IAS Mains Electrical Engineering 1992

8/22/2019 IAS Mains Electrical Engineering 1992

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(b) Using Laplace transforms find the current i1 (t) that enters the network of Fig. Q2.(b) whenthe switch is closed at t=O with zero initial current in the inductor. Nwnerical values of the

(c)

(a)

(b)

(c)

circuit elements areas shown

Synthesize the followmg dnvmg pomt ImpedanceIn Foster-! and caO msz(s)- (S+2)(S+4) ,., +- (S+l)(S+3)(S+5) '(.)

The core (radius a) of a coaxial cylindrical cable l ' : ~ by an insulating sheath ofconductivity o1 and outer radius b, and a t 6 ~ n d u c t i v i t y o2 extending to theouter metallic conductor of radius c. F i n d ~ per unit length of the cable betweenthe core and the outer conductor.Plane mono chromatic waves ~ r o e arallel to the axis in both the positive andnegative directions .At the origin, dd s !n s are given byEx= A cos (rot); Ey = 0Hx=O ; H y = A ~Calculate the mean ~ ; V radiation in each of the two directions in terms of A, Band the constants of .A parallel piat2i.o is made with a composite dielectric. A sheet of dielectric ofpermittivity S!, n s d1 is pi aced on top of a second elielectric sheet (permittivity 92thickness ~ T mbination is placed between parallel conducting plates which are

s e p a r a t { i i ~ ) { ~ t a n c e (d1+d2). What is the capacitanceper unit plate area of he capacitor?( ~ w t t of the construction of a cathode-ray osc11lo scope and explam the functions of

1111 ~ s t l t u e n t s The honzontal deflection sensitiVIty of a CRO 1s 0 02 mmN Wanwn voltage 1s applied to the honzontal plates, the spot sh1fts 4 0 mm honzontally Fmde of the unknown voltagewn m F1g Q 4 (b), an uncompensated electro -dynamic wattmeter has 1ts voltage co1l

connected directly across the load Show that the reading of the mstrument 1s approXImately(P1+ P2 P3), where P1 IS the power diSSipal lon m the voltage co1l c1rcwt, P2 IS that m the loadand P31s an error term due to the presence ofmductance 1n the voltage co1l arcu1t.r.,lp I~ i

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(a) A voltmeter has a sensitivity of I 00ON and three scales; 50 V, 150 V and 30 0 V. Whenconnected in the circuit of Fig. Q. 4(c), the voltmeter reads 4.65 Von its lowest (0-50V)scale. Calculate R..

(a)

(b)

(c)

(d)

Fig.Q 4(e) &Answer any three of the followmgA waveform produced by an electrontc orcutt ts to be attenuated 1Mth formdtstortlon (maxtmum bandwtdth) Usmg the htgh tmpedance attenuatton c cutt n m FtgQ 5 (a), draw m any addi nona! components requtred to fulfill theo qu ements anddetermme dies values of he reqwred components.II, (l)

~ ' ~ 1 I CJL ~

.fi&.Q .S.(a)

A silicon p -n junction diode is om a 0. I 0 -m p-material and 0.020-m n-material. Calculate the valu!8o tial erfor the unbiased diode at 300 K. For silicon,~ J . n = O . I 4 m N - sec. Ill'= 0.0 -s , andnt=1.4xl016/m3(Electronic charge e ~ I 0 c oltzmann' s constant K = 1.38x Io-23 JK-1)Using NAND gates, 1 the equation:K = (AB C D ) ~A 100 kW 250 g e n e r a t o r has a field circuit resistance, including a rheostat of 600and a tot atjfe circuit resistance of 0.0 650. Cal cui ate (i) field circuit current, (ii) full-load li nt [ owing to the load, (iii) armature circuit current and (iv) full-load generatedv t equired to deliver rated voltage to the load, Ignore armature reaction.

d voltage drop across resistance Ro(=8000) connected in the collector circuit is 0.5V.= 0.96, determine (i) collector-emitter voltage and (ii) base-current.A transistor in common -emitter configuration employs a 4 kO Ioad and Vee = 13 V. Estimatethe maximum peak to peak value of the input signal for linear operation if = I 00. Given:Vm, = IV and a change of I V in VBE causes a change of 5 mAin collector current.

(c) In a CE transistor amplifier, when the signal changes by 0.0 2 V. the base current changes byI 0 IJ.A and collector current changes by I mA. If the collector resistance RC = 5k0 and Ioadresistor fu. = I00, find

(a)(i) current gain (ii) input impedance, (iii) ac. load resistance, (iv) voltage gain and (v) powergam.The primary of a transformer consists of I 00 0 turns of wire and pro duces a maximum flux of0.0 3 Wb alternating at 5OHz in the iron core. The secondary winding has 40 turns. Calculate


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the r.p .m. values of primary and secondary e.m. f s on the assumption that the flux change ussinusoidal.(b) A d. c. series motor supplied from 23 0 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 10. The torque required to drivethe load varies as N12, where N is the speed. What should be applied voltage to the motor ifthe speed is to be raised to 1750 r.p.m.? Assume that the field is unsaturated.(c) Calculate the sui ab!e 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 withrated applied voltage is 5 times the full-load current and full-load slip is 5%. Fin the(a)

(b)

(c)

starting torque in terms of the full-! oad torque, when the auto-transformer is emplFind the percentage voltage drop for a transformer with a percentage resistanc f 2 d apercentage reactance of 4%, of rating 550 kVA, when it is delivering 40 0.6 p.f.lagging.A 3300 V, 24 pole, 50 Hz 3-phase star-connected induction mo r 1p-ring rotorresistance of 0.0 16 0 and stand-still reactance of 0. 265 0 per phas Cal late (i) speed atobtained at 247 r.p.m. +A 3-phase synchronous motor, when working on coG t - e mains at constant Ioad,takes a current which varies with the excitation. Exp n t result. How is the power factoraffected? ~

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(30)(b) Discuss the disadvantages ofharmonics and suggest methods for reducing a. c. harmonics in aconverter 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.(I 0)

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

(b) Describe with a neat di agrarn a static version of theWordLonard system o

(c)

(a)(b)(c)(d)

d.c. drive.

The a.c. supply voltage is 250 volt. The motor voltarafdian/min. For a fimg angle oc = 30 and rated m o t o r ! : . l : ! f l ' ! ~ . . 1 ; ~motor and (ii) motor torque, assuming continuous ee armature current.any formula used.

(25)

With V/f speed con ase induction motor the maximum torque and slip at whichmaximum t o r q u spectively, increases and decreased with increase in frequency.The higher the f machine, the heavier is the rotor of he machine.For econ ic oper on, the generator with highest positive incremental transmission losswill o te t thMowest positive incremental cost ofproduction.

~ .... ~ f n ' transmission line the string efficiency is 70%. Now if d. c. voltage is supplied- setup, the string efficiency will be 90%,~ ~ ~ principle of operation and applications of an induction generator.

(I 0)Describe with neat sketches the construction and principle of operation of a fast breederreactor.

(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 ts rated frequency.Determine (i) the maximum torque at the reduced value of supply as the a fraction of itsnormal value (ii) the starting torque at the reduced value of supply as a fraction of its normalvalue.(20)


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(d) A synchronous motor is connected to an infinite bus and draws I.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, oand P. If with the same excitation the loading on the motor is increased by I0%, determinethe new values ofE,Q,I, o, P and cos .

(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

(15)Explain with the he! p of Flowchart the procedure for solution of Coordination Equations.

(I 0)Explain with neat diagrams the pro cess of current chopping in circuit b r e a k e r s ~ " st asui table method to minimise its effect on power systems.(I 0)

Explain the carrier system of protection in power systems. With a blo ~ d neatsketches discuss how the phase comparison scheme can be used foo tin feeder fedfrom (i) one end and (ii) both the ends.Differentiate between a current transformer and a power transform. ss various errorsassociated with a current transformer and the methods to ~ e th'+e errors.

(15)Explain clearly 'Pinch effect' and 'Skin effect'. Comreth:Ger ormance ofAjax Wyatt andcareless mductlon furnaces State the apph a t l o ~ ~ of furnaces

(15)What ts regenserattve braking? Explam rn t ,p: of (1) d c shunt motor (u) d c senesmotor and (111) mductlon motor when ~ ~ r e g e n e r a t i v e braking

(15)A motor works on 2 mm ~ c ons uted as follows (1) 0 to 15 sec-load nsmg from 0to 1050kW, (u) 15 to 85 se co ad of 600 kW, (111) 85 to 95 sec-regenerattve brakmgwtth power retumedi.f!ll g o ;hy from 200 to 0 kW, (tv) 95 to 20 sec-motor ts at restDeternune the contln u rati f the motor, assunung the rating to depend upon (I ) r m svalue of loadmg and value of loading~ . . (20)

eff""ed by a locomotive wlulehauhng a tram on level track at 50 kmph tsns. ~ locomotive has to haul the same train on a gradient and the tractiveis 55000 newtons, determine the power required by the locomotive when the.. .""',. (i) d. c. series motors (ii) induction motors.

(I 0)

SECnONCQuestion No.9 is compulsory

9. Comment with justification on the correctness of any three of the following statements restrictingyour 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 micro strip compared with stri pline is that microstrip is more likely toradiate.


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(d) A parametric amplifier is used for lownoise high frequency amplificalion.(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.

(c)

(d)

(a)

(b)

(c)

(d)

(a)

(b)

Under what conditions is equivalent noise temperature a more useful quantity than noisefigure?DofiM md dooaibo pol. ,poOti" modWtii.,. E,.ta, wi w w o O O ~ M ~rom pulse width modulation. Compare the types of modulation. 0 (15)Differentiate between telex and telegraphy. Describe briefly the systu nes used intelegraphy.

(15)Explam wtth the help of neat dtagrams the pnnctple or ~ ~ ~ two-Ca111ty Klystronamphfier "-/r ~ (20)Descnbewtth a neat eliagram the pnnctple of ~ p h c a t t o n of Gunn-effect dtode

v ~ (15)ofSurface Acoust:l c Wave devtces

(15)D"ro"md o m p ~ ~ ~ fWruog " ' "" " " " " (IO)Derive the imp edan property of a quarter w < r > ~ e transformer and explain some ofits a p p l i c a t i o n s . ~ ' ( .1 (15)couple +

(I 0)~ h you would measure coupling factor and directivity for a two-hole directional

(c) .. n h w you would measure the common mode rejection ratio of a differential amplifier.(I 0)

time scaling:(25)

d2x dx2 dt 2 +30 dt + 200x= 0x(O) = 5x(O) = 9.

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IAS Mains Electrical Engineering 1992