BASICELECTRICALENGINEERING PREVIOUSYEARQUESTIONS · 2016-11-11 · CAH–WeBELIEVEinWORK,notinWORDS. [email protected] BASICELECTRICALENGINEERING PREVIOUSYEARQUESTIONS

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BASIC ELECTRICAL ENGINEERINGPREVIOUS YEAR QUESTIONS

CREDITSANMOL K MATHUR

ECE, II YEARCHARAN VEGIMCE, II YEAR

ANDCHANGE ACCHA HAI TEAM

UNIT IIntroduction: Role and importance of circuits in Engineering, concept of fields, charge,current, voltage, energy and their interrelationships. V- I characteristics of ideal voltageand ideal current sources, various types of controlled sources, passive circuit components,V-l characteristics and ratings of different types of R, L, C elements. DC Network: Seriesand parallel circuits, power and energy, Kirchhoff’s Laws, delta-star transformation,superposition theorem, Thevenin's theorem, Norton's theorem, maximum power transfertheorem, Tellgen's theorem.

1. Derive a relation for obtaining the delta connected formation of three resistancesequivalent to star connected formation of three resistances.

( NOVEMBER 2014 )2. A current of 20 A flows through two ammeters A and B joined in series. The potential

difference between A and B is 0.2 V and 0.3 V respectively. Find how the same currentwill divide between A and B when they are joined in parallel.

( NOVEMBER 2014 )

3. State and illustrate with the help of an example the Tellgen’s Theorem for electricalnetworks.

( NOVEMBER 2014 )State and prove Tellgen's theorem with the help of some example.

(MAY 2011)Write short notes on Tellgen’s theorem and Maximum power transfer theorem.

(NOVEMBER 2010)4. Determine the loop current of the network as shown in Fig. 1.

( NOVEMBER 2014 )


5. Write short notes on the following:(a) Dependent sources(b) Thevenin's theorem

( NOVEMBER 2014 )6. find the power consumed by 1.5 ohm resistance in the circuit Fig. 3(a), using Thevenin’

Theorem.

(NOVEMBER 2012)

7. Find the power consumed by resistance 5Ω, in the circuit Fig. 3(b).

(NOVEMBER 2012)

8. Calculate the current flowing through a 60 W lamp on a 230V supply when just switchedon at an ambient temperature of 250C. The operating temperature of the filament materialis 20000C, and its temperature coefficient of resistance is 0.005 per degree C at 0OC.

(NOVEMBER 2012)


9. Two capacitors of4µF and 6µF capacitance respectively are connected in series across apotential difference of 250V. Calculate the potential difference across each and charge onit. If the capacitors are disconnected from the supply potential difference and reconnectedin parallel with each other, with terminals of similar polarity being joined together,calculate the new p.d. and charge for each capacitor. What would happen if, in makingthe parallel connection, the connections of one of the capacitors had been reversed?

(NOVEMBER 2012)10. Show that a practical voltage source can be converted into a current source and Vice-

Versa.(NOVEMBER 2012)

11.Why energy stored in a mutual inductance can never be negative?(NOVEMBER 2012)

12. Find the phasor form of the following:(i) i(t) = 10 cos(10t + 600) + 15 cos(10t - 300) A(ii) i(t) = 460 cos(500∏t —250) — 220sin(500∏t + 150)A

(MAY 2012)13. In the circuit shown below find the voltage, V using Norton's theorem

(MAY 2012)14. A variable load RL= (0 - 25 ohms), is connected across the two batteries, which are

connected in parallel. The open circuit voltages of the two batteries are 120V and 65 V,respectively, and their internal resistances are 40Ω and 60Ω respectively. What is themaximum power that can be absorbed by the load if it is varied? Find the efficiency ofthe system.

(MAY 2012)

15. Find the power consumed/generated by each element for the circuit Figure given below.


(MAY 2012)

16. The metro-station is supplied 7.5kW electric lightening and fan load by a small dieselgenerator set during power supply failure. The efficiency of diesel engine and electricalgenerator are 50% and 90% respectively. If the set runs average 70 hours/month. The costof diesel is Rs—35 per litre and calorific value of diesel is 51, 500 kJ/hr, calculate the perKWh unit cost of electricity.

(MAY 2012)17. Two capacitors A and B having capacitances of 20µf and 30µf, respectively are

connected in series to a 600 V dc supply. Now, a third capacitor C is connected inparallel with A and it is found that the p.d. across B is 400V. Calculate the capacitance ofC and the energy stored in it.

(MAY 2012)

18. Draw V-I characteristics of the following (i) Ideal voltage and current sources (ii) Ohmicand non-ohmic elements.

(NOVEMBER 2011)19. The current in a 10 Henry inductor is shown in Fig.1. Sketch wave forms for the voltage

v(t), the instantaneous power p(t) and the energy stored w(t) as a function of time.

(NOVEMBER 2011)

20. Find currents I1 and I2 in the circuit shown in Fig.2

(NOVEMBER 2011)21. Obtain a Thevenin's equivalent at terminals a-b for the circuit shown in Fig.3


(NOVEMBER 2011)22. The filament of a 240 V metal filament lamp is to be constructed from a wire having a

diameter of 0.04 mm and a resistivity at 200 C Of 4.3 µcm. If α=0.005/0C , what lengthOf filament is necessary if the lamp is to dissipated 120 W at a filament temperatureof24200C.

(MAY 2011)

23. (i)State and prove maximum power transfer theorem for D.C. circuits,(ii) Derive the condition for maximum power transfer efficiency.(iii) Draw the graph between power and load resistance and power transfer efficiency andload resistance.

(MAY 2011)

24. Calculate the current in the 6Ω resistor of the circuit of Fig.2 by Thevenin's theorem.

(MAY 2011)

25. Using nodal voltage method, compute the power dissipated in 6Ω resistor of Fig.3.

(MAY 2011)


26. If the interconnection of sources shown in figure 1 is valid, find the total powerdeveloped in the circuit. If the interconnection is not valid, explain why.

(NNOVEMBER2010)

27. The current i0 in the circuit given in fig2 is 4A.

(a). Find i1.

(b). Find the power dissipated in each resistor.

(NOVEMBER 2010)

28. For the circuit given in fig3., Find the Thevenin’s equivalent with respect to the terminalsa, b.


(NOVEMBER 2010)

29. For the circuit given in fig 4., determine I2 using principle of superposition.

(NOVEMBER 2010)

UNIT IISingle Phase AC Circuits: Single phase emf generation, average and effective values ofsinusoids, complex representation of impedance, series and parallel circuits, concept ofphasor, phasor diagram, power factor, complex power, real power, reactive power andapparent power, resonance in series and parallel circuits, Q-factor, bandwidth and theirrelationship , half power points.

1. Enumerate advantages of using sinusoidal waveform for electric power supply.( NOVEMBER 2014 )


2. In the circuit shown in Fig. 2. What Value Of voltage (V∠θ) applied across A-B will a

current of 10 ∠00A to flow in the capacitor. Assume that the frequency of voltage applied

as 50Hz.

( NOVEMBER 2014 )3. Define series resonance in AC circuits. Draw the graphs showing variation of resistance,

inductive reactance, capacitive reactance, total impedance and current with frequency.( NOVEMBER 2014 )

Derive the relation between quality factor, Bandwidth and resonance frequency for aparallel RLC circuit.

(MAY 2012)

Derive the relation between Q-factor, bandwidth and resonance frequency for a series R-L-C circuit.

(MAY 2011)What is meant by resonance in an R-L-C circuit? Derive the expression for resonantfrequency for a parallel RLC circuit. Also discuss the similarities and dissimilaritiesbetween series and parallel resonance.

(NOVEMBER 2010)

4. A coil of resistance 40Ω and inductance of 0.75H forms part of series circuit for whichresonant frequency is 55 Hz. If the supply voltage is 250 V, 50Hz find the line current,power factor and voltage across the coil.

( NOVEMBER 2014 )


5. A coil of inductance 100 mH and resistance 800 ohm is connected with a variablecapacitor across a 10 V, 5 kHz supply. Find (a) capacitance value when supply current isminimum, (b) Quality factor, and (c) minimum supply current.

(NOVEMBER 2012)

6. For the series circuit of Fig.4(b), with the current and voltages indicated, find value of R,r, L and the frequency of applied voltage and its magnitude.

(NOVEMBER 2012)

7. Explain with the help of wave form diagrams, the concept of leading / lagging phaseangle w.r.t to sinusoidal waves. Draw the corresponding phasor diagrams also.

(NOVEMBER 2012)8. Draw the phasor diagram of the following circuit (Fig.1g) showing voltage V, I and IC .

(NOVEMBER 2012)

9. A resistance of 12 ohm, an inductance of 0.15H and a capacitance of 100µF areconnected in series across a I00V, 50 Hz supply. Calculate the impedance, current,voltage across R, L, C the pf of the circuit active and reactive power consumed.

(MAY 2012)10. An alternative voltage (80 + j 60) v is applied to a circuit and the resulting current

flowing in the circuit is (-4 + j 10) A. Find (i) Impedance and admittance of the circuit (ii)Power consumed (iii) Phase angle (iv) Resistive and reactive components. (v) Apparentpower.

(NOVEMBER 2011)


11. Two impedances Z1 and Z2 are connected in parallel. The first branch takes a leadingcurrent of 16 A and has a resistance of 6Ω, while second branch takes a lagging current atpower factor 0.8. The total power is 5 kW. The applied voltage being (100 + j 200)V.Determine : (i) Circuit Constant Of the network (ii)branch currents (iii) total current.

(NOVEMBER 2011)12. In a series RLC circuit consisting of R = 2Ω, L=1mH and C = 0.4µF. Find

(i) Resonance frequency(ii) Half power frequencies(iii) Quality factor(iv) Bandwidth(v) Amplitude of currents at ωo ω1 and ω2 if applied voltage is 20sinωt

(NOVEMBER 2011)13. For a circuit shown in Fig.4 fed from 100V, 50Hz supply. Determine(i) Current drawn from supply (ii) Power and Power factor (iii) Draw the vector diagramshowing all voltages and all current in the circuit.(iv) Determine the frequency at which thecircuit would become resonant.

(MAY 2011)14. A non-inductive resistance of 10 ohms is connected in series with an inductive coil across

200V, 50Hz ac supply. The current drawn by the series combination is 10A. Theresistance of the coil is 2 ohms. Determine

(a). Inductance of the coil

(b). power factor of the coil

(c). voltage across the coil.

Also draw the phasor diagram showing all the voltages.

(NOVEMBER 2010)

UNIT IIIThree-Phase AC Circuits: Three phase emf generation, delta and star connection, line andphase quantities, solution of three phase circuits: balanced supply and balanced load,phasor diagram, three phase power measurement by two wattmeter method.


1. Answer the following questions in reference to measurement of power using two Singlephase wattmeter in 3-Φ circuits.

a. How will you measure reactive power in three phase circuits?b. What would be the readings of two wattmeters in this experiment if load is purely

resistive?c. What would be the readings of two wattmeters in this experiment if load is purely

reactive?d. If one of the wattmeters reads zero, what would be the power factor of the load?

( NOVEMBER 2014 )What are various methods for 3 phase power measurement? Explain two wattmetersmethod with the help of suitable circuit diagram and phasor diagram. What are the effectsof power factor on the reading of wattmeters?

(MAY 2011)How the total power in a balanced 3 phase load can be measured? Discuss the twowattmeter method for power measurement for a star connected load.

A 3-phase motor load has a power factor of 0.397 lagging. The two wattmeters connectedto measure power show as 30 kW. Find the reading on each wattmeter.

(NOVEMBER 2010)

2. A balanced 3-Φ star connected load of 150 kW takes a leading current of 100 A with linevoltage of 1100 V, 50Hz. Find the constants of load.

( NOVEMBER 2014)

3. A 400V, 3-Φ, 50 Hz, power supply is applied across the three terminals of a deltaconnected three phase load. The resistance and reactance of each phase is 6Ω and 8Ωrespectively. Calculate the line current, phase current, active power, reactive power andapparent power of the circuit.

(MAY 2012)

4. A star connected 3-phase load has a resistance of 8Ω and inductance 0.0191H in eachphase. It is fed from 400V, 50 Hz 3-phase balanced supply. Determinei. Line currents in all phasesii. Power factoriii. Poweriv. Apparent powerv. If power is measured by two wattmeter method. Determine W1and W2

vi. Draw neatly phasor diagram.(NOVEMBER 2011)


5. Derive the relationship between line current and phase current in deltaconnected 3-phase circuit.

(MAY 2011)

6. A balanced 3-phase Y (star) connected load has an impedance of 4∠600Ω from line ‘a’ to

neutral. If the voltage from line ’a’ to neutral, = 20∠300

Determine

(a). current in phase ‘b’ and ‘c’

(b). voltage from line ‘b’ to neutral,

(c). The phasor expression for the voltage from line ‘a’ to line ‘c’, . The phase sequenceis abc. Also draw the phasor diagram showing the line and phase quantities.

(NOVEMBER 2010)

UNIT IVMagnetic Circuits and Transformers: Amperes circuital law, B-H curve, concept ofreluctance, flux and mmf, analogies between electrical and magnetic quantities, solution ofmagnetic circuits, hysteresis and eddy current losses, mutual inductance and dotconvention, single phase transformer- construction and principle of working, autotransformer and their applications.

1. What are the different losses in a transformer? Explain how these losses can beminimized?

( NOVEMBER 2014 )2. A 2500/250 V, 25 kVA transformer has a core loss Of 130 W and full load copper loss

of 320 W. Calculate the maximum efficiency this transformer can exhibit. Also.Determine the load kVA under this condition; consider the power factor of the load asunity.

( NOVEMBER 2014 )


3. Write short notes:Analogy between electric and magnetic circuits

( NOVEMBER 2014 )

4. A toroidal core of radius 6 cm is having 1000 turns on it. The radius of the core cross-section is 1 cm. Find the current required to establish a total magnetic flux of 0.4 mWb if(a) the core is non-Magnetic (b) the core is made of iron having relative permeability of 4000.

(NOVEMBER 2012)5. Explain why ferromagnetic materials exhibit typical B-H behavior under cyclic excitation?

(Saturation & hysteresis). Also discuss, what do you understand by magnetostriction?(NOVEMBER 2012)

6. A single phase 10 MVA, 132kV/ 220 kV, 50 Hz transformer (ideal) is connected to200kV—supply system. The secondary side of transformer is connected to a load of (300+ j400) Ω. If the number of turns on low voltage (L, V) side is 1000, find

a. Secondary side voltage;b. Number of turns on high voltage side,c. Maximum value of core flux,d. Primary (source) and secondary (load) currents,e. Power supplied by sourcef. Value of impedance seen by the sourceg. Phasor diagram taking source voltage as reference.

(NOVEMBER 2012)7. A two winding, 20 kVA, 2000/200V transformer is connected to form auto-transformer

to obtain voltage rating of (a) 2200 / 200V (b) 2000/1800 V. Find the kVA rating, VAtransferred magnetically and conductively in two cases.

(NOVEMBER 2012)8. Explain why the no load current transformer w.r.t the primary voltage lags by slightly

less than 900?(NOVEMBER 2012)

9. Discuss why the single phase induction motor is not self-starting.(NOVEMBER 2012)

10. A magnetic circuit with a tapered slug of steel supported between two H pole pieces isshown in Fig. below. The coil has 500 turns 'and µ0=4∏ x 10-7. The permeability of thecore is infinity and fringing in the air gap is neglected.

a. How much current is required in the coil to setup a flux density of 1 wb/m2, in theleft hand air-gap?

b. What is the inductance of coil?c. If there is a dc current of I amperes in the coil, what is the Net force on the steel

slug between the poles?


(MAY 2012)11. A 1000/200V, 25 k VA a transformer is connected as an autotransformer to yield a

transformation ratio of 1000/200V. Calculate its KVA rating as an auto transformer. Alsocalculate currents in the two windings and line currents when the transformer is fullyloaded.

(MAY 2012)12. Calculate the power supplied to the 1OΩ resistor in the ideal transformer shown in Fig.4

Compare electrical and magnetic quantities.(NOVEMBER 2011)

13. Efficiency of 400/200V, 200 kVA single phase transformer is 98.5% at full load at 0.8 pflagging At half load and at 0.8 pf lagging the efficiency is 97.5% calculate the values ofcoreloss and copper loss at full load.

(NOVEMBER 2011)14. Consider the circuit in Fig.5 determine coupling coefficient. Calculate the energy stored

in the coupled inductors at time t =1sec if V = 60 cos (4t + 300) volt.


(NOVEMBER 2011)15. Draw the phasor diagram of ideal transformer at no load and at load.

(MAY 2011)16. Explain the production of rotating magnetic field in poly phase system and principle of

working of 3 phase induction motor. What are various applications of 3 phase inductionmotors?

(MAY 2011)Explain the working principle of 3-Φ, induction motor. Why 1-phase induction motor isnot self- starting?

(MAY 2012)Explain the principle of operation of 3-phase induction motor.

(NOVEMBER 2011)

17. What is dot convention?(MAY 2011)

18. Magnetic circuit of cast iron having the configuration and dimensions shown in Fig.5below has cross sectional area of 0.005m2 throughout and an air gap of 0.002 m. Find themmf required to be produced by the coil in order to establish a flux of 4x10-4 wb in theair gap.length aefd = 0.25mlength (ab + cd)= 0.248 mlength (bc) = 0.002 mlength (ad) = 0.15 m

Magnetic characteristics of cast ironB 0.08 0.38 0.46H 190 1163.5 1400


(MAY 2011)19. Write a short note on Single phase induction motors

(MAY 2011)

20. What is an ideal transformer? How is a practical transformer different from ideal one?Draw and discuss the phasor diagram of a real transformer on load.

In a 50kVA 1-phase transformer the iron loss is 500 and full load copper loss is 800W.Find the efficiency of the transformer at one half of full load.

(NOVEMBER 2010)

21. A toroid is composed of three parts of different materials as shown in figure 5. The mainlengths of the flux in the core along with the relative permeability for different sectionsare given as under:

L1 = 0.15m, µr1= 1447

L2 = 0.30m µr2= 5969

L3= 0.45m µr3= 47750

It is required to establish a flux of 0.6 mWb in the core. Calculate

(a) MMF required for each section to establish the required flux.(b) The excitation current of the coil.


(NOVEMBER 2010)

22.Write short notes on Auto transformer.(NOVEMBER 2010)

UNIT VMeasuring instruments: Analog indicating instruments, PMMC ammeters and voltmeters,damping in indicating instruments, shunt and multipliers, moving iron ammeter andvoltmeters, dynamometer type instruments, multimeters, AC watt-hour meters. Digitalvoltmeters, ammeters and watt meters.

1. Explain the construction and principle of working of any two of following:(a) Permanent magnet moving coil ammeter(b) Multimeter(c) Digital Voltmeter(d) AC watt-hour meter

( NOVEMBER 2014 )Discuss advantages of digital instruments.

(NOVEMBER 2011)

Discuss the construction and principle of operation of a PMMC type instrument. Why itcan’t be used for measurement of AC currents and voltages?

(NOVEMBER 2010)

2. Write short notes:Extension of range of voltmeter and ammeter

( NOVEMBER 2014 )

3. Explain why current coil (Fixed coil) is split in two portions, in case ofelectrodynamic instruments?

(NOVEMBER 2012)


4. Discuss the working principle of digital electronic ammeter. Draw a block diagramscheme for it and bring out the functions of each block.

(MAY 2012)5. A 60 mV /120mV dual range Multimeter when used to measure the voltage across

two points in a dc circuit gives reading of 27.5 mV to 30 mV when 60 mV and 120mV ranges respectively, are employed. Assuming that meter has been correctlycalibrated, estimate the true value of the voltage existing across the two points in thedc circuit. The millivoltmeter has sensitivity of 7.5KΩ/V.

(MAY 2012)

6. Write short notes:(i) Repulsion type moving iron instruments(ii) Extension of moving coil instruments for ammeter and voltmeter(iii) Digital voltmeter.

(MAY 2011)

7. Write short notes on Moving iron type instrument

(NOVEMBER 2010)


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BASICELECTRICALENGINEERING PREVIOUSYEARQUESTIONS · 2016-11-11 · CAH–WeBELIEVEinWORK,notinWORDS. [email protected] BASICELECTRICALENGINEERING PREVIOUSYEARQUESTIONS