IAS Mains Electrical Engineering 1993

7/27/2019 IAS Mains Electrical Engineering 1993

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(b)

(c)

3. (a)

(b)

(c)

4. (a)

(b)

l.toig. Q.2(a)

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

i(t)=LOA and d v d ~ t )=I 0 VI sec. Find C.

r ~1; : : ~ : ~ : . ~p ~ & i : ~ < = M o

O M ' ' " ' ' 0r o ~o~ o0 S H md

resistance 20 0 and the other, capacitance C in sen es w t t h~ t a n c e etermme C and Rso that the combined impedance shall be independent o ct and have zero phaseangle.

A uniform plane wave is described by the phasor H t .~ ay, Aim. If the velocity of thewave is 2 x 108 mls and the relative permeab r z i . ; ! ~ ~t e r m i n ethe frequency, relativepermittivity, wavelength and electric intensity.

A point charge q is situated at a d i s t a n c~ o grounded conducting plane of infiniteextent. Obtain the total charge induced ane by direct integration of the surface chargedensity.

Two parallel conducting plates od by the distance d and maintained at the potentialdifference ll.U. A d i e l e c t r i c~ electric constant K and of uniform thickness t


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6.

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

(c) Discuss the advantages of a junction field-effect transistor over a conventional bi-polartransistor. Calculate the value of drain load resistance, fu. required in the circuit shown in Fig.Q.5(c) at audio frequencies to obtain a voltage gain o f 60. Th e PET used has g , = 5 x 10 - 3 Sand rds = I 00 kO.

(d) What are th e functions of a commutator m a d c machme? Explat , V ' - methods ofo btam effect:l ve commutation

(a)gatn ~ ~Another transt stor T 3 has current gatn ~3 Estabhs r n e t w e e nh an d Io

~ CJ&'(j

(b) A transistor used m common- CE) configuration ha s the following se t of hp arameters when th e d c op9 ! f tI S Vu = I 0 V and Ic = I mA

h n = I 0 -3

0

h . = 2000 0 , ~If the a c 'load ~ } th e tranststortsrL = 6 00 0 , d e t e~ ' pu t impedance, current gain and voltage gain.

(c) In a single p p l~ i s t a b l emul ivibrator, emitter biasing resistance RE = I 00 0 , RL = I, 0 00 0.

Rt = I 2 = 50 kO, and Vee = 18 V. Find (i) the output voltages Et and ~ and (ii) the

;

d ase currents for each transistor. Make necessa ry assumptions an d state them.

7. ( a )~ n how a 2 -winding transformer can be converted into an autotransformer. Two sing! e-as ransformers T 1 and T 2 rated at 250 kV A each are operated in parallel on both sides.

entage impedan ces forT 1 and T 2 are ( I + j6 ) and (1.2 + j4.8 ) respectively. Calculate th e

load sharedby

eachwhen the totallo

adis

5 00kV

Aat

0. 8 p .f. lagging.

8.

Explain the principle of operation o f a polyphase induction motor. A 4-p ole, 50 Hz polyphaseinduction motor ha s a full-load speed o f 1440 r.p .m. Es timate the full-load slip.

(c) Two transformers connected in open delta supply a 40 0 WA b a! anced load operating at 0. 86 6p.f. lagging. The load voltage is 440 V. Calculate th e values of kV A and kW supplied byeach transformer.

(a) A 4 -pole, 50 Hz, 3-phase inductio n motor has motor resistance and standstill rotor reactanceof 0.04 0 and 0.16 0 pe r phase respectively. Calculate th e value of the external rotorresistance pe r phase to be inserted to obtain 70 % of maximum torque at starting.


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(b) Two identical 30 kW, 500 V, 1200 r.p.m. d.c. series motors A and 13 are co!Ulected in seriesaero ss a 5 00 V d. c. sup pi y. They drive a common load through reduction gearing of a: I andb : I respectively. When the load shaft has a speed of 120 r.p.m., motor A has 225 V across itand runs at I 0 80 r.p .m. Find for this condition the speed of the second motor, the currenttaken from the mains and total I oad torque on the shaft. Neglect all losses and saturationeffects.

(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 I x I o- 31 kg

Electronic charge, e = I. 6 x 10- 19 C

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

Boltzmann constant, k = 1.3 8 x 10- 23 J/K

Permittivity of free space, sa = 8.8 5 x 10- 12 F/m

Permeability of free space, ~ o= 4 1t x I o- 1 H/m

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Time Allowed: Three Hours Maximum Marks: 300Candidates should attempt five questions only stiecting at l e a ~two Questions including th e

compulsory question from each of Section A an d B or Section A an d C.Asmme suitable data i f considered necessary and indicate th e same clearly. ~The number of marks carried by each question is indicated at the md of th e quemon.

SEC110NA 0Qu2stion 1 iS compulsory 6I Select any three of the followmg statements, read them carefully and entl the correct and

mcorrect ones Justtfy your answer usmg not more than 200 words m each cas

2.

~ .. + (20x3=60)(a) The performance of a contort system is affected bu ~ e r i s t i c sof the measuringelement.(b) In a system which uses a gear train to coup! e otor, the equivalent moment of

(c)

(d)

(a)

(b)

inertia referred to the motor shaft and t h~ I J t e load shaft is independent of gearratio. ~In the operation of a three-phase full r ~ e r both in the inverter-mode and convertermode, the direction of current and ty voltage must rem am same at different pomts ofthe ctrcwt

Enumerate the m8J or ties good adJustabl e-sp eed dnve system

soltd-state de111 ces m 2 P s .- ed motor dnves

!.">... (10)A half-wave r e~ r c u t tusmg thyn stors feeds a d c sen es motor from a three-phases o u~c e + (!Ox2=20)

w e ctrcwt eli agram and the waveforms showmg motor voltage and armaturec nts for finng (delay) angles 0 f o and 9 o

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

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

(20)

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

(I 0)

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,


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4.

One amplifier,

Gear train having gear ratio of I I 0.

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

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

(a) Draw a diagram showing th e scheme of connection to form a p o sitiona! servomechanismwith the above components, indicating the load on th e motor.

(b) Write down the equations describing th e operation of (i) th e error detect eamplifier.

(I 0)

(c) Obtain the transfer function relating armature voltage and angular d.

(d)

(e)

(a)

(b)

(c)

shaft.

(15)

In connection wtth d c m o~ ~ la m what IS a chopper and how 1t controls th e speedofthednve

~ " ' ( F(15)

G1ve the s1mpl8 i agrarn for a chopper controlled drive scheme usmg a d c senesmotor and sk \ !h oltage and current waveforms Define "pu lse rat1o' and "pu lsef r e q u e n~ x x p l+ w the hm1ts of current pui s al l on are detemuned

~ . , (30)

SECnONB

5 ent W l th JUsllficalion on th e correctness or otherwtse of any three of the followtng statements,ctmg your answer to wtthtn 2 00 words for each

(20x3=60)

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

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

(c) Protection techniques have failed to devise means for protection of alternators against intertu m faults.


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8. (a)

(b)

(c)

(15)

Explain the functions of:

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

(20)

Draw the speed-time curve for the following traction service on level track without any freerunning period, and estimate the maximum power inp ut to the traction motors:

The train having a dead weight of 400 tonnes accelerates for 30 seconds attaining a maximumspeed of 70 km per hour. It takes 2 0 sees to bring it at dead stop, the brake being atthe speed of 55 km ph when the coasting ends. Before starting again it is all owe eck r2 0 sees. Assume the train resistance to be 4 0 Nw/tonne and all ow I 0% of de werotational inertia. ~Given: efficiency of the transmission gear is 9 0% and efficiency of o ~

(20)

Give brie f answers to the following:

(1) What ts the data used m load-flow studtes? (u) What s~e y + swmg Bus? (111) Howts an A-C Calculatmg Board set up? (tv) What m o o >n obtatned from load-flowstudies?

f1j (20)~ N C

Qu2stion 9 is compulsory.

9.

10.

Select any three of the followmg stf t j an dtcate wtth JUst:lftcat:lon whether they are corrector wrong Your answer must not ex e 0 rth for each statement

(20x3=60)

(a) There wtll be no h b lem m an ampltfiertf 1 s mput can be made harmorucs free

(b) The charactens 0 ennas, whether of resonant type or of non -resonant type, arepractically same.

(c) In designi c h o~ e r sfor d.c. amplifiers it is necessary to ensure proper damping.

(d) Fillers used in a Visual M adulator, a component of TV transmitter.

account of the sampltng theorem

(I 0)

Describe the Noise-generator Method for the measurement of Noise Figure. Explain why thismethod 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 impedancefor co -axial lines, and for parallel-wire lines in terms of line dimensions, as well as for ausual radio-frequency line in terms ofline parameters.

(20)


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II.

12.

(a) What is a Radar? Draw a block diagram showing the components on which radar systems arebased. Explain function of radar as a navigational aid.

(25)

(b) A standing wave indicator used with a square law detector the wave guides having internaldimensions of width 3.75 em and depth I. 25 em (the electrical field being parallel to depth)shows adjacent minima at a distance of 2. 5 em ho m each other.

(c)

(a)

(b)

(c)

(d)

Assuming that the guide walls are perfectly conducting calculate (i) the free spacewavelength of the sin gal in the wave guide and (ii) the reflection co-efficient and n o~ a l i z e dload impedance when the I oad impedance is 75 + j 0 ohm and the guide impedance . ~ +j 23.7ohm. '

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IAS Mains Electrical Engineering 1993

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