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).

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

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 5s s

z ss 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 (050V) scale. Calculate Rz .

5. Answer any three of the following:

(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 = 1.6x1019C; Boltzmanns constant K = 1.38x1023 JK1)

(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.fs 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 0.05KG 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 21 2 1 4

2 10K s s

G ss s s

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

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

(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 0d x dx xdt dt

x(0) = 5

x(0) = 9.

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.

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

SSEECCTTIIOONN AA

1. Answer any three of the following:

(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 Thevenins theorem. Determine Thevenins equivalent circuit which may be

used to represent the network shown in Fig. Q. 2(a) at the terminals CD.

CC..SS..EE..--((MMaaiinnss))--11999933

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

(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 = 2ej 0.1 Z y, 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

(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 103 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 = 104 S hre = 103; 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 1031 kg

Electronic charge, e = 1.6 x 1019 C

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

Boltzmann constant, k = 1.38 x 1023 J/K

Permittivity of free space, 0 = 8.85 x 1012 F/m Permeability of free space, 0 = 4 x 107 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. 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 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,

CC..SS..EE..--((MMaaiinnss))--11999933

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

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)

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

SSEECCTTIIOONN AA

1. Answer any three of the following:

(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

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

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 cdvdt

and Ldvdt

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 e2t

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 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

5. Answer any three of the following:

(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 JFETs 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 Mdt

.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 BJTs 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.

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.

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. 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:

(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, fs

E s

. Find fs

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

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

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 4KG 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

VZI

, 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 X11 = 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)

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

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

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 (et + e3t) amps; i2 = 0.1 (3e3t et) 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

20 0

15exp /r zr zD i i c mx 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 103 x 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 300C and 500C 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 1019 C

Permeability of free space : o = 4 x 107 H/m Permittivity of free space : o = 8.85 x 1012 F/m

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

KG s

s 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 1s

As 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

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

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 12 2

2 0 01 1 1

x xu t

x x

12

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 SCRs.

(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 106 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 rsistance 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 107 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)

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

1. Answer any three of the following:

(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

raB i bi Tr

Find the magnitude and direction of torque on the conductor.

(20)

(c) Using Thevenins 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

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

(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 (25C) as 1000 , design a Wheatstone bridge with an OP-AMP arrangement to provide a 0 to 10 V signal for a temperature span of 25C to 125C.

(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)

SECTION B 5. Answer any three of the following:

(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

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, 4pole, 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, rbe = 1 k, rbb = 90 , Cbe = 20 pF and Cbe = 3 pF while RE = 1 k.

(c) Design a combinational logic circuit to generate the 2s 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 1019 C

Permeability of free space : o = 4 x 107 H/m Permittivity of free space : o = 8.85 x 1012 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) 104 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 106 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 23 5 74 5 18 29 35s sG s

s s s s

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

EELLEECCTTRRIICCAALL EENNGGIINNEEEERRIINNGG

(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 5KG 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 105 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

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 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

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 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 2 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 di