the different types of Read- Only-Memories and briefly discuss their attributes in terms of programmability volatility, erasability, technology used and utility.

(20)

(c) A 3-phase star-connected alternator, rated at 11kV, 1600k VA, has negligible armature resistance and a synchronous reactance of 30 per phase. When delivering full-load current at a certain power factor, the voltage regulation of the machine is zero. Estimate the power factor of the load and the power delivered by the machine.

(25)

7. (a) Two similar 200 k VA, single-phase transformers gave the following results when tested by

the back-to-back method:

(i) Wattmeter W1 in the supply line reads 4kW; and (ii) Wattmeter W2 in the secondary sends circuit reads 6 kW, when the full-load current is circulated through the secondaries. Show the diagram of connections and calculate the full-load efficiency of each transformer.

(15)

(b) A 4-pole D-C Series motor runs normally at 800 rpm taking 22 A on 260 V supply, when all the four field coils are connected in series. If now the field coils are reconnected in two parallel groups of two coils in series, estimate the speed and current taken by the motor. Assume that, (i) the load torque is proportional to square of speed, (ii) the field flux is directly proportional to the current, and (iii) the tosses are negligible.

(25)

(c) A 3-phase, star-connected, 1200 kW. 3.3 kV, 20-pole, 50 Hz synchronous motor has a synchronous reactance of 4 per phase and negligible resistance. The motor is fed from infinite bus-bar at 3.3 kV and is operated at unity power factor by so adjusting its excitation, Calculate the maximum power and torque that the motor can deliver for the same excitation.

(20)

8. (a) The normal full-load slip and shaft torque of a 373 kW, 50 Hz, 3-phase induction motor are respectively 1.9% and 2400 N-m. The rotor winding has a resistance of 0.25 and a standstill reactance of 1.5 per phase Estimate the slip and power output for the same full-load stator current when external resistors of 2 per phase are inserted in the rotor circuit Neglect no-load current.

(20)

(b) Two 100 kVa, 1-phase transformers are connected in parallel on both primary and secondary sides. One transformer has an ohmic drop of 0.5% and reactive drop of 8% of the voltage on full load. The other has the corresponding drops of 0.75% and 4% respectively. How will the total load of 200 kW at unity power factor be shared?

(20)

(c) The bulk n-type region of a particular Germanium junction has conductivity of 104 S/m at 300 K and that the for the p-region is 102 S/m. Find the voltage drop across the junction in equilibrium at 300K, assuming intrinsic carrier concentration as 2.5 x 1019 m3 mobility of electron and holes as 0.36m2/v-s and 0.17 m2/v-s respectively.

(20)

Some Useful Data:

Electron charge : 1.6 x 10–19 Coulomb

Permeability of free space : 4 x 10–7 H/m

Permittivity of free space : 8.85 x 10–12 F/m

Planck’s constant : 6.625 x 10–34 J-sec

Velocity of light in free space : 3 x 108 m/sec

Boltzmann constant : 1.38 x 10–23 J/K


Candidates should attempt five questions in all selecting at least two questions including the compulsory question from each of Section A and B or Section A and C. Assume suitable data if considered necessary and indicate the same clearly. The number of marks carried by each question is indicated at the end of the questions.

SSEECCTTIIOONN AA


1. Select any three of the following statements, read them carefully and identify the correct and the incorrect ones. Justify your answer using not more than 200 words in each case.

(20x3 = 60)

(a) If a system is stable with unity feedback, the open loop system is also stable.

(b) A two-phase a.c. Servomotor is similar in all respects to a conventional induction motor.

(c) If in the state variable description of a system, A matrix is singular, then the system is unstable.

(d) Commutation has to be specially taken care of, While using Thyristor-Chopper for motor speed control.

2. (a) Sketch the under-damped time response of a second order control system to a step input. Define clearly the “Delay time”, “Rise time”, Peak time”, Peak-overshoot”, “Settling time” and “Steady state error” and also show them in the sketch.

(20)

(b) A system is represented by the following equations:

(20)

x = x1 + 3u

x1 = –3x1 + x2 + 2u

x2 = –2x1 + u

Find the transfer function

X s

U s by signal flow technique.

(c) State Nyquist Stability Criteria and investigate the stability of the closed loop system with the following open loop transfer function.

2 3

1

sG s H s

s s

by drawing Nyquist plot

Wherever the Nyquist plot crosses the real or imaginary axis determine the frequency and the intercept value.

(20)

CC..SS..EE..--((MMaaiinn))--11999977


3. (a) The response of an initially relaxed time invariant constant parameter system to a unit

impulse applied at t = 0 is 4 e–2t u (t).

(i) Find through time domain analysis the response to the same system to an input e–tu(t)

(ii) Find the time function which when applied to the system would yield a response sin 2t. u(t).

(20)

(b) An open loop transfer function of unity feedback system is given by

25

2G s

s s

.Determine the damping factor, undamped natural frequency, damped natural

frequency and time response for a unit step input.

(20)

(c) What do you understand by the term controllability of a system? State the necessary and sufficient conditions to be satisfied by the coefficient matrices of a system, so that the system is completely controllable. Hence check the controllability of a system having for its state description, the matrices:

0 1 0 0

0 0 1 0

1 0.5 2 1

A B

(20)

4. (a) Discuss the volt-ampere characteristic of a Triac. Explain its usefulness in speed control applications.

(20)

(b)

The above figure shows a chopper circuit of perating at 100 Hz with a Kd = 0.5. The load current at steady state is continuous but varies between 3 amps and 10 amps. Sketch the wave shape of

(i) the load current, iL

(ii) the current if through the free wheeling diode Df

(iii) the current iC through the commutating capacitor

(20)

(c) The load connected at the output of a single phase bridge inverter is a series combination of R = 10 and L = 0.05H. Determine the load voltage and current waveforms for the first two half cycles with rectangular wave output at 50 Hz. The input of the inverter is Vdc = 250 V.

Also find the expression for the steady state current in each half cycle.

(20)

SSEECCTTIIOONN BB

Question no.5 is compulsory.

5. Comment with justification on the correctness or otherwise of any three of the following statements, restricting your answer to within 200 words for each.

(20 x 3 = 60)

(a) Load frequency control is used to control the reactive power flow in a power system.

(b) Ground wires protect transmission lines against direct lightning strokes.

(c) Sequence networks are connected in parallel to represent a single line-to-ground fault.

(d) The secondary of a current transformer should never be kept open circuited.

6. (a) Distinguish between Restriking voltage and Recovery voltage as applied in case of a circuit breaker. On what factors the Active Recovery voltage depends? What is the maximum value of the peak Restriking voltage transient?

(20)

(b) With a neat sketch explain the working principle of an Axial blast circuit to breaker Explain why such a circuit breaker is prone to current chopping. What is the voltage expected across the contacts of the circuit breaker under such a condition?

(20)

(c) A generator operating at 50Hz delivers 1 P.U. power to an infinite bus through a transmission circuit in which the resistance is ignored. A fault place reducing the maximum power transferable to 0.5 P.U., whereas before the fault this power was 2.0 P.U. and after the clearance of the fault it is 1.5 P.U. By the use of Equal Area criteria determine the critical clearing angle.

(20)

7. (a) Discuss the advantages and disadvantages of HVDC transmission over a.c. transmission. Explain with suitable circuit diagram how power flow in an HVDC transmission system neither direction facilitated.

(20)

(b) A 2000 WA, 11 kV, 3-phase star-connected alternator has a resistance of 0.3 ohm and a reactance of 5 ohms per phase. It delivers full load current at a power factor of 0.8 lagging and normal rated voltage. Compute the terminal Voltage for the same excitation and load current at 0.8 p.f. leading.

(20)

(c) What is a synchronous condenser? Distinguish between a synchronous condenser and a static var compensator. Why is static var compensation necessary in a power system?

8. (a) An electric train has an average speed of 42 kmph on a level track between stops 1400 m apart. It is accelerated at 1.7 kmphps and is broken at 3.3 kmphps. Draw the speed-time curve for the run. Estimate the energy consumption at the axle of the train per ton km. Take tractive resistance constant at 50 N/ton and allow 10 per cent for rotational inertia.

(20)

(b) Explain the working principle of a time graded protective scheme for radial feeders. Explain further with a sketch of a ring main feeder, why the time graded protective scheme as to be supplemented by directional control.

(20)

(c) An 11 kV, 3-phase radial feeder is equipped with two current transformers along with their

associated I.D.M.T.L. relays at two different locations. The c.t. ratios are 600/5 and 200/5 respectively and the relay associated with each c.t. has the following characteristic,

Plug Setting Multiplier (PSM) Times in secs

2 10

3 6

5 4.1

10 3.0

15 2.5

20 2.2

The relay on 200/5 ct. has a 150% plug setting and 0.2 time multiplier setting. Determine the time of operation of this relay, if the fault current through the feeder is 3000amps. Also calculate the time setting and time of operation of the other relay connected to 600/5 c.t. assuming a plug setting of 200% and taking into consideration the adequate discrimination margin of 0.5 sec.

(20)

SSEECCTTIIOONN BB


9. Select any three of the following statements and indicate with justification whether they are correct or incorrect. Your answer must not exceed 200 words for each statement.

(20 x 3 = 60)

(a) VHF band is most suited for communication between a ship and a submarine.

(b) Round trip propagation time with geostationary satellites is very large and poses a peculiar problem for telephone service.

(c) Colour, television transmitter has to transmit more information than black and white but has to mange within the same bandwidth.

(d) FM broadcast systems use wide-band FM.

10. (a) An MTI radar operating at 6GHz is having a PEW of 900 pps. Find the two lowest blind speeds for this redar.

(20)

(b) Why is companding used in PCM Systems? Describe any one companding law.

(20)

(c) A silica fibre has a large core diameter. The refractive indices of core and cladding are 1.49 and 1.46 respectively. Find the NA of the fibre, the acceptance angle in air and the critical angle at the core-cladding interface.

(20)

11. (a) How is stereophonic FM broadcast made compatible with a monophonic system?

(20)

(b) What is a three-port circulator? Write its S-matrix. One of the ports of such a circulator is

terminated in a load having a reflection coefficient p. If the system is now viewed as a two-part at the other pair of ports, find the scattering matrix of the two-port.

(20)

(c) A deep space probe radar operating at 3 GHz is using a 64 m dish and peak pulse power of 20MW. The receiver noise figure is 1.0 dB, the target cross section is 1.5 m2 and the receiver bandwidth 8kHz. Find the maximum range under threshold detection conditions. Take antenna noise temperature to be 290K.

(20)

12. (a) Find the required size of a cubic cavity resonator to have a dominant mode at 8 GHz. Name the dominant modes for this cavity.

(20)

(b) Describe a balanced off-resonant (slope) discriminator for FM signals.

(20)

(c) A broadside antenna array has 20 identical radiators, equally fed and spaced 1/2 apart. Find the directivity of the array and express it in decibels.

(20)



Some useful data is given at the end of the questions. Assume any other data if considered necessary and indicate the same clearly. All questions carry equal marks.

Marks assigned to each part of the question is indicated at the end of the respective part.

SSEECCTTIIOONN AA


(a) There are three concentric conducting spherical surfaces of radii a, b and c (a < b < c). The sphere of radius b carries a charge Q and the surfaces of spheres of radii a and d c are grounded. Find the potential of the sphere of radius b.

(20)

(b) Find the output voltage in steady state condition for the operational amplifier shown in the figure below when (i) switch S is open, (ii) switch S is closed.

(20)

(c) Fig. (a) below shows a singly excited electromagnetic system consisting of a winding of N turns on a core carrying a flux . (N = 1000). For the flux waveform of Fig. (b) below draw the waveform of induced e.m.f.e = 314 rad/s.

CC..SS..EE..--((MMaaiinn))--11999988


(20)

(d) Draw a simple experimental set up to measure the Hall effect and show that the Hall field can be written as Ey = VXBZ where VX and BZ are the applied voltage and magnetic fields respectively. What are the typical applications of Hall effect ?

(20)

2. (a) Briefly explain the following statements on 3-phase induction motors:

(i) Crawling is observed in squirrel cage motors.

(ii) Rotor resistance method of speed control is employed only in wound rotor (slip ring) motors.

(iii) Pole changing method is used in squirrel cage motors.

(iv) Variable frequency method of speed control is used below base speed keeping the ratio of supply voltage to frequency constant.

(b) A half wave rectifier which prevents current flowing in one direction is connected in series with A.C. ammeter and a permanent magnet moving coil ammeter. The supply is sinusoidal. The reading on the A.C. ammeter is 10 A. Find the reading of the other ammeter. What would be the reading on the ammeters if the other half wave were rectified instead of being cut off?

(20)

(c) Reduce the following four variable function using Karnaugh map and implement it in NAND logic, F = m (0, 4, 12, 8, 13, 7, 15)

(20)

3. (a) Two coils A (R1 = 100 ohm, L1 = 0.2 H) and B (R2 = 100 ohm, L2 = 0.4 H) are coupled by an inductance of 0.2 H. Find the voltage V at a frequency of 1000 rad/s to be applied across coil A to cause a current 0f 10 milliamperes through coil B across whose terminals a resistance of 100 ohms is connected.

(20)

(b) Two identical 7.5 KW, 230 V 36 A d.c. shunt wound machines are used for speed control using Ward Leonard arrangement. One of them used as a generator is driven by a 6-pole, 50 Hz synchronous motor. Armature resistance of each d.c. machine is 0.22 ohm. Data for magnetisation curve of each machine is given below at 1000 r.p.m.:

Field current (A) 0.3 0.6 0.9 1.2 1.5 1.8

Armature voltage (V) 108 183 230 254 267 276

Compute the maximum and minimum values of generator field current to give the motor a speed range from 250 to 1500 r.p.m. at full load armature current (36 A) with the field current held at 0.80 A.

(25)

(c) State Faraday’s Law of induction. A rectangular loop of sides a, b has its plane normal to a magnetic flux density of strength B0, sin t. What is the voltage induced in the loop?

(15)

4. (a) Explain the concept of Fermi-level. Sketch the Fermi-energy level for an n-type semi conductor as a function of temperature beginning at T = 0°K and going to a very “high” temp. Qualitatively discuss the nature of the characteristic.

(20)

(b) The figure below shows a full wave rectifier circuit. For a sinusoidal imput voltage, plot the voltage wave forms across R and D1.

(20)

(c) The circuit arrangement of an Owen Bridge for the measurement of the effective self inductance and resistance of an iron cored coil is shown in the figure below. Determine the balance conditions for the bridge and calculate the effective inductance and resistance of the coil if, for a frequency of 1000 Hz, the values of the circuit elements at balance are as follows:

R1 = 10, R2 = 842

C1 = 1F, C2 = 0.135 F

(20)

SECTION B


(a) a single element rectifying device is connected in series with a moving coil ammeter and a thermal ammeter and it is found that the circuit has a resistance of 100 to

current in one direction and infinitely large resistance to current in the opposite direction. If a sinusoidal e.m.f. of 10 V (r.m.s.) is applied to this circuit, what will be the reading of each instrument and what will be the power taken from the supply?

(20)

(b) Find X and; in terms of R and R2 to give maximum power dissipation in R2 shown in the figure.

(20)

(c) A 3-phase delta connected squirrel cage induction motor has certain equivalent circuit

parameters. With the same stator core, the motor is rewound with star connected windings of same poles for operation at the same supply voltage and frequency to give same rated power output at the same rated slip. By what factors each of the equivalent circuit parameters would be charged?

(20)

(d) An infinitesimal dipole (Hertzian) antenna of 1 cm length radiates a power of 1 mW at 100 MHz. Calculate (i) the current in the dipole, (ii) E and H fields at 100 meters. =90°, = 0o

(20)

6. (a) 415 V star connected 3-phase, 4-pole synchronous generator has 36 stator slots carrying double layer 3-phase windings formed through uniformly distributed coils each short chorded by one slot. The diameter at the airgap is 20 cm and the effective axial length is 60 cm. The rotor driven at 1500 r.p.m is excited to result in a sinusoidally varying airgap flux density distribution in space with a peak value of 0.5 T. Calculate the number of series turns per phase to generate rated no load voltage.

(20)

(b) What are the starting methods used for 1-phase induction motors. Draw typical torque-speed characteristics in each case both under starting and running conditions.

(20)

(c) Two perfectly insulated condensers are connected in series. One is an air condenser with a plate area of 100 cm2 and the plates being 1 mm apart; the other has a plate area of 10 cm2, the plates being separated by dielectric of 0.1 mm thick with a permittivity of 5. Find the voltage across the combination if the potential gradient in the air condenser is 20 KV per cm.

(20)

7. (a) 11 MV, 415 V, 3-phase 50 Hz distribution transformer has its NV and LV windings connected in delta and star respectively. It is delivering 100 KVA load at 0.8 PP (lag). Calculate all HV and LV currents. Draw a phasor diagram showing all voltages and currents.

(20)

(b) For the circuit shown in the figure below, draw the ac. equivalent circuit and find the voltage gain of the amplifier.

(20)

(c) Draw the circuit diagram of an R-C phase shift oscillator using a transistor with CE configuration and derive an expression for its frequency of oscillation. Prove that

1

23 29 4 Lfe

L

RRH

R R

(20)

8. (a) Consider the set up shown in the figure below for measuring Hall effect.

Assume that L = 10–1 cm, W = 10–2 cm. The following parameters are measured:

IX = 1.0 mA, VX = 12.5V

BZ = 5 x 10–2 T, VH = –6.2 mV

Find the type of the semiconductor, majority carrier concentration and the mobility.

(20)

(b) Determine the condition under which the input impedance of the network shown in the figure below will be equal to R.

(20)

(c) A 250 V, d.c. shunt motor has Rf = 150 and Ra = 0.6 . The motor operates on no load with a full field flux at its base speed of 1000 r.p.m. with Ia = 5A. If the machine drives a load requiring a torque of 100 Nm. Calculate the armature Current and speed of the motor if the motor is required to develop 10KW at 1200 r.p.m. What is the required value of the external series resistance in the field circuit? Neglect saturation and armature reaction.

(20)


Candidates should attempt five questions in all selecting at east two questions including the compulsory question from each of Sections A and B or Sections A and C.

Assume suitable data if considered necessary and indicate the same clearly.

The number of marks carried by each question is indicated at the end of the question.

SSEECCTTIIOONN AA Question No. 1 is compulsory.

1. Select any Three of the following statements, read them carefully and identify correct and incorrect ones. Justify your answer using not more than 200 words in each case:

(20 x 3 = 60)

(a) In analog simulation of physical system, Integrators are used rather than Differentiators.

(b) Phase lead compensation decreases the band width of a system.

(c) Synchro error detectors are used in dc servo system.

(d) The required PIV capacity of diodes in single phase bridge rectifier is one half than that required in rectifier using centre tap transformer.

2. (a) The roots of a second order system are located at –1+j2 and –1–j2. Determine the followings if the system is excited by unit step input:

(i) Damping ratio

(ii) Maximum % overshoot

(iii) Natural frequency of oscillation

(iv) Damped frequency of oscillation.

(20)

(b) Discuss briefly the speed-torque characteristics of a 2 ac servomotor and compare it with that of ordinary 3 induction motor. Hence describe the laboratory method to obtain its speed-torque characteristic.

(20)

(c) The unity feedback system has forward path transfer function

1 2

KG s

s s s

Sketch the Nyquist diagram. If K = 4, determine whether the given system is stable or not. If the system is stable, determine its gain margin. What is the maximum value of K that will make its gain margin zero ?

(20)

3. (a) Obtain the transfer function of phase lead network. Show that

CC..SS..EE..--((MMaaiinn))--11999988


1

sin1m

a

a

and 1

maT

where m, w, a and T have usual meanings.

(20)

(b) The unity feedback system has forward path transfer function

2

1

2 4

SG s

s s s

Determine steady state error coefficients Kp, Kv and Ka.

Hence find the steady state error in output if

(i) unit step input,

(ii) nit ramp input and

(iii) nit parabolic input are given.

(20)

(c) The state equations for a system are given by

X AX Bu

Y CX Du

where

1 0 0

0 2 0

0 0 3

A

,

1

2

1

B

, 1 1 1C and

0

0

0

D

Determine the output response of a system to unit step input.

(20)

4. (a) Explain the working of a timer using general purpose OP AMP IC 741. Design such a timer for a time period of 60 seconds.

(20)

(b) Obtain

(i) average load voltage and

(ii) rms load voltage in case of 3 full wave rectifier. Hence calculate ripple factor.

(c) A simple voltage regulator circuit is shown in Fig. 4(c). The zener voltage is given by Vz = (10 + 0.20iz) volts where iz is in mA. For a given transistor, assume = 100 and VBE = 0.3 V (constant). Determine % load regulation if maximum load current of 500 mA is switched off.

(20)

SSEECCTTIIOONN BB Question No. 5 is compulsory

5. Select any three of the following statements and justify the correctness or otherwise of them, restricting your answer to within 200 words for each.

(3 x 20 = 60)

(a) An amplidyne is an alternating current machine with a low gain.

(b) Impedance relays used for transmission line protection work in conjunction with directional relays.

(c) The power transfer capability of a transmission system can be enhanced by series-capacitor compensation.

(d) An axial blast circuit breaker is not prone to current chopping.

6. (a) Explain the term ‘Critical clearing time” in the context of transient stability of a power system, as a sequel to a fault. A synchronous generator with an induced emf of E volts per phase is operating on constant-voltage, constant frequency bus bars, the bus-bar voltage being V volts per phase. If the impedance of the generator is Zohms, obtain an expression for the maximum power generated.

(20)

(b) What are the factors affecting corona power loss in a transmission line ?

An inductance L connects two sections of a transmission line with surge impedances Z1, and Z2. A rectangular surge with a crest value ‘V” and of “t” microseconds travels along the line-section 1 towards the inductance. What is the maximum value of the transmitted wave?

(20)

(c) Explain the need for time = grading in the protective scheme for radial feeders. Explain how the multiplier setting, plug-setting affect the time of operation of the relay.

(20)

7. (a) What are the most important requirements of a lightning arrester ?

A thyrite lightning arrester has a characteristic RI0.7 = 72000.

Draw the Volt-ampere characteristic of the arrester.

What is the ratio of the voltage appearing at the end of a line with a surge impedance of 500 ohms, due to a 500 kV surge when the line is open circuited to that when the line is terminated by the arrester?

(20)

(b) Explain with the help of a neat phasor diagram, the manner in which a synchronous condenser located at the receiving end of a short transmission line improves the power factor and maintains the voltages at both ends at the same level.

(20)

(c) Distinguish the characteristic features of alternators employed in Hydel and thermal power stations. A three-Phase 400 V, 6-pole, 50 Hz star-connected synchronous motor has a resistance and synchronous reactance of 0.5 and 5 per phase respectively. It takes a current of 15 A at Unity Power factor with a certain excitation. If the load torque is increased

until the line current increases to 60A, with the excitation maintained constant, find the gross torque developed and the new power factor.

(20)

8. (a) What are the essential requirements of motors employed in electric traction ?

A three-phase induction motor has a 4-pole, star- connected stator winding and runs on a 400 V 50 Hz supply. The rotor resistance is 0.1 and reactance 0.9. The stator to rotor turns ratio is 1.75 and the full load slip is 5%. Calculate the full-load Horse-Power output. Determine also the maximum torque and the corresponding speed.

(20)

(b) Describe with the help of a neat diagram the operation of a reluctance-type stepper motor along with its characteristics. What are its possible applications ?

(20)

(c) What re the limitations on the application of the “Equal area criterion” for examining transient stability ? The maximum power that can be transmitted by a Synchronous Generator operating on to an infinite bus is 1.00 pu. The Generator initially was delivering under stable steady-state 0.4pu. What is the extra load that can be suddenly thrown on the generator for it to become critically stable ? Ignore active resistance.

(20)

SSEECCTTIIOONN BB

Q. No 9 is compulsory.

9. Select any Three of the following statements and indicate with justification whether they are correct or incorrect. Your answer must not exceed 200 words for each statement:

(20 x 3 = 60)

(a) Pulse Code Modulation (PCM) systems for voice communication employ companding,

(b) With a sinusoidal modulating signal and modulation indices of 0.1 each, the spectra of AM, FM and PM signals are identical.

(c) The dominant mode of propagation in a rectangular waveguide is TEM.

(d) For a given data rate, QPSK signals and BPSK signals require equal bandwidths.

10. (a) An antenna having a 10°K noise temperature feeds a low noise amplifier of 4oK noise temperature and 20 dB gain. This is followed by another amplifier of 4 dB noise figure and 30 dB gain. Find the overall noise figure and noise temperature of the system.

(20)

(b) Discuss the relative merits and shortcomings of BPSK and BFSK signals.

(20)

(c) Explain a scheme for demodulation of PPM signals.

(20)

11. (a) Consider a DSB-SC signal corresponding to a modulating signal x(t) of bandwidth B and a carrier signal A cos 2fct, This DSB-SC signal forms the input in Fig. 11(a). Find the output and comment on the result.

Fig. 11(a)

(20)

(b) Draw the input impedance variation of an o.c. lossless line of characteristic impedance R0 (purely resistive) when the length varies from zero to . Indicate the nature of the impedance at every 0.2 separation.

(20)

(c) Describe a method of measuring impedance at microwave frequencies.

(20)

12. (a) Discuss the significance of G/T ratio of an earth station.

(20)

(b) Give the frequency range of a gunn diode and order of available power. How many junctions does it have?

(20)

(c) Describe a DME system for measurement of distance.

(20)



SSEECCTTIIOONN AA


(a) Explain the concept of complex frequency. Discuss its usefulness in specifying network functions.

(20)

(b)

Determine the current in the 5 ohm resistor using Thevenin’s theorem.

(20)

(c)

Determine the electric field in the overlap region cit two spheres A and B shown in the above figure. Sphere A carries a charge – c/m3 and the sphere B, a charge of – c/m3.

(20)

(d) Discuss the merits and demerits of using counters in digital measuring instruments.

(20)

2. (a)

CC..SS..EE..--((MMaaiinn))--11999999


A loss-less transmission of length m and characteristic impedance Z0 ohm is excited at both the ends as shown in the above figure. Determine the currents delivered by the two sources. Determine also the current at the mid-point of the line.

(20)

(b)

Determine

(i)

V s

I s and

(ii)

ABV s

V s for the network shown in the above figure.

(10 + 10)

(c)

The bridge circuit, shown in the above figure, is balanced when S = 10 milli ohm, P = 1100 ohm, Q = 1000 ohm, q = 500 ohm and r = 0.1 ohm. Find the value of Rx.

(20)

3. (a)

Determine V so that the circuits (i) and (ii) shown in the above figure are equivalent.

(20)

(b)

Determine the emf induced in the triangular loop ABC due to the flux set up by a sinusoidal current Im sin t flowing in an infinitely long wire shown in the above figure.

(20)

(c) Two conductors have a potential coefficient matrix 2 1

1 2

Each conductor is given a charge

1 C. One of the conductors is then momentarily grounded. Thereafter it is connected to the other conductor. Determine the final potential of the composite conductor.

(20)

4. (a) (i) Why does an induction type energy meter creep? How is the creep prevented?

(10)

(ii) The calibration constant of a 3-phase energy meter is 0.14 revolutions of disc per kwh. It is used with a potential transformer of ratio 11 KV/110 V and a current transformer of ratio 500 A/5A. The test results are:

Line voltage = 100

Line current = 5.25 A,

Power Factor = 0.9 lagging,

Time to complete 30 revolutions of discs =0 sec.

Determine the percentage error in the meter in terms of the actual energy consumed.

(10)

(b) The output of a LVDT transducer is 0.5 V at a frequency of 50

2 103 rad./sec. Determine the

output if the frequency deviates by 10%. The time constants of primary and secondary circuits are 5 ms and 2 ms respectively.

(20)

(c)

Determine the resistance across the terminals A and B of the network shown in the above figure.

(20)


(a)

The above figure shows the circuit of a voltage follower. Calculate its closed loop voltage gain, input impedance and output impedance. Determine also the output offset voltage. The parameters of 741 C are:

Open loop gain AOL = 105 ,

rin = 2M, rout = 75, Vin(off) 2mV,

Iin (bias) = 80 nA and Im(off) = 20 nA.

(20)

(b) (i) Simplify the Boolean function

. .Z A B B A C

and realise it using NAND gates.

(10)

(ii) What is a race condition ? How is it prevented?

(10)

(c) A 400 V, 3-phase, star connected synchronous motor has xd = 8 ohm and xq = 6 . It is running on infinite bus. If its field current is reduced to zero, determine the maximum load that the synchronous motor can carry. Determine also the corresponding armature current.

(20)

(d) A d.c. shunt motor draws an armature current of 50 A from 250 V mains. It is required to increase its speed by 40% by weakening of the field flux. If the torque at the increased speed is also increased by 40%, find the per cent change in the field flux. The armature resistance is 0.2 ohm.

(20)

6. (a)

For the class C tuned amplifier shown in the above figure, calculate the maximum load power and the transistor power rating if VCE(sat) = 0.75 V.

(20)

(b)

The transistor shown in the above figure has following h parameters:

hie = 1000 , hfe = 50, hre = 3 x 10–4 and hoe = 5 x 10–5

Find the voltage gain aid ac. input impedance of the amplifier. Neglect capacitor impedances.

(20)

(c) Describe any two types of A to D converters. Discuss their merits and demerits.

(20)

7. (a) A 500 KVA, 11 K 3-phase, star connected alternator has the following data:

Friction and windage loss = 1500 W

Open circuit core loss = 2500 W

Effective armature resistance per phase 4 ohm

Field copper loss = 1000 W

Find

(i) alternator efficiency at half-full load and 0.8 power factor lagging

(ii) maximum efficiency of the alternator.

(10 + 10)

(b) Describe, giving reasons, the choice of materials for the following applications:

(i) Magnetic tapes

(ii) Recording heads

(iii) Super conductors

(iv) Insulation for 11 KV cable

(5 + 5 + 5 + 5)

(c) Three 600 KVA, 33000/11000 V, 50Hz, 3-phase, Delta/Star transformers are connected in parallel. These transformers give the following test results when operated at the rated full load current with their low voltage windings short-circuited:

Transformer A–300 V, 3000 W

Transformer B–400V, 3500 W

Transformer C–450 V, 4000 W

Determine the maximum load at unity power factor which can be supplied by the parallel combination without overloading any of them.

(20)

8. (a)

Describe the constructional features of E-MOSFET.

Calculate the voltage gain from gate to drain of the circuit shown in the above figure.

(10 + 10)

(b)

Explain the operation of colpitts oscillator. A colpitts oscillator has a tank circuit as shown in the above figure. Calculate its frequency of oscillation. What should be the minimum gain of the amplifier for the oscillations to start ?

(20)

(c) (i) Show that the use of a synchronous condenser improves the efficiency and regulation of a system.

(10)

(ii) A consumer has a total load of 2000 KW at a p.f. of 0.8 lagging. If it is required to improve the power factor to 0.95 lagging, determine the KVA rating of the synchronous condenser required for the purpose.

(10)

PPAAPPEERR -- IIII

SSEECCTTIIOONN AA Question No. 1 is compulsory

1. Select any three of the following statements, read them carefully and identify correct and incorrect ones. Justify your answer using not more than 200 words in each case:

(20 x 3 = 60)

Case

(a) For a stable system, the gain margin and phase margin should be positive.

(b) Shaft encoders are extremely useful for the measurement of singular displacement.

(c) An SCR is derated when it handles pulsed anode current.

(d) An operation amplifier has features suitable for radio frequency timers.

2. (a) What is Bode-plot? Discuss how a Bode-plot can be drawn.

Derive the step response of a second order system expressed by:

2d da b c F i

dt dt

.

(20)

(b) (i) Enumerate the salient features of phase-lead and phase lag networks.

(10)

(ii) Design an astable multivibrator with LM 555. Calculate also the duty cycle and frequency of oscillation.

(10)

(c) (i) The pole-zero configuration of a closed-loop control system is given by (s1, s2) = –2 ± j2. Calculate the undamped resonance frequency and the damping ratio.

(10)

(ii) Explain Hurwitz stability criteria.

The characteristic equation of a system is represented by

4s3 + 3s2 + 2s + 3 = 0.

Verify the stability of the system through expansion by confined fraction.

(10)

3. (a) Fig. 3(a) shows a circuit employing class-C commutation. The circuit is initially relaxed. Sketch the waveform for VT1, VT2, ic iT1, vc and iT2 for one cycle for explaining the commutation process of both the thyristors.

CC..SS..EE..--((MMaaiinn))--11999999


(20)

(b) Describe the working of a 3-phase full converter when connected to a resistive load. The firing angle is approximately 70°.

Derive an expression for its average output voltage in terms of firing angle, input supply voltage.

(c) A fixed frequency chopper feeds a separately-excited dc motor supplied from 80 V dc source. The rated speed is 900 rpm and the rated armature current is 40 A. Armature circuit resistance is 0.25. Find the duty cycle of the chopper at half the rated motor torque for a speed of 300 rpm ignoring the current pulsations.

(20)

4. (a) Enumerate states and state variable representation of a dynamic system.

Determine, by Routh array, whether a system represented by characteristic equation.

s3 + 4s2 + 8s + 11 = 0

is stable or not.

(20)

(b) Discuss the principle of working of a 3-phase bridge inverter with an appropriate circuit Draw phase and line voltage waveforms on the assumption that each thyristor conducts for 180° and the resistive load is star connected. Indicate the sequence of firing for thyristors as well.

(25)

(c) Explain the mechanism for the dissipation of stored energy in a SCR during its commutation process.

(15)

SSEECCTTIIOONN BB Question No. 5 is compulsory

5. Select any three of the following statements and justify the correctness or otherwise of them, restricting your answer to within 200 words for each:

(3 x 20 = 60)

(a) Restriking transients can be damped by connecting a resistance across the contracts of a circuit breaker.

(20)

(b) Gauss-Siedel method is preferred for load flow studies of large power system due to its quadratic convergence.

(20)

(c) An electrical machine has cylindrical stator and salient- pole rotor. Reluctance torque is produced when exciting winding is on the rotor.

(20)

(d) If the rotor of a 3-phase induction motor is assumed purely inductive, the electromagnetic torque would be optimum with load angle equal to 90°.

(20)

6. (a) Which type of distance relay is preferred for EHV transmission lines ? Discuss its principle of operation with its salient features.

(20)

(b) Develop necessary equations and explain the Newton-Raphson method in polar co-ordinates for solving the load flow problem when both load and voltage-controlled buses are present in the system.

(20)

(c) A 3-phase, 4-pole, 50 Hz, star-connected induction motor, during short circuit test, took 100 A and 30 kW. In case stator resistance is equal to equivalent standstill rotor resistance, calculate the starting torque.

(20)

7. (a) An overhead transmission line, with a surge impedance of 500 , has a load comprising of a 10 kilo-ohm resistor is parallel with a 0.005 F capacitor connected across the far end. A surge voltage of 10 kV magnitude and unit step travels along the line. Determine an expression for the time variation of the voltage across the load and calculate this voltage 5 s after the arrival of the wave front of the surge.

State any assumptions made.

(20)

(b) A 3-phase generator, an open circuit, is excited to give a voltage of 2200 V between lines. The absolute values of fault currents for various types of faults are:

(i) 3-phase fault : 127.0 Amps

(ii) L–L fault : 129.5 Amps

(iii) L–G fault : 190.0 Amps

Find the positive, negative and zero-sequence reactances of the generator. Assume the resistance to be negligible.

(20)

(c) Explain how the power factor of a 3-phase induction motor is controlled by static capacitors. Show that for a constant capacitance, the degree of power factor correction is not the same at different loads.

(20)

8. (a) Describe the constructional features and working of a 2-phase, 4/6 pole hybrid stepping motor.

Discuss its torque-displacement characteristics also.

(20)

(b) A salient-pole synchronous motor; with ra = 0, Xd = 1.0 pu and Xq = 0.5 pu, is operated on

infinite bus-bar of 1.00 pu voltage. Show that for one per-unit synchronous power, the excitation voltage is given by E1 = cosec – cos .

Also derive the condition for load angle when synchronous power is maximum.

(20)

(c) The generalized constants A and B of a transmission line are 0.96 1° and 100 80° respectively. If the line to line voltages at the sending and receiving ends are both 110 kV and phase angle between them is 30°, find the receiving end power factor and current. With the sending end voltage maintained at 110 kV, if the load is suddenly thrown off, find the corresponding receiving end voltage.

(20)

SSEECCTTIIOONN CC

Question No. 9 is compulsory.

9. Select any three of the following statements and indicate with justification whether they are correct or incorrect. Your answer must not exceed 200 words for each statements:

(20 x 3 = 60)

(a) The characteristic of a twin–T network is similar to that of a notch filter in audio frequency range.

(b) The transposition of telephone lines is desirable.

(c) Magnetrons are capable of delivering low pulsed power output.

(d) Only one kind of dielectric substrate is used in microwave integrated circuits (MlCs).

10. (a) Define spectral density and auto-correlation function for the measurement of noise.

Find out the spectral density and auto-correlation function of the output when white noise with spectral density is applied to a low pass RC filter. Derive the expressions used.

(30)

(b) Discuss the microwave spectrum in satellite communication.

(c) Describe the technique of making stereophonic FM broadcast compatible with monophonic system.

(20)

11. (a) Discuss the realization techniques, with circuit diagrams, for inductor and capacitor in microwave communication.

(20)

(b) Derive the equivalent circuits and find out ‘Q’ factor of the inductor and capacitor. State any assumptions made.

(20)

(c) Enumerate the salient features of wave guides with diagrams based on band width and power handling capabilities.

(20)

12. (a) Describe the relative merits of single mode and multimode fibres used in communication system.

(20)

(b) Describe the schematic arrangement of pulse-code modulation by using optical fibre.

(20)

(c) Draw a schematic diagram and discuss how the thickness of foil can be measured by microwave sensors.

(20)

PAPER – I Candidates should attempt questions 1 and 5 which are compulsory and any three of the remaining

questions selection at least one question selecting at least one question from each Section. SECTION A


(a) Determine the relationship between R1, R2, L and C of the circuit shown below so that the poles of the function V(s)/I(s) are complex.

20

(b) (i) Justify that a thin sheet of a conducting material acts as a low-pass filter for electromagnetic waves.

10

(ii) A radio signal at a frequency of 20 MHz is transmitted through sea-water having = 4 /m and r = 100. Find the distance at which 80% of the wave amplitude is attenuated.

10

(c) Justify the statement Commentator in a d.c. machine keeps the armature mmf stationary in space along the interpolar axis even though the armature rotates.

20

(d) (i) What factors influence the performance of Yagi—Uda antenna? Explain it clearly with the help of a neat sketch showing the direction of e.m.w.

10

(ii) Determine field strength at a distance of 50 kin of a short vertical monopole radiating power of 20 kW at a frequency 1 MHz over ground with mean conductivity of 10 mS/metre

10

2. (a) The clock frequency of 1000 ns is connected to a 3-bit asynchronous counter with each flip-flip having 50 ns propagating delay. Which slate will never occur in it, if the clock frequency is decreased to 100 ns? Explain it by drawing wave- shapes for 1000 ns and 100 ns clock frequencies separately.

20

(b) A 230 V D.c. source feeds a separately-excited d.c. motor through a chopper operating at 400 Hz. The load torque at 1200 r.p.m. is 32.5 Nm. The motor has ra = 0, La = 3 mH and Km = 1.3 V-s/rad. All motor and chopper losses are neglected.

C.S.E-Mains 2000


Calculate (i) the armature current excursion and (ii) the armature current expressions during on and off periods.

20

(c) Draw the equivalent circuit of a 1-phase induction motor based on two-revolving field theory. Identify the various parameters involved in it. Prove, there from, that the forward flux wave is several times greater than the backward flux wave b at normal rotor speed, hut at standstill. f = b.

20

3. (a) Describe the principle of working of linearized model of phase lock loop shown in the figure given below and show that the output voltage is proportional to the change in frequency in the FM signal

20

(b) Two transformers A and B, operating in parallel have equal voltage ratios and leakage impedances. Their reactance to resistance ratios are 10 and 4 respectively. Determine the ratio of the full-load kVA delivered by the parallel combination to the sum of their individual kVA ratings. Comment on the result.

20

(c)

For the, circuit shown above, thyristor T is turned on at a triggering angle greater than that given by sin-1 (E/Vm). Draw waveforms for u s, i0 and T. Derive an expression for the average charging current I0.

For supply voltage of 30 V, 50 Hz and constant battery e.m.f. of 6 V find the resistance to he inserted in series with the battery to limit the average charging current to 3 A in case thyristor T is turned on at a firing angle of 20°.

20

4. (a) Why do the maximum voltages across the inductance and capacitance in a series R - L - C resonant circuit occur at different values of frequencies? Explain.

20

(b) (i) Calculate the quiescent collector current ICQ or room temperature for the amplifier circuit shown in the figure given below.

10

(ii) Describe the applications of clipping and clamping circuits.

10

(c) What is the difference between TWT and Klystron? How TWT operates as microwave oscillator?

Determine (i) the gain parameter; (ii) the power gain, and (iii) any two propagation constants in a traveling wave tube having the following parameters

Beam voltage V0 = 2 kV

Beam current I0 = 20 mA

Frequency f = 10 GH

Characteristic impedance of helix Z0 = 10

Circuit length = 50 (wavelength)

20

SECTION B 5. (a) Answer any three of the following

A non-sinusoidal current i(t) as shown in the above figure is passed through a capacitance C = 100 F. Determine the voltage across the capacitance when = 1000 rad/sec.

20

(b) (i) Design a combinational circuit having 3 inputs and 6 outputs with the condition that the output number is square of the input number.

10

(ii) A computer uses RAM chips of capacity 1024 × 1. How many chips are required and how their address and data bus be connected to provide a memory capacity of 1024 bytes? How chips would be connected to provide memory of 16 K bytes?

10

(c) For a single-phase one-pulse controlled converter system, sketch waveforms for load voltage and load current for (i) RL load and (ii) RL load with freewheeling diode across RL. Compare these waveforms and discuss the advantages of using a freewheeling diode. State the assumptions made.

20

(d) Justify the statement pulse modulation is not digital whereas pulse code modulation is digital

Obtain the bandwidth for 128 quantization levels and 8 kHz sampling frequency of a practical PCM system consisting of 24 telephone channels, each band limited to 3.4 kHz in time division multiplexing. Deduce formulae used.

20

6. (a) (i) Explain the phenomenon of attenuation in TE and TEM waves.

10

(ii) Determine the guide wavelength to propagate a 20 GHz signal through a parallel plate waveguide having a plate separation of 5 cm.

10

(b) (i) Which type of communication system will you prefer for speech processing? Why?

10

(ii) With the help of a suitable schematic diagram describe the working of a delta modulation system. Discuss its limitation.

10

(c) (i) Derive relationship between critical frequency and the maximum density of a layer N.

10

(ii) Calculate skip distance for a wave of frequency 4.5 MHz when the maximum ionization in the E - region has value of 1010 electrons/m3 at a height of 105 km.

10

7. (a)

Obtain Z-parameters of the 2-port network given above and deduce Y-parameters from the result.

20

(b) (i) A n-type Gats Gunn diode of length 10 m, doping concentration 2 × 1014/m3 and threshold field 3 kV/cm operates at a frequency of 10 GH with applied voltage of 3.5 kV/cm. Calculate electron drift velocity, current density and negative mobility of electron.

10

(ii) A circular loop of 20 cm radius made of 3 mm dia. of copper wire is used as an antenna. Calculate its radiation resistance. ohmic resistance, input resistance and radiation efficiency at 1 MHz.

10

(c) A signal i(t) = k{1 + m f(t)} cos ct is applied to the input of an average (diode) detector with load resistance R. The diode is characterized as

ib = 0 for b 0

ib = b/rd for b > 0

Obtain an expression or low frequency component of the output voltage of the detector for |m f (t)| < 1

20

Determine the output c(t) at t = 2 sec for the system shown in the above figure.

20

(b) A star - connected synchronous machine with ZS = 1 + j 10 ohm per phase, is synchronized with an infinite bus of 11 kV. The machine is made to operate at a leading power factor of 0.8 with an armature current of 50 A and with terminal voltage ahead of the excitation e.m.f. Calculate the magnitude of excitation e.m.f.

How can this machine be made to operate at unity p1? Under this condition, find the armature current and load angle,

20

(c) A single-phase full bridge inverter feeds a load that allows load commutation. Explain how load commutation is achieved. Describe the conduction of various elements of the inverter circuit.

20

PAPER – II SECTION A


20 × 3 = 60

(a) State variable approach for analysis is applicable only to linear time invariant systems.

(b) The problem of drift may assume very serious problems in electronic instrumentation system.

(c) Power carrying capability of transmission lines decreases with increase in the length of transmission, and can be improved by providing shunt reactors.

(d) Optical fibre is the best choice for heavy demand long line telephone communication systems.

2. (a) The open hop transfer function of a unity feedback system is given by

1 21 1

kG s

s st st

(i) Using the Routh—Hutwitz method, determine the necessary conditions for the system to be stable.

(ii) Sketch the root locus diagram for the system described for positive values of k, t1 and t2. Show how the root locus gets modified with addition of a zero.

20

(b) Incremental cost characteristics of two thermal plants is given by

11

1

60 0.2 . /GG

dCP Rs MW hr

dP

22

2

40 0.3 . /GG

dCP Rs MW hr

dP

Evaluate the sharing of a load of 200 MW for most economic operation. If the plants are rated 150 MW and 250 NW. what will be the saving in cost in Rs./hour in comparison to loading in the same proportion to rating?

20

(c) Describe the construction and principle of working of a solid-state and over-current relay.

20

3. (a) Boron is doped in n-type Si wafer having 2 × 1022 phosphorus atoms m3 through a circular window of 0.1 m diameter for 10 minutes at 1100° C The gaseous atmosphere of BH3 used for doping provides the B concentration of 1 × 1023 atoms m3 at the surface of the wafer Estimate the location of the p-n junction beneath the surface. Given the diffusivity of B in Si at 1100°C is 4 × 10-17 m2 s-1.

20

C.S.E-Mains 2000


(b) What is load flow solution? What is its significance in power system Classify various types

of buses in a power system for load flow studies. Explain, with a flow-chart, the computational procedure for load-flow solution using Gauss-Siedel method.

20

(c) What are solar concentrators? Describe various components of a solar concentrator giving their functions.

20

4. (a) One method of measuring a small capacitance Cx is shown in fig. 1. C1 and C2 are equal high-quality variable air capacitors. C3 is a fixed high quality capacitance of much smaller value than maximum value of C2 (about 1/10 of C2). The following two balances are obtained

(i) With switch S open, C2 at its maximum value and C1 is adjusted for balance.

(ii) With switch S closed, C1 left unaltered and C2 is adjusted to C2 to get a new balance

Prove that

23 2 2

2 2 2 3 2 3' '

C C CCx

C C C C C C

If R = 1000, C1 and C2 are 1000 pF and C3 = 50 pF and assuming the variable capacitors are readable to ± 5 pF with what accuracy could a capacitance of 1 pF be measured?

20

(b) Discuss three kinds of error-detection codes and explain how they detect errors.

20

(c) Explain with the help of block diagram a CW Doppler radar using an intermediate frequency in the receiver. Give its advantages applications and limitations.

20

SECTION B 5. Select any three of the following statements, read them carefully and identify the correct and the


20 × 3 = 60

(a) Feedback provides same control on the steady state error to standard inputs by adjustment of open-loop gain.

(b) One disadvantage of magnetic tape recorder is that its characteristics limit its use at higher frequencies.

(c) The cost of fabrication and installation of system for utilizing solar energy is too high to be viable.

(d) ICs, although cheaper, axe less reliable as compared to discrete components interconnected

by conventional techniques.

6. (a) A system is described by the following state and output equations

0 1 0

2 3 1X X U

And Y = [1 0]X

where u(t) is a unit step input and the initial conditions X1(0) = 0, X2(0) = 0. Obtain the time response of the system.

20

(b) Briefly explain the principle of PCM, differential PCM and delta modulation.

20

(c) Discuss salient features of a fibre optic communication system. List some of the optical components used to interconnect a digital voice or data systems.

20

7. (a) Discuss the functions of various Registers and ALU of 8085.

20

(b) Discuss multiple access techniques for satellite communications. Explain the meaning of the following

FDMA, TOMA, COMA and SOMA

20

(c) Briefly describe two different types of phase array radars. How can phased arrays overcome the difficulties occasioned by the use of moving radar antennae?

20

8. (a) Give the construction, principle of working and apperceptions of an LVDT.

20

(b) Develop receiver end power circle diagram of a transmission tine. Explain, how from the power circle diagram, capacity of a synchronous phase modifier be determined.

20

(c) Establish block diagram for load-frequency control of an isolated power system.

(d) 20


questions selection at least one question selecting at least one question from each Section.

SECTION A 1. Answer any THREE of the following

(a) Prove that an ideal transformer shown below for which v1/v2 = n can be identical with a perfectly coupled transformer

if n = 1 2/L L , where L1 and L2 are the self inductances of the primary and secondary

windings of the transformer and M is the mutual inductance.

Discuss about the current ratio. Obtain the expression for i1 (t) and conditions that the perfect transformer will be an ideal transformer.

20

(b) A farmer (F) has two sheds (N1 & S1) in his farm to keep his farm produce and belongings. He keeps vegetables (V) as farm produce, a goat (G) which can eat the vegetables and a dog (D) that can bite the goat in the absence of the farmer.

The farmer along has to manage both sheds without loosing either vegetables or the goat. Suggest a logic circuit that can alarm him under any catastrophic condition by means of a buzzer.

20

(c) A 3-phase induction motor has an efficiency of 0.9 when the load is 37 kW. At this load, the stator copper and rotor copper loss each equals the iron loss. The mechanical losses are one-third of the no load loss. Calculate the slip.

20

(d) A two-tone modulating signal em(t) = 5 cos 2.103 (t) + 4 cos 4.103t modulates a carrier voltage e(t) = 10 cos 2.106t. Find various frequency components present and corresponding modulation indices. Also obtain the amplitude of the signals present in each sideband and the bandwidths.

20

2. (a) Show that the multiplication of a sinusoidal signal by a signal f(t) translates the whole frequency spectrum. Can we retranslate the spectrum to its original position? If yes how?

20

(b) A separately excited d.c. motor supplied with rated voltage and rated field current is driving a fan delivering rated torque. The load torque of the fan is proportional to square of speed. The motor exhibited linear load characteristics such that the speed dropped by 10% from no load

C.S.E-Mains 2001


to full load torque at rated voltage and field current. If the supply voltage is halved at the same field current develops an expression for the new speed in terms of no Load speed (at rated voltage), full load torque and other constants.

20

(c) While the Thyristors is used in Power Electronic convertors it is being replaced by other devices recently. Mention any two of these devices and explain how they differ from the Thyristors. Name typical convertors in which the Thyristors are being replaced.

20

3. (a) (i) Draw the truth table of an octal-to-binary encoder with the condition that the output F = 1 when at least one of the inputs is a 1. The octal input with highest number has the highest priority. What is the condition of binary outputs and F output, if the inputs D5 = D3 = 1?

10

(ii) Design a combinational circuit using AND gates and half adders that can multiply two 2-bit numbers a1a0 and b1b0 to produce 4-bit number S3, S2, S1 and S0.

10

(b) Figure below shows a singly excited electro mechanical system with a stationary member and a rotating member. The stationary member is an electromagnet excited by a coil of N turns carrying a current i wound on a ferromagnetic core of infinite permeability. The rotating member is an unwound ferromagnetic core of infinite permeability causing variable permeance at changing rotor position from a maximum permeance of P1 at = 0 to a minimum permeance of P2 at = 90°. Assuming this variation of permeance to be sinusoidal develop an expression for the developed torque on the rotating member as a function of .

20

(c) In an electric traction d.c. series motor has been used with single-phase a.c. overhead lines. Gradually it is being replaced by another motor along with suitable speed control mechanism. Explain this new system comprising the motor and its control from the same overhead supply system and compare with the one using d.c. series motor.

20

4. (a) Define a uniform plane wave, Show that a uniform plane wave progressing in x-direction does not have the x-component of E. Explain what does this mean physically.

20

(b)

Obtain the voltage gain and output resistance of the composite amplifier shown in Fig. above.

20

(c) A rectangular wave guide has cross-sectional area a = 7 cm and b = 4 cm. Obtain all TE modes that will propagate frequency less than 5 GHz. How these modes are designated and what are the corresponding cut-off frequencies.

20

SECTION B 5. Answer any THREE of the following

(a) How do you determine the Laplace transform of a function f(t)? Are there any restrictions on f(t) for this ? State them.

Determine the Laplace transforms of

(i) signal in the form of a triangular pulse and

(ii) impulse train as shown below.

The strength of each impulse is unity.

20

(b) Figure below shows a coil of N turns placed in a time varying flux (t), The coil direction is such that for the direction of current i (as shown) in the flux direction is an indicated in the figure. Assume infinite conductivity of the conductor of the coil. Using relevant Maxwell’s equation obtains an expression for e. Indicate how this expression will be in conformity with Lenz’s law.

20

(c) A 3-phase induction motor is driving a constant torque load. If one of the supply lines to the

motor is disconnected discuss the changes you would observe in supply current, speed and efficiency.

20

(d) (i) Differentiate between FDM and TDM pictorially. Enumerate the advantages of one over the other.

10

(ii) 24-telephones, each band limited to 3.4 kHz, are to be time division multiplexed using PCM. Calculate the bandwidth of the PCM system for 128 quantization levels with 8 kHz sampling frequency

10

6. (a) What is the order of magnitude of velocity of an electromagnetic wave in a good conductor as compared to sound wave in air ? Discuss.

Calculate the depth of penetration of a MHz wave into copper which has = 5.8 × 107 /m and permeability equal to that of free space.

20

(b)

The simplified circuit of a radio oscillator is shown in Fig. above. Draw its equivalent circuit and obtain the condition of oscillation. Obtain the frequency of oscillation and the value of hfe required for the oscillation in the case of Hartley oscillator

20

(c) A transmitting antenna at a height of h2 above a smooth perfectly flat-earth surface sends a signal to a receiving antenna of height h1 with ground range between them equal to a. Obtain the path difference between the direct and reflected signals reaching the receiving antenna. Assume h2 > h1.

20

7. (a) What is cut-set in linear graph theory? Explain with an example.

In the graph

check if the sets of branches {1, 3, 4, 6} {1, 2, 3}, {1, 2, 3, 5} constitute cut-sets.

20

(b) Show that the output of a balanced modulator using semiconductor diode is DSB-SC.

20

(c) Obtain relationship between the skip distance and maximum usable frequency over the flat earth surface.

20

8. (a) For the pole-zero configuration of an impedance function shown below draw the schematic diagrams of the two Foster and two Cauer networks without determining element values.

What more information do you need to determine the element values?

20

(b) Measurements on a small permanent magnet d.c. motor indicate that it has an armature resistance of 7.5 ohm. With an applied armature voltage of 6 V. the motor is observed to achieve a no load speed to 10570 r/min while drawing an armature current of 13 mA.

(i) Calculate the motor torque constant

(ii) Find the no load rotational losses of the motor.

(iii) Find the stall current and torque of the motor at an applied armature voltage of 6 V.

20

(c) How are the Power Semiconductor devices specified?

What do these specifications signify?

20

1.

PAPER - II SECTION A

1. Select any THREE of the following statements, read them carefully and identify the correct and the incorrect ones. Justify your answer using not more than 200 words in each case.

20 × 3 = 60

(a) While a lead compensator reduces system bandwidth, a lag compensator on the other hand increases the system bandwidth.

(b) It is necessary to use preamplification and filtering before data processing.

(c) Tap changing transformers arc used in power systems, especially to control reactive volt-amperes.

(d) For the correct reproduction of the picture on the screen of a television, it is desired that the scanning at the receiver and transmitter are synchronized.

2. (a) What is the fundamental difference between a subroutine call and an interrupt request ? Is it possible to employ a common memory stack for both?

20

(b) For the network shown in Fig. 2 (b), obtain the complex bus bar voltage at bus 2 at the end of the first iteration. Use the Gauss -Seidel method. Line impedances are in pu. Given

20

Bus 1 is slack bus with V1 = 1.00°.

P2 + j Q2 = -5.96 + j 1.46; |V3| = 1.2

Assume V 03 1.02 0V and 0

2 1 0V

(c) Explain how a Buchholz relay protects a transformer.

20

3. (a) What is the proportion in which n-type and p-type Si both having conductivity of 10 ohm-1 m-

1 be mixed so as to produce a fully compensated semiconductor at room temperature? Mobility of electrons and holes in Si are 0.14 and 0.05 m2V-1s-1 respectively at room temperature.

20

(b) Explain the following in connection with power systems:

Ferranti effects, infinite line, Peterson coil, load bus.

20

(c) Calculate the maximum power by a solar cell at an intensity of 200 W/m2. Given Vmax = 0.14 V and Imax = -6 mA. Also calculate the cell efficiency if the area is 4 cm2.

20

C.S.E-Mains 2001


4. (a) While manufacturing IC, the Si wafer is oxidized time and again. What functions can an

oxide layer perform during and after the manufacture? What role is played by dislocations which get generated during the growth of the single crystal and during processing of the wafer? 20

(b) Explain, what is meant by the term Blind Speed in MTI (Moving Target Indicator) radar. What is the method of overcoming the problems of blind speed in radar?

20

(c) What do you understand by the term Satellite Communication Package? Explain in brief the working of satellite repeaters in use.

20

SECTION B 5. Select any THREE of the following statements, read them carefully and identify the correct and the

incorrect ones. Justify your answer using not more than 200 words in each case. .

20 × 3 = 60

(a) In order to increase the distance between poles, carrying long distance transmission line, aluminum conductors should be strengthened by strain-hardening.

(b) Routh-Hurwitz and continuous fraction stability criteria tell whether the system is stable or unstable. They do not tell anything about the relative stability of the system.

(c) Using energy audit and energy management techniques, one avoid or postpone the installation of costly peaky generation.

(d) In pulse compression radar, one gets a higher maximum range capability and a better range resolution.

6. (a) The conductivity of intrinsic Si at room temperature is 3.3 × 10-4 ohm-1 m-1. A bridge is available which can measure 1 in 104 changes in resistance. With what accuracy can this Si be used to measure the temperature in the range from 0°C to 100°C? The energy band gap in Si is 1.1 eV. The Boltzmann constant, k = 1.38 × 10-23 J K-1 and the electronic charge, e = 1.6 × 10-19C.

20

(b) Great a block diagram for a differential encoding and decoding system. Explain how it works by showing the encoding and decoding for the sequence 001111010001. Assume that the reference digit is a binary 1. Show that error proportion cannot occur.

20

(c) With reference to the colour television transmission and reception, explain (i) hue and saturation, and (ii) colour subscriber.

20

7. (a) Discuss the effect of temperature on the relative dielectric constant of a material.

20

(b) The semi-major axis and the semi-minor axis of an elliptical satellite orbit are 25000 km and 20000 km respectively.

Determine the apogee and perigee distances.

20

(c) Explain; how energy is lost at a sharp bend in a fibre. What do you understand by the term Dispersion, as applied to optical fibre?

20

8. (a) (i) If the program counter is always one count ahead of the memory location from which the machine code is being fetched, how does the microprocessor change the sequence of program execution with a jump instruction ?

(ii) List the sequence of memory transfers required to process an indirect-addressing mode instruction, when the instruction is a control type (for example Jump Unconditionally). How many memory cycles are needed?

20

(b) Explain with the help of flow chart procedure for solution of exact coordination equations.

20

(c) Discuss the basic principles involved in distance protection and describe how it is applied to transformer feeders.

20



SECTION A 1. Answer any three of the following

(a) The figure shows an equivalent circuit of a small two winding transformer in terms of the primary.

If the self inductance L2 of the secondary is 16 H, estimate the following

(i) The actual winding resistance of the secondary.

(ii) The secondary open circuit voltage with primary supplied with 220 V a.c.

(iii) The mutual inductance M. between the primary and the secondary.

6 + 6 + 8

(b) Write the integral form of Maxwell’s equations and identify each equation with the proper experimental law.

20

(c) A synchronous motor is running idle on an infinite bus at its rated voltage and frequency. It is excited to have an inducted voltage of 125% and its synchronous impedance is

(1 + j 20)%. Calculate

(i) the stator current, and

(ii) its phase with respect to the bus voltage.

Where would such idle runs be useful?

15 + 5

(d) A super heterodyne receiver uses an IF frequency of 455 kHz. The receiver is tuned to a transmitter having a carrier frequency of 1350 kHz. Give two permissible frequencies of the local oscillator and the image frequency for each.

20

2. (a) A filter has transfer function H(f) = rect(f/2B).

Where

1 1/ 2

0

xrect x

elsewhere

C.S.E-Mains 2002


An input x(t) 2 W sinc (2Wt) is applied to this filter, where

sinsin

xc x

x

(i) Find the output y(t) for W < B.

(i) Find the output y(t) for W > B.

(iii) In which case does the output suffer distortion? Justify your answer

7 + 7 + 6

(b) A d.c. shunt generator supplies the full load current of 50 A at 500 V Assume its field is drawing 2A, and the armature voltage drop is 5%. The generator is being driven at the rated speed of 1500 rpm, and is working at an efficiency of 80%.

Estimate (i) the speed at which it will run and (ii) the efficiency when the machine is working as a motor, drawing 50 A from a 500 V d.c. source

Assume the rotational losses, (the iron, friction and windage losses) vary as the square of the speed.

10 + 10

(c) (i) Germanium and silicon are both semi—conducting materials and we have both diodes and triodes with germanium as well as silicon. However, in respect of controlled rectifiers, we have only silicon-controlled rectifiers hut no germanium-controlled rectifiers. Explain why.

(ii) Use the two transistor equivalent circuit of the SCR and show that when reverse biased the SCR cannot contact even if it is triggered.

10 + 10

3. (a) A locker can be opened by A and B when they are both there, as the locker has 2 keys, one of which is with A and the other with B. C has both the keys, but he is permitted to open the locker only when B is not there. Show that the locker can be opened always if both A and C are there. Give the Boolean identity that symbolisms this situation.

20

(b) With respect to full load current why does an induction motor take higher no-load current than a transformer?

A 3-phase induction motor draws 25 A from 460 V 3- phase line at a p.f. of 085 lagging. The stator copper loss is 1 kW, and the rotor copper loss is 500 W. The rotational losses due to windage and friction are 250 W, core loss is 800 W, and stray load loss is 200W. Calculate (i) the air gap power Pg, (ii) the developed mechanical power, (iii) the output horse power, and (iv) the efficiency.

5 + 15

(c) Explain the principle of pulse with modulated inverters for induction motor control. What is the purpose of using pulse width modulation in this context?

4. (a) Obtain the conditions under which the circuit below produced oscillations

What is the frequency of oscillations?

20

(b) Define directive gain and directivity of an antenna. The radiation intensity of a certain antenna is

34sin sin 0 ,0

,0

Uelsewhere

Determine the directivity of the antenna.

5 + 15

(c) (i) An AM modulator has output

30cos 2 200 6 cos 2 180 6cos 2 220x t t t t

Determine the modulation index and efficiency.

(ii) An upper sideband SSB modulator has the message signal m(t) = Am cos mt. The unmodulated carrier is given by

c(t) = Ac sin t. Sketch the modulator output x(t).

10 + 10


(a) Consider the op-amp circuit shown.

In the circuit make the usual assumption that (i) the inverting and non-inverting terminals are virtually at the same potential and (ii) the op - amp does not take any input current at its

terminals. You would arrive at the normal conclusion that 0 21

.ivv R

R . Is this conclusion

valid? If yes give reasons why operational amplifiers are never used in this manner with inverting input grounded and input to be amplified given to the non-inverting terminal.

20

(b) Use the properties of the unit impulse function (t) to evaluate the following integrals

10 + 10

(i) 2 cos 5t t t dt

(ii) 10cos 2 1te t t dt

(c) Describe any brushless excitation system for large alter nators. Explain the working.

20

(d) What is a Hertzian dipole? Find the radiation resistance of a Hertzian dipole.

20

6. (a) A standard air-filled rectangular waveguide with dimensions a = 8.6 cm and b 4.3 cm is fed by 4 GHz carrier from a coaxial cable. Determine if a TE10 mode will be propagated. If so, calculate the phase velocity and the group velocity.

20

(b) An FM modulator has input m(t) = 4 cos 10 t. The peak frequency deviation is 25 Hz. The modulator is followed by an ideal band pass filter with a center frequency given by the carrier frequency and a bandwidth of 54 Hz. Determine the power at the filter output assuming that the modulator output power is 100 W. Express the answer in terms of appropriate Bessel function values.

20

(c) In a certain region with = 0, = 0 and = 6.25 0, the magnetic field of an EM wave is 80.6cos cos10 /zH x ta A m

Find the phase constant and the corresponding E (electric field) using Maxwell’s equations.

20

7. (a) Devise a 4-bit odd parity generator function using 3-level NOT-NOR-NOR logic.

(b) A delta modulator has the message signal

m(t) = 6 sin 2 (10)t + 4 sin 2 (20) t.

Determine the minimum sampling frequency required to prevent slope overload, assuming the step size to be 0.1 m.

20

(c) In a non-magnetic medium

75sin 2 10 0.8 /zE t x a V m

Find

(i) relative permittivity and intrinsic impedance.

(ii) the time average power carried by the wave.

10 + 10

N1 and N2 are two 2-port networks connected in parallel on both input port side as well as output port side, to form a composite 2 - port network N as indicated

N1 and N2 are defined by the Z-parameters as below

1 2

4 3 3 2,

3 5 2 4n nZ Z

Obtain the Z-parameters for the composite 2-port network N.

20

(b) What is all day efficiency of a transformer? Why is it necessary to calculate such efficiency?

A 300 KVA transformer has a core loss of 1.5 kW and a full load copper loss of 4.5 kW. Calculate the efficiency at 75% of full load output at a p.f. of 0.8 lag. Calculate the maximum efficiency and the load at which it occurs.

5 + 15

(c) (i) Explain the main advantage of power MOSPETs over SCRs

(ii) A fully controlled bridge rectifier has a voltage of 200 V d.c applied to its input terminals as shown.

Identify the forwarded and reverse biased SCRs in the circuit. Assume the forward biased SCRs have a leakage resistance of 100 M and the reverse biased SCRs have 200 M. Show that each of the devices is blocking a voltage of approximately 100 V in the circuit, irrespective of whether it is forward biased or reverse biased

5 + 15


1. Select any three of the following statements read them carefully and identify the correct and the incorrect ones. Justify your answer using not more than 200 words in each case

20 × 3 = 60

(a) The magnitude of a transfer function whose poles are only in the left half of ‘s’ plane and whose zeros are mirror images of the poles about the ‘jw’ axis increases with increase in frequency.

(b) For measurement of 3 power using two wattmeters one of the wattmeters reads negative when the power factor angle is more than 60°

(c) In a large interconnected power system, consider three buses having short circuit capacities 1600 MVA (1), 1200 MVA (2) and 1000 MVA (3) respectively, the voltages of all the buses are 1 p.u. If a 3 fault takes place on bus 2, the change in bus voltage is described as V1 > V2 > V3.

(d) A satellite earth station must have as many receive chains as there are carriers transmitted to it.

2. (a) A system is represented by state equation X = AX.

If for 1

0 3

2

X

the response is

2

23

2

t

t

eX t

e

And for 1

01

X

t

t

eX t

e

Determine the state transition matrix.

20

(b) Station ‘A’ transmits 50 MW power to station ‘B’ through a tie-line. Maximum steady state capacity of the line is 100 MW. Determine the allowable sudden load that can be switched on without loss of stability. Derive any formulae used.

20

(c) Explain clearly the construction of a p-n junction and its use to convert sun-light directly into electricity. What distinguishes a solar cell from a conventional p-n junction diode?

20

3. (a) Discuss briefly the microprocessor architecture with special reference to the following components-

C.S.E-Mains 2002


(i) Data bus, (ii) Address bus and (iii) Control bus.

20

(b) Explain briefly various abnormal operating conditions in a large alternator against which protection is necessary. Explain clearly with neat diagram the protection of altemator against failure of excitation.

20

(c) What is wave power? Describe some of the devices used for converting wave power into electrical power. Discuss briefly its feasibility with reference to Indian sub-continent.

20

4. (a) Differentiate between para-magnetic, diamagnetic and ferro-magnetic materials. Discuss various factors which affect permeability and hysteresis loss

20

(b) Explain how television is capable of displaying complete moving pictures, despite the fact that at any instant of time, only a tiny portion of the picture tube screen is active.

20

(c) How do the functions of a communication satellite compare with those of a microwave link repeater? What is the most significant difference in their functions?

20


incorrect ones. Justify your answer using not more than 200 words in each case

20 × 3 = 60

(a) The steady - state errors for unit step input to type-0, type-1 and type-2, unity feedback systems are finite non-zero, zero and infinite respectively.

(b) “Tan ” of an insulating material is an indication of its health

(c) For the same rupturing capacity the actual current to be interrupted by an HRC fuse is much greater than that of any circuit breaker.

(d) PCM is considered very well, but other modulation systems are also used

6. (a) Explain with suitable diagrams, how one single phase wattmeter can be used to measure 3 active and reactive power. Discuss the limitation of this method.

20

(b) Find the signal-to-quantization noise ratio in a binary PCM system, if the number of quantization levels is 64:

20

(c) In a TV reveiving system the pie-amplifier (mounted on the aerial) has a gain of 20 dB and a noise factor of 6 dB and the cable between the pre-amplifier and the receiver has an attenuation factor of 3 dB. While the RF and IF stages or the receiver have a total gain of 90 dB and a noise figures of 3 dB

(RF stage) and 13 dB (IF stage) Determine the system overall noise figure.

20

7. (a) Show that the feedback reduces the effect of parameter variation on the performance of the

control system, but adversely affects the gain of the systems Also discuss briefly the effect of feedback on the transient performance of the control system. .

20

(b) Derive a simple expression for the orbital period of & satellite in terms of the mechanical constants of the satellite and the earth.

20

(c) A multimode fiber has a core refractive index 1.5 and the relative refractive index difference is 3 per cent and the operating wavelength is 0.8 in. Calculate the critical radius of curvature at which large bending losses occur.

20

8. (a) First manually place OOs in the accumulator and all general purpose registers.

Then write a programme which will

(i) place the hex number FE in the accumulator.

(ii) copy the contents of the accumulator ‘A’ into resister ‘B’

(iii) copy the contents of the accumulator ‘A’ into register ‘C’

(iv) stop.

Show the programme in 8085 mnemonics.

20

(b) Derive swing equation and discuss its application in the study of power system stability.

20

(c) Discuss one scheme each of (i) constant speed constant frequency (CSCF) (ii) variable speed constant frequency (VSCF) of a wind-electrical energy conversion system.

20



SECTION A 1. Attempt any THREE of the following

(a) For the circuit given below,

(i) Find the frequency w0 at which the impedance across X, Y reaches a maximum.

10

(ii) Find the impedance across X, Y at frequency w0 and the current through the network at that condition.

5 + 5

(b) (i) Solve the difference function given by

y(K + 2) - 5y(K + 1) + 6y(K) = 3x(K + 1) + 5x(K)

If the initial conditions are

111

6y and 37

236

y

and the input function is x(K) = 2. u(K).

10

(ii) Enumerate different properties of the ROC for Laplace

transform and find out the I.L.T of 1

1 2X s

s s

for

the different ROC’s Re(s) > -1, Re(s) < -2, -2 < Re(s) < -1

10

(c) (i) A full-pitched coil having N turns rotates with an angular velocity of wr rad/sec with respect of field flux. Obtain an expression for the flux Linkages with the coil as a function of time t. Derive there from an expression for the emf generated in the coil. Discuss about the two components of the emf expression so obtained.

10

C.S.E-Mains 2003


(ii) Show that, in case the field flux is time-invariant, the generated emf lags by 90° the

flux that generates it.

10

(d) (i) Can you detect a DSBSC signal using an envelope detector? By a suitable analysis, compare the same with coherent detection

(ii) A modulating signal, m(t) is given by cos 100t + 2 cos 300t. Determine the LSB and USB relations + LSB and USB if the carrier frequency is 1000 rad.

15+ 5

2. (a) (i) The following two amplifier circuit is a generalized immitance converter (GIC).

Analyze the above circuit to get an expression for Zin. Assume the two op-amps to be ideal.

(ii) With Z1 = R1, Z3 = R3, Z2 = 1/C2s, Z4 = R4 and Z5 = R5, what does the GIC circuit above realize?

(iii) Z1 and Z5 are capacitors, Z2, Z3 and Z4 are resistors. How does the input impedance Zin, behave?

(iv) What are energy and power signals? Compute the energy and power contents of the following signals and indicate whether they are energy or power signals

xT(n) = {(-0.5)n u(n) 0.01s}

yT(n) = {2e3jn u(n) 0.2s}.

10 + 2 + 2 + 6

(b) (i) Discuss why forward break over in a thyristor occurs at a lower anode-cathode voltage if its gate current is increased.

(ii) What are the advantages of IGST over BJT, MOSFET and thyristor?

(iii) Discuss why the speed control with constant supply voltage and reduced supply frequency is rarely used in practice in a three-phase induction motor.

(iv) A dc machine has total armature ampere conductors of 4500 and total flux in the machine is 0.14 Wb. Calculate the torque developed in the machine.

5 + 5 + 5 + 5

(c) A three-phase full-converter feeds a resistive load of 150. For a firing-angle delay of 75°, the load takes 6 RW. Find the magnitude of per phase input supply voltage to the three-phase kill-converter.

Derive the relevant expression used if any.

20

3. (a) (i) A rectangular pulse g(t) exists between T = - (T/2) and t = + (T/2). Sketch the pulse

obtained by convoluting g(t) with itself. The Fourier transform of g(t) is a sine function Write down the Fourier transform of the pulse obtained by the above convolution.

(ii) Find the Laplace transform L(t) of the following function

f(t) = t cos wt.

Given L(sin wt) = 2 2

w

s w

Using the result of L(t cos wt), find

122 2

1L

s w

10 + 10

(b) Explain the development of electromagnetic torque in a three-phase squirrel-cage induction motor through the interaction of stator and rotor mmf waves. Hence introduce the concept of load angle.

Show that rotor must rotate in the direction of rotating field. Discuss the conditions under which optimum torque is developed in a 3-phase induction motor.

20

(c) A 12-V battery feeds a single-phase full-bridge inverter whose output is connected to an ideal 1-phase transformer. Its primary has 10 turns and the load voltage is 230 V. For a load resistance of 100, calculate

(i) the number of turns in the transformer secondary winding,

(ii) rms value of primary current, and

(iii) rms value of thyristor current.

Consider only the fundamental component of inverter output voltage.

20

4. (a) In the circuit shown below the switch ‘S’ closes at t = 0, and open after 10 ms. What will be the currents in R, and L1 and L2, and voltage across C1 8 ms after switch ‘S’ opens? Assume D, to be an ideal diode and a 0.7 V drop across D2 whenever it conducts.

(b) Show that the field strength at a point close to the antenna (at distances of unto 200-300 miles from the transmitting antenna) in terms of the power is given by

9.49/

Pvolts m

r

Where:

is the field strength in volts/m

P is the power radiated in watts

r is the distance in meters.

Calculate the field strength at 20 km away from a transmitting station of 50 kW power

20

(c) (i) A sine wave carrier is frequency modulated by a triangular modulating signal m(t) as shown in figure below. If the carrier frequency is 100 MHz and modulating signal frequency is 5 kHz, determine the bandwidth of the FM signal. Assume frequency deviation constant, Kf = 105 Hz/r.

(ii) In a commercial FM broadcast system, the modulation index is 5, the 3 dB frequency of the pre-emphasis/de-emphasis filter is 2100 Hz and message signal frequency is 15 kHz. If the average to peak power ratio of the message signal is 05, what is the improvement in output SNR of FM with pre-emphasis and de-emphasis filtering compared to a baseband system?

10 + 10

SECTION B 5. Attempt any THREE of the following:

(a) (i) What are the criteria for a material to be an imperfect di-electric or a good conductor? Write Maxwell’s equations in terms of phasors for sinusoidally varying electromagnetic fields. Consider a uniform plane wave in an isotropic and homogeneous medium, and show that the electric and magnetic field are not in time phase.

15

(ii) A uniform plane wave is traveling at a velocity of 3.0 10 m/s in a non-magnetic good conductor and its wavelength is 0.35 mm. Find the frequency of the wave, and the conductivity and the intrinsic impedance of the conductor.

5

(b) A series resonant circuit has its impedance

20 1 10 1 10s j s jZ s

s

Find

(i) Resonant frequency

(ii) Q factor

(iii) A and width

(iv) Impedance under resonance condition.

5+5+5+5

(c) Two single-phase full converters are used for control ling the speed of a separately-excited dc motor. One full-converter in the field circuit has zero degree firing-angle delay. Both the converters are connected to 1-phase, 220 V, 50 Hz source. DC motor armature resistance is 0.6 and field circuit resistance is 150 and its constant is 0.6 V-s/A-rad. Armature and field currents are ripple free. For a firing angle delay of 40° and armature current of 40 A, determine

(i) The motor speed in rpm,

6

(ii) Power factor of the armature converter, and

6

(iii) Power factor of the drive scheme.

6

(d) (i) Define sensitivity, selectivity and fidelity with reference to a radio receiver. What is double spotting and its effects on a receiver?

(ii) A superhet receiver having no RF amplifier, the loaded Q of the antenna coupling circuit is 100. If the superhet is to be improved for HF reception so that its image rejection at 25 MHz is as good as at 1100 kHz, determine the loaded Q which an RF amplifier for this receiver would have to have and the new IF that would be needed in the absence of the RF amplifier. (Assume IF of 455 kHz).

8 + 12

6. (a) Discuss the criteria for a circuit to oscillate. Explain the operation of a phase shift oscillator and derive an expression for the frequency of oscillation of an RC phase shift oscillator.

20

(b) (i) A rectangular hollow metal waveguide of internal cross-section 7366 cm × 3.556 cm carries a 3 GHz signal in the TE10 mode. Calculate the maximum power handling capability of the waveguide assuming the maximum permissible electric field inside the waveguide to be 30 kV/cm.

10

(ii) Explain how a PLL can be used for demodulation of FM signals. Define capture range and lock range of a PLL.

10

(c) (i) White Gaussian noise of two sided spectral density 10-12 V2/Hz is applied to a low pass (RC) filter having a 3 dB cut-off frequency of 1 kHz. Find the output noise power.

(ii) The received signal is r(t) = s(t) + n(t) where s(t) is the signal and n(t) is the noise signal. The signal process is random with an autocorrelation Rs() = 2e-||. The noise is a sample function of a random process with auto-correlation Pn() e-2||. If the two processes are having zero mean value and are independent of each other determine the autocorrelation and total power of r(t).

5 + 15

7. (a) What is uniform plane wave? Define all the parameters associated with it. What is meant by group velocity?

A uniform plane electro-magnetic wave travelling in region 1(1, 1, 1) is incident normally on a plane boundary separating the region 2(2, 2, 2), How are the reflected and the transmitted waves at the interface related to the incident wave?

10 + 10

(b) Distinguish AM and NBFM signals. Obtain an expression for NBFM and WBFM signals. Determine the approximate band of frequencies occupied by an FM wave with carrier frequency of 5 kHz, Kf = 10 Hz/V and f(t) 100 cos 200 t volts.

(c) Distinguish between envelope detection and synchronous detection. Explain a Costas loop receiver for demodulating a DSBSC signal. Explain how the squaring circuit can also be used for synchronous detection.

20

8. (a) (i) Using multiplexers, implement a full adder circuit.

(ii) It is desired to generate the following three Boolean functions

1F abc abc bc

2F abc ab abc

3F abc abc ac

By using an OR gate array as shown in Figure, where P1 to P5 are the product terms in one or more of the variables, , , , ,a a b b c and c . Write down the terms P1, P2, P3, P4 and P5.

10 + 10

(b) A 50 kW, 440 V, 50 Hz, star-connected synchronous generator with per phase impedance of 0.8 + j 4.5 is working with rated terminal voltage. It is required to supply a power of 25 kW to the bus. Estimate the minimum excitation emf. Find out the corresponding stator current.

20

(c) Consider the circuit shown below. Main thyristor T1 and auxiliary thyristor TA are off and capacitor is charged to voltage Vs with upper plate positive. Sketch the waveforms for io, ic, iT1, vc and vT1 (assuming constant toad current), after

(i) thyristor T1is turned on

(ii) steady slate is reached in part (i) and then thynistor TA is turned on

From these waveforms, obtain the expressions for circuit turn-off time for both SCRs.

In case Vs = 220 V, R = 10 and maximum value of current through main SCR is 3 times the load current, calculate the value of commutating inductance L for C = 20F and the circuit turn-off time for the auxiliary SCR.

20


1. Select any three of the following statements, read them carefully and identity the correct and the incorrect ones. Justify your answer using not more than 200 words in each case:

20 × 3 = 60

(a) Routh-Hurwitz criterion is an algebraic method that provides information on the absolute stability of linear time invariant system that has a characteristic equation with constant co-efficient.

(b) If a polyphase, load is supplied through ‘m’ conductors, the total power consumed is measured by adding the readings of ‘m’ wattmeters so arranged that current coil of a wattmeter is in each line and the corresponding pressure coil is connected between that line and a common point. If the common point is located on one of the lines, then the power may be measured by (m - 1) wattmeters only, irrespective of balanced/unbalanced load.

(c) For economic operation, the generator with highest positive incremental transmission loss will operate at the highest positive incremental cost of production.

(d) Considering state of the art, the communication links of satellites for weather forecasting are at present very effective.

2. (a) Explain the effects of negative feedback in control systems on the following

20

(i) Stability (ii) External disturbances

(b) Define surge impedance of a line and discuss the advantages and disadvantages of operating an overhead line corresponding to surge impedance loading. Also explain the terms infinite line, flat line.

20

(c) Describe a few features of a wind wheel for efficient generation of electrical energy and discuss economic viability of this form of energy vis-a-vis conventional method. Which generator do you recommend and why?

20

3. (a) Discuss briefly the architecture of 8086 microprocessor with special reference to the following

20

(i) Memory organization

(ii) Register structure

(iii) Addressing modes

(b) Explain with reasons the connection of C.T.s for protecting a detta/star transformer Justify your scheme of protection for (i) internal fault and (ii) external fault, using Merz-Price protection.

20

(c) What is solar power? Describe very briefly with neat diagram the scheme for (i) residential cooling and heating and (ii) solar power plant, using solar energy.

C.S.E-Mains 2003


20

4. (a) Describe the factors on which the conductivity of intrinsic semiconductor depends.

A semiconductor is made of ‘n’-type germanium. From the following data, calculate the conductivity of ‘n’-type germanium:

(i) Density of Ge = 5.32 × 103 kg/m3

(ii) At. wt. of Ge = 72.6 kg/k.mol

(iii) Charge of electron = 1.6 × 10-9 C

(iv) Mobility of holes = 0.18 m2/V.s

(v) Mobility of electrons = 3.8 m2/V.s

(Assume one donor atom in each 108 atoms)

20

(b) Describe the basic modulation schemes in digital data transmission.

20

(c) Explain what is meant by saying that a satellite is stationary. Why are such satellites used for worldwide communications in preference to any other kind?

20


incorrect ones. Justify your answer using not more than 200 words in each casa

20 × 3 = 60

(a) If the roots of characteristic equation of a linear digital control system lie within unity circle in z-plane, the system is stable.

(b) The magnetic property of a material is completely lost above its characteristic Curie temperature.

(c) The chances of arc interruption in subsequent current zero increase in case of air blast CS. but decrease in oil C.B.

(d) The bandwidth of a PCM signal is much larger than that of original base band.

6. (a) Explain how frequency can be measured using four arm bridge arrangement. Derive balance condition for such a bridge. Mention the range of frequency measurement and accuracy of such measurement. If applied voltage is non- sinusoidal, will it be possible to balance this bridge?

20

(b) Describe a communication circuit using modem and explain how modems are classified,

20

(c) Explain the significance of blind speed in moving target indicator (MTI) by radar.

A MTI operates at 5 GHz with a pulse repetition frequency (PRF) of 400 pps. Calculate the lowest blind speed of radar.

20

7. (a) Discuss the performances of proportional plus derivative and proportional plus integral controllers.

20

(b) Describe radar beacons and their application.

20

(c) What do you mean by bandwidth of single mode fibre and describe the factors on which it depends?

20

8. (a) Explain the following

(i) Gain margin (ii) Phase margin

How are they determined from Nyquist plot and Bode plot?

20

(b) A generator is connected to a motor through a Y/ transformer. The generator is connected to the star side of the transformer. A fault occurs between the motor terminal and the transformer. The symmetrical components of the sub-transient current in motor flowing towards the fault are

1aI : - 0.8 - j2.6 p.u., 2aI : -j2.0 p.u. and

0aI -j3.0. From the transformer

towards fault1aI : 0.8 – j0.4 p.u.,

2aI -j1.0 p.u. and0

: 0aI . Assume X” = X2 for both the motor

and the generator. Describe (i) the type of fault, (ii) the prefault current, if any, in line ‘a’.

20

(c) Explain the terms energy management, energy audit and energy economics. Explain how these techniques can be used to improve the performance of an energy starved inter connected power system.

20



SECTION A 1. Answer any three of the following

(a) (i) What will be the impedance of a parallel connected LCR network at resonance frequency?

4

(ii) If an impedance ZL is connected across a voltage source V with source impedance Zs then for maximum power transfer, what will be the load impedance?

4

(iii) The voltage across an impedance in a network is V(s) = Z(s) . I(s) where V(s), Z(s) and I(s) are the Laplace transforms of the corresponding time functions v(t), z(t) and i(t). What will be the relation among them in the time domain of v(t), z(t) and i(t)?

4

(iv) Develop the relation between unit step sequence and unit sample sequence.

4

(v) How can you express a discrete signal as a sequence of a sum of scaled delayed unit sample sequence?

4

(b) (i) Determine Laplace transform of the waveform f(t) as shown in the figure below:

4

(ii) Draw the block diagram of the system represented by the following constant coefficient difference equation

y(n) = b0x (n) + b1x (n -1) + a1y (n-1)

where x(n) input sequence and y(n) = output sequence.

10

(c) (i) What is the principle of operation of a repulsion motor? What are its main features, advantages and applications?

15

C.S.E-Mains 2004


(ii) Describe the working of a stepper motor.

5

(d) (i) A cable transmission system with a loss of (L) 240 dB and has m = 6 equal-length repeater sections and the SNR at the destination (S/N)D 30 dB. Find the new value of (S/N)D if (a) m is increased to 12 and (b) m is decreased to 4. Express your answer both in ratio and in dB.

10

(ii) A helix travelling wave tube operates at 4 GHz with a beam voltage of 10 kV and a beam current of 500 mA. The helix impedance is 25 ohm and interaction length is 15 cm. Determine the output power gain in dB.

10

2. (a) (i) Use final value theorem to find f() for the function (1 - e-t) u(t), where > 0.

(ii) Find the current i(t) through the 8 resistor of the circuit diagram shown below with initial current i(0-) = 5 Amp.

6

(iii) Obtain the frequency response of the first-order recursive system represented by constant coefficient difference equation

y(n) ay(n - 1) + x(n).

Consider the system is initially relaxed.

10

(b) What are commutation circuits? Design a commutation circuit for an inverter.

5 + 15 = 20

(c) (i) Discuss transformer noise and means adopted to reduce it.

5

(ii) A 30-kVA, single phase transformer has an iron loss of 457 Wand copper loss of 125W, when delivering half the full load. At what percent of the full load will the transformer have maximum efficiency?

15

3. (a) (i) Consider the function of complex frequency s of the following expression

2

3

4 5

sX s

s s

Find the Laplace inverse function of the above expression.

8

(ii) What do you understand by the mathematical relation between continuous and discrete

system as given below?

1 2j TA

m

X e X mT T

where X(ejT) is the frequency response of the sequence x(nT), T is the sampling period and XA (j 0) is the Fourier transform of the continuous-time waveform. If the analog frequency response is band limited to the range | | /T what will the new relation be?

2 × 8 = 12

(b) (i) Illustrate and compare speed-current and torque-current characteristics of a compound wound d.c. motor with that of series and shunt motor.

5

(ii) A 3-phase, 50 Hz, 16 pole generators with connected windings has 144 slots with 10 conductor’s star slot. The flux/pole of 24.8 mWb is sinusoidally distributed. The coil is full pitched. Find the line emf. 15

(d) The speed of a 20 HP 460 V, 1000 rpm separately excited d.c. motor is controlled by a single-phase full- wave bridge circuit as shown in the figure:

The rated motor armature current is 35 A and the armature resistance is 0.15 0. The a.c. supply voltage is 480V. The motor back emf constant is kb = 0.45 V/rpm. The motor current is continuous and ripples free. For rectifier operation (d.c. machine running as a motor) and for a firing angle = 60° and rate armature current, calculate (i) torque, (ii) speed and (iii) supply power factor.

20

4. (a) (i) A sinusoidal voltage source is applied to the parallel RC network as shown in the figure below. Find the energy stored and energy dissipated in the resistor over the interval 0 < t < 0.5 sec.

10

(ii) Find the output response to an input x(n) = u(n) - u(n - p) for the system whose unit sample response is as follows

0

0 0

na for nh n

for n

10

(b) The joint probability density of the random variables X and Y is f(x, y) = K e-(x+y) in the range 0 x and 0 y and f(x, y) = 0 otherwise.

(i) Find the value of the constant K.

(ii) Find the probability density f(x), the probability density of X independently of Y.

(iii) Find the probability P 0 2;

2 3

X

Y

(iv) Are the random variables dependent or independent? Justify.

20

(c) What is troposphere? State the parameters that characterize troposphere. Write empirical formulae for the variation of these parameters within the first 1 - 2 km of the troposphere. Write expressions for the permittivity (r) and refractive index (n) of the tropospheric medium. What is the value of n at an altitude of 1 km?

20

SECTION B 5. Answer any three of the following

(a) (i) Circuit diagram shown below s initially under steady-state condition, the switch is moved from position I to position 2 at t = 0. Find the current after switching.

8

(ii) Find the z-transform of cos (n/2). u(n).

12

(b) (i) For plane waves in free space, show that the intrinsic impedance of free space is given by 120 .

10

(ii) An EM wave travels in free space with the electic field component

0.866 0.5100 /j y zs xE e a V m

Determine and .

5 + 5 = 10

(c) (i) In the circuit of the figure shown below, the source voltage F = 120 V and the current

through loads R1 and R2 is 20 A. The turn-off time of both the SCRs is 60 sec. find the value of C for successful commutation.

10

(ii) Give interpretation of negative, unity and > unit slips of induction motor.

10

(d) A paraboloidal reflector antenna of cross-sectional area of 8000 cm2 works at 3 GHz. Calculate the (i) beam width between first nulls, (ii) half power beam width, and (iii) gain of the antenna.

20

6. (a) (i) Design a second-order Butterworth low- pass filter with cut off frequency of 1 kHz. Use only Sallen and Key network configuration as shown below:

8

(ii) Find the impulse response of the system whose transfer function is given as follows

1 2 2

1

1 2 cos .H z

r z r z

Which converges |z| > r, it has a pair of poles at z = r.e±j0 and a double zero at z = 0.

12

(b) It is desired to transmit an audio signal which has a bandwidth of 10 kHz with an output SNR of 40 dB. The channel introduces a 30 dB power loss and channel noise is AWON with power spectral density /2 = 10-9 W/Hz. Assume = 2Sx = 0.5 for AM.

Calculate the (i) transmission bandwidth (ii) required transmitter power ST for DSB, SSB and AM systems.

20

(c) A microstrip line is to be designed on alumina substrate having relative dielectric constant r = 8.2. The W/h ratio 0.95.

Compute

(i) effective relative dielectric constant

(ii) characteristic impedance Z0

(iii) phase velocity

(iv) guide wavelength

20

7. (a) Draw the circuit diagram of a half wave rectifier with a reservoir capacitor that smooths the output. Draw its output waveform. Determine the reservoir capacitor value for a HWR and smoothing circuit to supply 20 V to a load of 500 . Maximum ripple amplitude is to be 10% of the average output voltage, and the input signal frequency is 50 Hz.

20

(b) In an Armstrong modulator, the crystal oscillator frequency is 200 kHz. The angular deviation is set to 0.2. The system is to accommodate modulation frequencies between 50 Hz and 15 kHz. The carrier frequency at the output is 108 MHz and the maximum frequency deviation is 80 kHz. Draw the block diagram of the modulator showing the details and select multiplier and mixer oscillator frequencies to accomplish this end.

20

(c) Find the field strength at distances of 500 m and 2,500 m from a dipole centre fed with Im = 0.1 A. Dipole has a length of 200 cm and the frequency of operation is 3 GHz. Consider E only in the maximum direction.

20

8. (a) The discrete time system is described by the constant coefficient difference equation

y(n) = x(n) + a y(n - 1) with initial condition y(-1) = K. If the input x(n) = ejn. u(n) is applied to the system, what will be the output response?

[Solve the problem using z-transform and its inverse transform operation only]

20

(b) (i) Derive conditions for maximum starting torque to be obtained in case of an induction motor.

5

(ii) A 4-pole, 50 Hz, 7.46 kW motor at rated voltage and frequency has a starting torque of 160% and a maximum torque of 200% of full-load torque. Determine (a) full load speed and (ii) speed at maximum torque.

15

(c) A 6 MW, 3-phase, 11 kV, Y-connected, 6-pole, 50 Hz, 0.9 (leading) power factor synchronous motor has Xs = 9 and Rs = 0. Rated field current is 50 amperes. Machine is controlled by variable frequency control at constant (V/f) ratio upto the base speed and at constant V above base speed. Determine the torque and field current for the rated armature current, 750 rpm and 0.8 leading power factors.

20


1. Select any THREE of the following statements, read them carefully and identify the correct and the incorrect ones. Justify your answer using not more than 200 words in each case

(a) At any point on the root locus with angle condition satisfied, the value of K-variable parameter (for this point to be a root) is obtained as

product of lengths of vectors from poles

product of lengths of vectors from zeroK

(b) Bridges are used to measure inductance of coils having different values of Q and capacitances are used therein to obtain the balance.

(c) Is voltage control necessary in power systems? Various current control methods are used. Permissible voltage variations are known. Any advantage of current control?

(d) Digital transmission has more advantages than disadvantages as compared to analog transmission.

20 × 3 = 60

2. (a) What do you understand by lead compensator? Draw the approximate Bode plot for a typical lead compensator. Design a suitable control system for a missile to introduce a lead of 35° and gain of 6.5 db at 2.8 rad/s. What will be the transfer function of this lead compensator that will satisfy the above requirement?

20

(b) A 25 MVA, 11 kV generator with X”d = 20% is connected through a transformer, line and a transformer to a bus that supplies three identical motors as shown in fig. below. Each motor has X”d 25% and X’d = 30% on a lease of 5 MVA, 6.6 kV The three-phase rating of the step-up transformer is 25 MVA, 11/66 kV with a leakage reactance of 10% and that of the step-down transformer is 25 MVA, 66/6.6 kV with a leakage reactance of 10%. The bus voltage at the motor is 6.6 kV when a three- phase fault occurs at the point F For the specific fault, calculate

(a) the subtransient current in the fault

(b) the subtransient current in the breaker B

(c) the momentary current in breaker B

Given Reactance of the transmission line = 15% on a base of 25 MVA, 66 kV Assume that the system is operating on no load when the fault occurs.

20

C.S.E-Mains 2004


(c) What is wave energy? Explain the difficulties facing wave-power development. Give a list of

devices developed for converting wave energy into electrical energy and fully describe one of these.

20

3. (a) Describe the various steps involved in the fabrication of a monolithic circuit shown below

20

(b) Draw a block diagram of Remote Terminal Unit (RTU), having different components/subsystems, communication bus etc. Write the functions it generally carries out. Briefly explain the distributed control system. Bring out the features of Data Highway.

20

(c) State and explain the basic requirements (i.e. essential qualities) of protective relaying.

20

4. (a) Justify that the stack and subroutine offer a great deal of flexibility in writing programs. Point out the differences between stack pointer and program counter, PUSH and POP, CALL and RET. Give examples for each. Write a program to add two numbers which generate the carry over, store the carry over in memory locations XX50 and sum XX60 in 8085 microprocessor.

20

(b) A CW radar operates at a frequency of 10 GHz. What is the doppler frequency produced by

(i) an air plane flying at a speed of 250 kms per hour

(ii) a man crawling at a speed of 2.5 cms per second?

20

(c) The guided missile tracking radar operates at 5 GHz and a 1.0 MW peak power output. If the antenna diameter is 4 meters, the receiver band-width 1.6 MHz and the receiver has a 10 dB noise figure, what is its maximum detection range for 1 m2 targets?

20

SECTION B 5. Select any THREE of the following statements, read them carefully and identify the correct and the

incorrect ones. Justify your answer using not more than 200 words in each case

(a) Latch is used at input and tristate is used for output purposes in the microprocessor based system.

(b) Capacitors in integrated circuits are obtained by using the transition capacitance of a reverse-biased p-n junction.

(c) An HVAC circuit breaker of a given voltage, current and MVA rating can be used as an HVDC circuit breaker of the same voltage, current and MVA rating.

(d) Do we have a system which makes a fuller utilization of the transmission capacity of an

optical fibre and needs multiplexing?

20 × 3 = 60

6. (a) (i) Explain the construction of single-phase induction-type energy meter.

(ii) A 230 V single-phase watt hour meter has a constant load of 4 A passing through it for 6 hours at unity p.f. If the meter disc makes 2208 revolutions during this period, what is the meter constant in revolutions per kWh? Calculate the power factor of the load if the number of revolutions made by the meter are 1472 when operating at 230 V, and 5 A for 4 hours.

20

(b) How is digital data converted into analog signals for telephone lines? Compare them with respect to

(i) Waveforms

(ii) Bit rate

(iii) Baud rate.

20

(c) State Kepler’s three laws of planetary motion. Illustrate in each case their relevance to artificial satellites orbiting the earth.

20

7. (a) Describe the band structure of an open-circuited P-N junction.

20

(b) How I, Q and Y signals are generated in a colour TV transmitter? Explain with tube and matrix arrangement.

20

(c) What is meant by carrier to noise ratio? How is it related to SNR? An FM system has modulation index of 10. Given that C/N is 25 dB, calculate the receiver processing gain and the S/N.

20

8. (a) Write the discrete forms of P, PI and PID controllers. What are various methods of tuning them?

Discuss in brief.

20

(b) Explain briefly the techno-economic advantages and disadvantages of HVDC transmission system.

20

(c) Explain on-line real-time computer system for protection of a line. Draw a sample fault program flow diagram.

20



SECTION A 1. Answer any three of the following:

(a) (i) A continuous time LTI system is described by

/2

/2

1 t T

t Ty t x d

T

Find the impulse response of the system. Is the system casual?

5

(ii) For the circuit shown below, find i1, i2, i3 and i4.

5

(iii) A transmission line of characteristics impedance Z0 = 50 is terminated by load impedance RL = ZL = 100 . Find voltage standing wave ratio VSWR, minimum impedance Zmin and maximum impedance Zmax.

5

(iv) Find the qualifactor Qp at resonance for the circuit shown below:

5

(b) (i) Design a combinational circuit that accepts a three-bit number and generates an output binary number equal to the square of the input number.

10

C.S.E-Mains 2005


(ii) Design a second-order active RC band-pass filter with a mid-band voltage gain –A0 =

34 dB, carrier frequency 160 Hz and 3 dB bandwidth 16 Hz. Assume C1 = C2 = 0.1 F.

10

(c) (i) Explain the principle of split-phase, single-phase induction motor. Discuss resistance split-phase motors and capacitor split-phase motors.

3 + 6 + 6

(ii) Explain why an induction motor is not run under conditions of maximum load.

5

(d) (i) Calculate the depth of penetration for copper at 1 MHz. The conductivity for copper is 58M /m and permeability is 1.26 H/m

5

(ii) Discuss the merits of delayed AGC as compared with simple ACC.

5

(iii) Show that the first minor lobe is -13.5 dB to principal maxima for broadside array for large n.

10

2. (a) (i) Find 0 , ,

c

L C

dV Oi V O

dt

and

Ldi O

dt

for the circuit shown below. Use

CdV O

dt

to denote

0

C

t

dV t

dt

Assume the switch 1 has been opened for switch 2 has been closed for along time and steady-state condition prevails at t = 0-.

(ii) A parallel plate waveguide consists of two parallel perfectly conducting infinite planes situated 8 cm apart. Determine the propagating TEmo for a wave of 6000 MHz, assuming free space between the plates. Find also the cut-off frequency fc, waveguide wavelength g, waveguide intrinsic impedance g.

9

(iii) List the three sources of instability of collector current Define the three stability factors. Which biasing arrangement gives you better stability?

5

(b) (i) Define voltage regulation of a transformer. Derive the expression for voltage

regulation at lagging power factor. Draw phasor diagram.

2+5+3

(ii) A single-phase transformer working at unity power factor as an efficiency of 90% at both half full load and full load. Full load is 500 W. Determine the efficiency at 60% of full load condition.

10

(c) (i) What is a TRIAC? Explain its characteristic. Discuss its applications, advantages and disadvantages.

3 + 6 + 2 + 2 + 2

(ii) A voltage across SCR is 1 V when it is conducting. It has a holding current of 2 mA when gate current (IG) = 0. If the SCR is triggered on by momentary pulse of gate current to what value must VA be reduced in order to turn the SCR off? The figure is shown below.

5

3. (a) (i) Using the difference equation design method with a backward difference, design a digital filter to simulate the analog filter whose transfer function is

2 5400 2 10

sH s

s s

using the sampling rate fs = 1 kHz

10

(ii) Find the poles and zeros of the networks shown below.

5

(iii) Implement the Boolean function

F(A, B, C, D) (0, 1, 3, 4, 8, 9, 15)

with an 8 × 1 multiplexer with A, B and D connected to selection lines s2, s1, s0, respectively.

5

(b) (i) Explain speed-current and torque-current characteristics of d.c. series motor.

4 + 4

(ii) A 4-pole d.c. generator has 1200 armature conductors and generates 250 volts on open circuit when running at a speed of 600 r.p.m. The diameter of the pole-shoe circle is 0-35 m and the ratio of pole arc to pole pitch is 0.7 while the length of pole-

shoes is 02 m. find the mean flux density in the air gap. Assume lap-connected armature winding.

12

(c) (i) What are the steps involved for the generation of gating signals for thyristors of ac. to d.c. converters? Draw a block diagram incorporating various stages of firing circuits for a single-phase fully controlled rectifier.

3 + 9

(ii) If the SCR in the following figure is continuously fired by a d.c. signal, what is average value of the current i(t)?

8

4. (a) (i) For the network shown below, Find the current ratio transfer function given by = I2/I1.

10

(ii) Explain boundary conditions for time-varying fields.

5

(iii) Draw the small signal model of FET. Define , gm and rd, and obtain relationship between them.

5

(b) (i) When a super heterodyne receiver is turned to 555 kHz, its local oscillator provides the mixer with an input at 1010 kHz. What is the image frequency? The antenna of this receiver is connected to the mixer via a tuned circuit whose loaded Q is 40. What will be the rejection ratio for the calculated image frequency?

10

(ii) Show that power contained in one sideband is 1/6 to the total power of amplitude-modulated signal at 100% modulation index.

10

(c) (i) Explain Faraday rotation isolator.

A matched isolator has insertion loss of 0-5 dB and isolation 25 dB, Calculate the scattering coefficient.

5 + 10

(ii) Explain construction of Ionosphere.

5

SECTION B 5. Answer any three of the following

(a) (i) Determine v (t) for the circuit shown below when vs = 10 cos 30t V

5

(ii) Find the Fourier transform of the signal

2

sin sin / 2t tx t

t

(iii) Find the Laplace transform of the stepped waveform shown below.

(iv) An air-filled rectangular waveguide of dimension a = 4 cm, b = 2 cm, transports energy in the dominant mode at a rate of 2 mW if the frequency of operation is 10 GHz. Determine the peak value of electric field in the waveguide.

(b) (i) The 741C op-amp having the following parameters is connected as shown in the figure.

A = 20000, Ri. = 2 M, R0 = 75, f0 = 5Hz, supply voltage = ± 15 V, output voltage swing ± 13V. Identify the circuit.

Compute the values of AF, RIF, ROF, fF & VOUT

10

(ii) How much noise voltage is generated in a 1 M resistance at 27°C over a 10 kHz bandwidth? List the sources of noise in an amplifier.

2+3

(iii) What are the conditions for distortion less transmission in continuous time LTI systems and discrete time LTI systems?

5

(c) (i) A three-phase star-connected synchronous generator is rated at 1.5 MVA and 11 kV. The armature effective resistance and synchronous reactance are 1.2 ohms and 25 ohms respectively per phase. Calculate the percentage voltage regulation for a load of 1.4375 MVA at (1) 0.8 power factor lagging and (2) 0.8 power factor leading. 10

(ii) Explain qualitatively with sketches as to how armature reaction effect in a synchronous machine can be modeled as an inductive reactance.

10

(d) (i) In a trapezoidal pattern displaying modulation, the length of the long vertical side is 5 cm, and of the short vertical side, 2cm. Determine the modulation depth.

5

(ii) Explain Foster-Seeley discriminator circuit with the help of phasor diagram.

10

(iii) Describe transmission & radiation modes of helix.

5

6. (a) (i) Find the inverse z-transform of

2

2

2

2

e zx z

z e z

Where |z| > 2 and e-2 < |z| < 2.

7

(ii) Obtain the three-dimensional wave equation for an absorbing medium, assumed to be both magnetically and electrically homogenous and isotropic. The charge density in the field may be assumed to be zero. Hence, determine the wave equation if the field is varying harmonically with time.

8

(iii) A sequential circuit has one input and one output. The state diagram is shown in the

figure. Design the sequential circuit using D flip-flop.

5

(b) (i) Three regular dice are thrown. Assign probabilities to the following events. The sum of the points appearing on three dice is 4; 9; and 15.

10

(ii) Explain central limit theorem.

10

(c) (i) Explain the term ‘virtual height’ with respect to ionospheric transmission.

Calculate the transmission-path distance for an ionospheric transmission that utilizes a layer of virtual height 200 km. The angle of elevation of the antenna beam is 20°. Consider flat earth.

6+4

(ii) Explain varactor diode. Give its equivalent circuit. Give its application.

10

7. (a) (i) Explain the different input statements in SPICE.

5

(ii) Design an m-derived -section high-pass filter having characteristic impedance 600 , cut-off frequency 5 kHz and infinite attenuation at 4 kHz.

10

(iii) Using a bipolar 32 × 8 PROM, implement the following equations simultaneously:

5

F1 = m(0, 1,3,5,7,21)

F2 = m(2, 3,4,7, 11, 12, 15, 26)

F3 = m(8, 9,11,13,20,23)

(b) (i) Describe ‘third method’ of generating an SSB-SC signal.

10

(ii) Explain envelop detector circuit with input and output waveforms.

10

(c) (i) Draw and explain construction details of reflex klystron.

Explain electronic tuning.

10

(ii) Explain test setup for VSWR measurement.

10

8. (a) (i) Find the CTFS harmonic function for the signal

x(t) = 4 + 2 cos (20t) -3 sin (40 t)

by sampling above the Nyquist rate over exactly one fundamental period and finding the DTFS harmonic function of the samples.

10

(ii) Find the input impedance Z of the circuit shown below, = 1000 rad/s.

5

(iii) Draw the circuit diagram of voltage-to-current converter with floating load using op-amp. Derive the necessary equation.

5

(b) (i) Describe the effect on the speed-torque characteristic of a 3-phase induction motor by (1) reducing the applied voltage to half the normal voltage but normal frequency and (2) reducing both applied voltage and frequency to half the normal value.

10

(ii) A 4-pole, 50 Hz, 3-phase induction motor has a rotor resistance of 4.5 ohms per phase

and a standstill reactance of 8.5 ohms per phase. With no external resistance in the circuit, the starting torque of the motor is 85 Nm. Calculate (1) the rotor voltage at standstill, (2) starting torque if a resistance of 2 ohms is added in each rotor phase.

10

(c) (i) Describe single-phase d.c. drive considering a phase-controlled converter supplying a d.c. motor load. With the help of a circuit diagram, explain the operation of a single-phase full converter circuit feeding a separately excited motor.

12

(ii) A d.c. drive runs at 900 r.p.m. on no load when fed at 200V d.c. from a full converter. Calculate the triggering angle of the converter thyristors if the motor has to run at 600 r.p.m. on no load. Input to the converter is 230 V a.c. Neglect IaRa drop.

8

1. (ii) Find the Fourier transform of the signal



3 × 20 = 60

(a) Jury stability test states that if the first element of descending order polynomial, viz, a0 > 0, then all roots of the polynomial Q(z) are inside the unit circle if and only if the first element in each odd-numbered row is positive in the derived array.

(b) Transducers are sensors having many classifications, advantages and applications.

(c) Very efficient operation and control of an interconnected power system is necessary and various levels of planning, and load control centers are required.

(d) 2 : 1 interlace ratio is used for interlaced scanning in television systems.

2. (a) State and explain the Mason’s Gain formula. Determine all input-output relationships of the system represented by the set of following linear equations using the Mason’s Gain formula:

x1 = a11 x1 + a12x2 + b1u1, and

x2 = a21 x1 + a22 x2 + b2u2

where u1 and u2 are the inputs and x1 and x2 are the states of the system.

20

3. (a) Sketch and discuss the fabrication of a planar transistor made for monolithic integrated circuits using epitaxy and diffusion with that of a discrete planar epitaxial transistor.

20

(b) Explain how the SCADA systems are techno economically feasible for Indian power systems. Point out the differences between loose-coupled SCADA system and tight-coupled SCADA system. Draw the diagrams of direct digital control system for power systems.

20

(c) The time-current characteristic of an induction-type over current relay is

C.S.E-Mains 2005


It is desired to determine the time of operation of a 5- ampere relay having a current setting of 125 per cent and give time-multiplier setting of 06 connected to a supply circuit through a C.T. ratio of 400/5 amperes when the circuit carries a fault current of 4000 amperes.

20

4. (a) What minimum functions are required in a programmable device? Point out the difference between control logic and control word with examples. Write the steps to compute the control word, say of 8255.

20

(b) The relevant data for closed-circuit TV system is given below. Calculate the highest modulating frequency that will be generated while scanning alternate black and white dots for equal vertical and horizontal resolution.

20

Data: Number of lines = 250,

Interlace ratio = 1:1

Aspect ratio = 4:3,

Picture repetition ratio = 50 per second,

Resolution factor = 0.8,

Vertical retrace time = 10% of picture frame time,

Horizontal retrace time = 20% of total line time.

(c) With the aid of a block diagram, explain the operation of FMCW radar altimeter.

20



3 × 20 = 60

(a) The memory mapping for the circuit shown below is 2000H to 2OFPH, assuming A8, A9, and A10 to be zero.

(b) Electrical isolation different elements of a monolithic integrated circuit is obtained by

creating isolation islands.

(c) In a long transmission line, the sending-end voltage is always greater than the receiving-end voltage and this effect is known as Ferranti effect.

(d) Transmission links using optical fibres need components with specific characteristics to fulfill the system requirements.

6. (a) (i) With a simple block diagram, explain basic swept receiver spectrum analyzer. List the spectra of different signals applied to the spectrum analyzer for analysis.

20

(ii) Draw basic elements of a function generator and its output waveforms.

20

(b) Describe the basic elements of a PCM system.

20

(c) Calculate the apogee and perigee heights for the orbital parameters given Eccentricity = 0002, Mean Earth’s radius = 6378 km, Mean distance to the semi-major axis = 7100 km.

20

7. (a) Distinguish between (i) spontaneous and stimulated emission, (ii) step index and graded index optical fibre.

20

(b) Bring cut the concept of TDM with a block schematic.

20

(c) A signal having a range of 300 to 3400 kHz is transmitted using binary PCM on a channel whose bit rate is 40,000 bits per second. Design the PCM system and draw the block diagram.

20

8. (a) Point out the advantages of frequency response studies of a system. Explain the Bode plots for the log magnitude versus frequency and phase angle versus frequency for the complex conjugate poles or zeros.

20

(b) Draw and briefly describe a neat diagram schematic diagram showing the components of a bipolar point- to-point HVDC transmission system. Explain how the control of power transfer and of power-flow direction is achieved.

20

(c) Describe microprocessor - based inverse - time over current (ITOC) relay with a neat schematic block diagram.

20


Candidates should attempt Questions 1 and 5 winch are compulsory and three of the remaining questions selecting at least one question from each Section.

SSEECCTTIIOONN AA


(a) (i) Determine the power delivered by 6 A source.

(10)

(ii) For the circuit shown in the figure determine V0/I5 using nodal analysis.

(10)

(b) (i) A partially filled truth table of a sequence generator (3-D flip-flops in cascade) is given below. Find the sequence generated. Q0 is LSB and Q2 is MSB.

State Q2 Q1 Q0

1 1 0

2 0

3

4 1

5 0 1

6

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7 1 1

1 0 0

(ii) A D/A converter has the lowest and the highest values of resistances 1 k and 4 k, respectively. If the bit-length of the converter is to be increased by 2, what would be the number of resistors required and their values ? Draw the complete circuit of the new converter.

(10)

(c) A 250 V, 50 hp, 1000-rpm d.c. shunt motor drives a load that requires a constant torque regardless of the speed of operation. The armature circuit resistance is 0.04 . When this motor delivers rated power, the armature current is 160 A.

(i) If the flux is reduced to 70°/o of its original value, find the new value of the armature current.

(10)

(ii) What is the new speed ?

(10)

(d) (i) List the sources of noise in bipolar transistor

(10)

(ii) A 100 kc resistor is connected in parallel with a 100 pE capacitor Determine the effective noise bandwidth and the noise voltage appearing at the terminals of the combination at 27° C.

(10)

2. (a) (i) Solve the following difference equation by use of the z transform method:

(10)

x(k+2)+3x(k+1)+2x(k)=0, x(0) = 0, x(1) = 1

(ii) Obtain the inverse z transform of:

3

2

2( )

2 1

z zX z

z z

(b) (i) For a three-phase induction motor, establish the ratio of maximum torque to full-load torque in the form:

21

,2

m T FL

FL FL

T S

T S

where '

2'2

r

x

(10)

(ii) An induction motor of squirrel cage type has starting current 5 times of full-load current and full-load slip, of 4%.

Evaluate maximum torque and corresponding slip and relation of starting torque to full-load torque.

Assume the stator impedance and magnetising current negligible.

(10)

(c) (i) What is the essential difference in commutation performance of triac and two thyristors connected in antiparallel ?

(8)

(ii) A battery is charged through a resistor R as shown:

If F is 220 V (rms), Eb = 100 V (dc) and R = 20 , calculate the battery charging current and power supplied to the battery.

(12)

3. (a) (i) Write the Maxwell’s equation in a lossy dielectric and explain that the electric field leads the magnetic field in time phase.

(10)

(ii) Explain the term intrinsic impedance and its value in free space.

(5)

(ii) From Maxwell’s equation state the significance of the term /. What will be its value for a good conductor and a good dielectric ?

(5)

(b) A three-phase 440 V, 60 Hz, star-connected eight-pole, 100-hp induction motor has the following parameters expressed per phase:

r1 = 0.06 , x1 = x2’ = 0.26

r2’ = 0.048 , rc = 107.5

x = 8.47 , s = 0.03

The rotational losses are 1600 W. Using the approximate equivalent circuit, determine

(i) the input me current and power factor,

(ii) the efficiency.

(12 + 8)

(c) (i) What are the advantages of half-controlled converters over fully-controlled converters?

Show the output voltage waveforms for controlled and uncontrolled halves of the converter and the resultant voltage.

(10)

(ii) The speed of a 20-hp, 220 V, 1000 rpm dc series motor is controlled by a single phase half-controlled full wave ideal converter Series field winding and armature winding have resistances 0.05 and 0.12 respectively. Input to converter is 230 V, 50 Hz supply.

Calculate the motor torque, current and supply power factor at rated load and speed for firing delay angle = 30°. Assume motor current to be continuous and ripple free. Residual back emf constant is 0.08 V/ rad/sec and armature current, dependent back emf (or torque) constant is 0.04 Nm/A2.

(10)

4. (a) (i) Derive the condition of oscillation and the expression for the frequency of oscillations

for the circuit shown. (Use mesh analysis and Barkhausen’s criteria). Draw actual oscillator circuit with one operational amplifier and minimum number of RC elements.

(15)

(ii) The ac equivalent circuit of an N-P-N transistor circuit is shown below. Draw the actual circuit with biasing and coupling capacitors.

(5)

(b) (i) Show that at 100% modulation index, power contained in one side band of AM system is 1/6th of total power transmitted.

(10)

(ii) A standard AM transmission, sinusoidally modulated to a depth of 30°/n, produces side frequencies of 4.928 and 4.914 MHz. The amplitude of each side frequency is 75 V. Determine the amplitude and frequency of the carrier

(5)

(iii) Define conditional probability and random variable.

(5)

(c) (i) Explain tropospheric scatter propagation.

(10)

(ii) A reflex klystron operates at the peak of the n = 2 mode. The dc power input is 40mW and V1/V0 = 0.278. If 20% of the power delivered by the beam is dissipated in the cavity walls, find the power delivered to the load. Assume 1 = 1 and J1 = (1.53) 0.562.

(10)

SECTION B

5. Answer any Three of the following:

(a) (i) State and prove the frequency-shift theorem.

(3)

Determine the Laplace transform of f(t) shown in the figure, using the frequency-shift theorem.

(7)

(ii) A system has impulse response e–at. What would be the response of the system, if it is excited by a delayed unit step function (delay = T) ?

(10)

(b) (i)

The above circuit is intended for NOT operation for input levels 0 and 10 V. Calculate the minimum value of R for which this operation is possible. Assume hFE = 30, VBE

(sat) = 0.8, VCE (sat) = 0.2 in respect of above silicon transistor.

(10)

(ii) Implement an OR gate with AND and NOT gates.

(5)

(iii) Draw a positive logic OR gate with two diodes and explain its operation.

(5)

(c) (i) Draw arid explain the speed, torque and power-time curves for a typical electrical traction drive.

Differentiate clearly between dynamic and regenerative braking.

(10)

(ii) A 3-phase, 400 V, six-pole, 50 Hz induction motor with parameters RS = Rr’ = 1, XS = Xr’ = 2 (referred to stator) is under regenerative braking action. Calculate (i) the maximum overhauling torque that it can hold and the range of speed for safe operation and (U) the speed at which it will hold the overhauling load with torque of 160 N-m.

(10)

(d) (i) List the differences between microwave transistors and transferred electron devices.

(10)

(ii) Explain the structure of IMPATT diode.

(5)

(iii) List the properties of S-parameters.

(5)

6. (a) (i) Use Thevenin’s theorem to replace the three-loop circuits of figure by a single-loop equivalent circuit in which the identity of RL is preserved.

(10)

(ii) Find the value of RL so that the maximum power is consumed in it.

(10)

(b) (i) Explain principle of super-heterodyning. List the advantages of super-heterodyne receiver over TRF receiver.

(10)

(ii) Explain PLL FM demodulator

(10)

(c) (i) A certain microstrip line has the following parameters:

r = 5.23, h = 8 mils

t = 2.8 mils, W = 10 mils.

Calculate the characteristic impedance Z0 of the line.

(10)

(ii) Compare linear broadside arrays having uniform, binomial and Dolph – Tchebyscheff distributions.

(10)

7. (a) (i) Draw the field lines and equipotential surface when a point charge is placed near an infinite grounded conducting plane.

(10)

(ii) Derive the expression for the force between the point charge and the conductor with the help of the above concept.

(5)

(iii) Explain the cause of polarisation in a dielectric.

(5)

(b) (i) Explain the difference between coherent and non coherent detection of ON/OFF keying signals.

(10)

(ii) State the Carson’s rule, Given that the peak deviation is 75 kHz, determine, using

Carson’s rule, the bandwidths required for sinusoidal EM indexes of 0.25, 2.4, 5.5 and 7.

(10)

(c) Show that the radiation resistance of a small loop antenna is 31200 (A/2)2 ohm.

(20)

8. (a) (i) Explain with the help of neat diagram, the operation of a Schmitt trigger employing OP-AMP.

(10)

(ii) With OP-AMP draw a circuit of a unity gain amplifier having no phase shift and give its one practical application.

(5)

(iii) Draw the circuit of a two input adder with OP-AMP and explain its operation.

(5)

(b) (i) ‘Transformer is a constant-flux machine’. Justify.

(10)

(ii) Explain logically why primary current increases when secondary side is loaded.

(5)

(iii) Derive a condition for maximum efficiency of a transformer

(5)

(c) An ideal chopper operating with a chopping period of 2 m sec is supplying a load of 5 resistance with inductance of 10 mHenry, from a battery of 100 volts. Assume the shunt diode to provide perfect commutation. Determine the load current waveform for the ratio TOFF/ON = 1/4 and average value of load voltage.

(20)

PPAAPPEERR -- IIII Time Allowed: 3 Hours Maximum marks: 300

Candidates should attempt Questions 1 and 5 which are compulsory, and any three of the remaining questions selecting at least one question from each Section.

SSEECCTTIIOONN AA

1. Answer any THREE of the following:

(3 x 20 = 60)

(a) Hicks clinical thermometer measures human body temperature. The instruction is to wait for half-a-minute before taking the reading. Is it because the measuring system is a:

(i) zero order system with a pure time delay of hall-a- minute ?

(ii) first order system with a time constant greater than half-a-minute ?

(iii) first order system with a time constant equal to half-a-minute ?

(iv) first order system with a time constant less than half-a-minute ?

Justify your answer.

(20)

(b) Enumerate the different kinds of head type flow meters used in the industry. What kinds of restrictions are placed in the flow path for flow measurement? Develop the relationship between flow and pressure differential from physical principles.

Why is it that a rotameter is considered a standard in flow measurement ?

(20)

(c) Give a brief account of application of computers in the study, analysis and control of power systems.

(20)

(d) Colour sub-carrier (3.58 MHz tar FCC standard and 4.43 MHz for CUR standard) is chosen as some odd multiple of one-half of line frequency. Justify the statement.

(20)

2. (a) What is the difference between a microcomputer and a micro-process ? What type of data is stored in the memory of a microcomputer ? How much memories can a typical 8-bit micro-process access? What are the condition flags and why are they important ? Write the advantage of the indexed address mode compared to the direct addressing mode. Draw the time diagram of out instruction for 8085 micro-process.

(20)

(b) A constant load of 330 MW is supplied by two 250 MW generators, 1 and 2, for which the respective incremental fuel costs are:

CC..SS..EE..--MMaaiinn--22000077


11

1

0.12 22,dF

PdP

and 22

2

0.10 16dF

PdP

with powers P n MW and costs F in Rs./hr Determine:

(i) most economic operating schedule of the generators, and

(ii) the saving in Rs./hr. thereby obtained compared to equal load sharing between them.

(20)

(c) Explain the sea waves phenomena and vortex motion of waves. Discuss the principles of two possible devices used for generating electrical energy from the wave energy. Draw the block diagram from wave energy device to utility grid. Write the relation for the potential power in terms of height of wave and its time period. If the time period of a wave is 10 secs having height 7 meters, estimate the power that can be generated.

(20)

3. (a) How is the surface layer of silicon dioxide formed ? How thick is this layer ? What are the reasons for forming the silicon dioxide layer?

(20)

(b) A 25 MVA, 11 kV generator has 1 2 0.2Z Z j pu and 0 0.05Z j pu . For a line-to-ground

fault on the generator terminals, find:

(i) the fault current and

(ii) the line-to-line voltages during the fault condition.

Assume that the generator neutral is solidly grounded and that the generator is operating at no load and at rated voltage at the time of occurrence of fault. Compare this fault current with the line current for a three-phase fault at the terminals.

(20)

(c) (i) A power transformer is connected in / fashion. Draw the arrangement of the current transformers (CTs) on either side of the transformer for Merz-Price circulating current protection.

(ii) A distribution transformer is to be protected by over- current relays against all kinds of fault. List the different kinds of fault and suggest the minimum number of over-current relays required. How would those over-current relays be placed in the circuit ?

(iii) How would one protect a direct-on-line (DOL) start induction motor against overload? Explain your choice.

(20)

4. (a) Discuss the physics of piezo-electric phenomena. Draw the equivalent circuit of a piezo-electric transducer and derive the output voltage, Vout of transducer with applied force F. Point out the four materials which exhibit the piezo-electric phenomena. Also write its applications.

A piezo-electric pressure transducer having a charge sensitivity of 2 x 10–12 C/N is connected to a charge amplifier, the gain being set to 5 mV/pC. The amplifier output is connected to an ultraviolet chart recorder whose sensitivity is set to 25 mm/volt. Determine the overall sensitivity.

(20)

(b) With the aid of a block schematic, describe the functioning of the receiving unit of a DBS TV/FM receiving system intended for home reception.

(c) Consider an optical fiber with refractive index of the core 1.8 and of cladding 1.7; calculate:

(i) Critical angle

(ii) Acceptance angle

(iii) Brewster angle

(iv) Numerical aperture.

(20)

SSEECCTTIIOONN BB

5. Answer any THREE of the following:

(3x20=60)

(a) What do you understand by dielectric films? State any two dielectric film material and point out their important properties and applications. Explain

(i) the orientation polarization,

(ii) electronic polarization; and

(iii) ionic polarization referred to polyatomic gas.

Discuss the effect of frequency on electronic polarization and ionic polarization. What is electronegative gases? State the important features of SF6 and its applications.

(20)

(b) Find the gain and approximate input impedance of the accompanying circuit. What is its bandwidth? What is this circuit called and for what purpose it is used as instrumentation ?

Analyze the following three Op-Amp based instrumentations amplifier and find its gain. What is its approximate input impedance ?

Assume that the Op-Amps are ideal.

(20)

(c) Explain why it is more difficult to interrupt short-circuit current in an HVDC circuit than that

in an HVAC circuit.

(20)

(d) In a certain communication system, the message is coded into binary digits 0 and 1. After coding, the probability of a 0 being sent is 0.45, while the probability of a 1 being sent is 0.55. In the communication channel, the probability of a transmitted 0 being converted into a 1 at the receiver is 0.1, while the probability of a 1 being distorted into a 0 is 0.2. Find:

(i) the probability that a received 0 was transmitted as a 0, and

(ii) the probability that a received I was transmitted as a 1.

(20)

6. (a)

1 0 0.375 0

0 1 0.375 0

1 1 1 1

x x u

Given that the above is a linear time-invariant (LTI) system, how would you check the stability of the system ? If there is an unstable mode in the system, is it controllable? If the unstable mode is controllable shift it to a stable position, say = –0.5, by state feedback.

(20)

(b) What are the design considerations for layering in data networks ? Discuss a seven-layer network architecture with a diagram.

(23)

(c) Why and to what degree do optical signals get degraded as they propagate along a fiber ? Discuss in detail.

(23)

7. (a) Which I/O technique provides the highest data transfer rate ? Explain the function of DMA. Draw the block diagram of a DMA controller with memory interfacing. What is meant by simultaneous DMA ? List three different DMA modes for transferring data. How is the DMA controller is programmed ? Are special buffers required when interfaces a DMA controller ?

(20)

(b) A radar receiver has a low noise RF amplifier of noise figure 6 dB and gain 20 dB followed by a mixer of noise figure 6 dB and conversion loss of 6 dB. If the noise figure of IF pre-amplifier is 3 dB with a gain of 26 dB and that of IF main amplifier is 10 dB, obtain the noise figure of the receiver and its noise temperature.

(20)

(c) Draw the waveform at the end of one of the vertical fields, showing a horizontal and a vertical blanking pulse. Indicate the duration and relative amplitudes of two pulses and explain their functions. Where are synchronizing pulses placed? Does it matter that there are no horizontal blanking purses during vertical blanking period ?

(23)

8. (a) A Wheatstone bridge has resistances R1, R2, R3, and R4 in its four arms. The bridge is excited by E volts dc. Assuming that the ratio of arms R1 and R2 are equal, the bridge is initially balanced with R3 equal to R4. A slight unbalance (R3 ± R3) is created by R3. Show that the unbalance voltage, V0, is directly proportional to (R3/R3) as long (R3/R3) << 1.

What is the condition for maximizing the sensitivity ?

(20)

(b) The figure below shows a generator-transformer unit feeding 1.0 pu power to the infinite bus

through a double-circuit transmission line with the generator terminal voltage of V1 = 1.0 pu. Compute the generator e.m.f. behind transient reactance. Also find the maximum power that can be transferred when

(i) the system is operating under healthy condition,

(ii) one line is shorted (three-phase) in the middle, and

(iii) one line is open.

(20)

(c) What do you understand by energy conservation ? What is the economics of energy conservation ? Point out the strategic measures for energy conservation.

Show that for a given load current I, if power factor is improved from 0.7 to 0.9, the useful power can be enhanced by 28.6% of the existing system. Comment on the losses due to power factor improvement.

(20)

Documents

Civil Service - Electrical Engineering Main Paper I & II - 1992 - 2007