CS MAINS
ELECTRICAL ENGINEERING (PAPER-I) 2014
(Please read each of the following instructions carefully before attempting questions)
QUESTION PAPER SPECIFIC INSTRUCTIONS
There are EIGHT questions divided in two Sections and printed both in HINDI and
in ENGLISH.
Candidate has to attempt FIVE questions in all.
Question Nos. 1 and 5 are compulsory and out of the remaining, THREE are to be attempted
choosing at least ONE question from each Section.
The number of marks carried by a question/part is indicated against it.
Answers must be written in the medium authorized in the Admission Certificate which must
be stated clearly on the cover of this Question-cum-Answer Booklet in the space
provided. No marks will be given for answers written in medium other than the authorized
one.
Assume suitable data, if considered necessary, and indicate the same clearly.
Unless otherwise mentioned, symbols and notations have their usual standard meanings.
Attempts of questions shall be counted in chronological order. Unless struck off, attempt of a
question shall be counted even if attempted partly. Any page or portion of the page left blank
in the Question-cum-Answer Booklet must be clearly struck off.
Time Allowed Three Hours IMaximum Marks 250

SECTION-A
1. Find the values of E and I in the circuit shown in Fig. 1. <img src='./qimages/268-1a.jpg'>
Starting from Maxwell's equations <img src='./qimages/268-1b.jpg'>
Determine the inverse z-transform of <img src='./qimages/268-1c.jpg'>
A comparator and limiter circuit is shown in Fig. 2. Develop transfer characteristics V0 vs.V1 and explain. <img src='./qimages/268-1d.jpg'>
A three-phase balanced delta-connected load gives wattmeter readings of 1050 W and 550 when the two-wattmeter method is applied. Calculate the impedance in each arm of the load. Given, line voltage 200 V. 10

2. (a)The net power flowing out of a given volume u is equal to the time rate of decrease in the energy stored within u minus the conduction losses. Derive equations to explain. 20
delta-connected balanced three-phase load is supplied from a three-phase, 400 V supply. The line current is 20 A and power taken by the load is 10000 W. Find impedance in each branch and the line current, power factor and power consumed if the same load is connected in star. 20
(c)Determine whether the following system is linear, time invariant, stable or dynamic: 5 10

3. Find the Laplace transform of the signal -3t -2t and find ROC. 20
Derive Helmholtz equation in terms of magnetic field.20
The z-parameters of a two-port network are Z11= 20 z22= 30 z12= z21= 10 Q. Find y and ABCD parameters of the network. 10

4. (i)Why the output voltage of an Op-Amp does not respond immediately to the fast-changing input? Develop the expression for the slew rate of Op-Amp. (iii)An open-loop configuration of Op-Amp 741C changes its output from +14 V to -14 Veach time its input crosses zero volt. If 741C has a typical slew rate of 0·5 VIlls, calculate the time between the zero crossings and the maximum frequency at which the output will be distorted. 20
Obtain the Norton's equivalent circuit at the terminals A and B for the network shown in Fig. 3. 20 <img src='./qimages/268-4b.jpg'>
Determine whether the following signal is periodic or aperiodic. Find the fundamental period, if it is periodic: =sin(3pie/7n+pie/4) cos pie/3 n 10

SECTION-B
5. A single-phase fully controlled bridge converter supplies an inductive load. Assuming that the output current is virtually constant and is equal to 20 determine the following per(ormance measures, if the supply voltage is 230 V and the firing angle is maintained at 30degrees 10
Average output voltage
Supply r.m.s. current
Supply fundamental r.m.s. current
Fundamental power factor
Supply power factor
(vi)Supply harmonic factor
Voltage ripple factor
(viii) Reactive power input

Prove that the torque produced in a shunt motor is proportional to armature current I a and in case of series motor, it is proportional to 10

single-phase converter feeds an R-L load having a resistance of 10 ohms in series with an inductance of 20 mHo The converter operates such that the d.c. voltage across the load is 250 V. The thyristor used in the converter has holding current of 300 rnA and a delay time of 5 j.lS. Determine the pulse width of gate current. 10

A random variable has an exponential probability function given by =ae-b1xl where a and b are constants. Find the relationship between a and and fii) the distribution function of x. 10

(e)Design a circuit that gives the input-output relationship shown in Fig. 4. <img src='./qimages/268-5e.jpg'> 10

A d.c. shunt motor takes a current of 80 A on a 480 V supply and runs at 960 r.p.m. The armature resistance is 0·25 ohm and the field resistance is 120 ohms. A chopper is used to control the speed of the motor in the range of 400-750 r.p.m. having constant torque. The on-period of the chopper is 3 ms. The field is supplied directly from 480 V supply. Detennine the range of frequencies of the chopper. 20


10010010
11011000
Find complement of the following numbers:
10010010
11011000

Convert the logical equation +CAl into product-of-sums fonn and design circuits using OR and AND gates and NOR gates.
Explain the function of Schottky diode connected between the collector and base of transistors of TTL gate. 20

550 55 kVA, single-phase alternator has effective resistance of 0·2 Q. A field current of 10 A produces an armature current of 200 A on short circuit and an e.m.f. of 450 V on open circuit. Calculate the synchronous reactance and the full-load regulation with power factor 0·8 lagging. 10

Describe all-day efficiency of a transformer. 10

The two random variables X and Y are independent and identically distributed each with a Gaussian density function with mean equal to zero and variance equal to 2. If these two random variables denote the coordinates of a point in the plane, find the probability density function of the magnitude and the phase of that in polar coordinates. 20

Describe the voltage-commutated chopper controlling the speed of a d.c. series motor with relevant schematic diagram and associated voltage and current waveforms as a function of time. 20

Discuss the effect of excitation on armature current for a synchronous motor and compare the performance of 31jl synchronous motor with 31jl induction motor. 20

Explain the method of frequency demodulation using PLL. 20
A three-phase, four-pole, 50 Hz induction motor has a chopper-controlled resistance in the rotor circuit for speed control. Load torque is 00 2 . When the thyristor is ON, the torque is 30 N-m at a slip average 0·03. If TON TOFF I compute the average torque and speed. The motor develops a torque of 80 percent of ON torque when the thyristor is OFF. The speed variation ranges ,down to 1200 r.p.m. from synchronous speed. Determine the ratio TON to give TOFF an average torque of 25 N-m. 15