C S MAIN 2015 ELECTRICAL ENGINEERING Paper—I

SECTION—A

Q. For the circuit shown in Figure, evaluate the current through and the voltage across each element 10

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Q. A voltage of a wave travelling in a non-magnetic medium is given by the equation

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Find
Er II
the time-average power carried by the wave. 10

Q. Derive the expressions for instantaneous and average power in a single-phase ac circuit. 10

Q. In the Figure shown what is the function of and determine its value when

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Q. Find the efficiency of a class A amplifier shown in Figure. Given that the voltage drop across Re is 1 the signal current swings from -0.5 A to +0.5 A arid the DC current is 1 A.

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Q. A 8-pole, 240 V lap-wound, series motor has armature and series field resistances of 0.2 ohm and 0.02 ohm respectively. There are 660 armature conductors. If the flux per pole is 0.03 Wb and the torque developed in the armature is 320 find the current taken by the motor and its speed. 20

Q. A single-phase full converter is connected to RLE load. The source voltage is 230 50 Hz. The average load current of 10 A is constant over the working range. For 0.4 Q and L 2 mH, compute

firing angle delay for E 120 V and input power factor.
firing angle delay for E -120 V and input power factor. 20

Q. Find the z transform of
sin wt for t 0.

Q. Find the Laplace transform of the function

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Q. A distortionless line has zQ 60 a 20 mNp/m, u 0.6 c where c is the velocity of the wave in vacuum. Find C and y(lamda) at 100 MHz. 20

Q. A two-port network has Z11 Z12 Z21 Z22 200 Q (Oms). Two such networks are connected in cascade. What are the overall z-parameters of the composite network 10

Q. 4(a)Explain the speed enhancement, gained in MOS device by using silicon-on-sapphire or spinel (SOS). What are the other good features of the process Mention the drawbacks. 20

Q. In the network shown in Figure, determine the value of current through 1 Q resistance connected between terminals A and B. Verify the answer using superposition theorem also. 20

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Q. The transfer function of a system is given as

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Find out the impulse response of the system. 10

SECTION—B

Q. A single-phase full converter is supplied from 230 50 Hz source as shown in Figure. The load consists of R 10 Q and a large inductance so as to render the load current constant. For firing delay angle of determine
average output voltage
average output current
average and rms values of thyristor current
input power factor. 10

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Q. A battery is charged through a resistor R as shown in Fig. For an ac source voltage of 230 50 Hz find the value of average charging current for R 8 Q and E 150 V. 10

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Q. Prove that mechanical power developed by a self excited DC shunt motor is maximum when back e.m.f. is equal to half the applied voltage, if the field loss is neglected. 10

Q. Consider an incoming narrow-band signal of bandwidth 10 kHz, and mid frequency that may lie in the range of 0.535 to 1.605 MHz. It is desired to translate this signal to a fixed frequency band centered at 0.455 MHz. Determine the range of tuning that must be provided in the local oscillator to obtain the frequency band centered at 0.455 MHz. 10

Q. Obtain all the possible minimal functions for
F E 7). 10

Q. A 440 V dc shunt motor has a no-load speed of 2000 rpm. It is running at 1000 rpm at full load torque, reduced armature voltage and full field current. If load torque is reduced to 50% of rated value with armature voltage and field voltage held constant at previous values, the speed increases to 1050 rpm. Find the armature voltage drop at full load.
Neglect the effect of armature reaction. 20


Q. Divide by N counter is shown in the figure below. If initially Q0 Q1 Q2 what is the value of N 20

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Q. Draw the circuit for the speed control of a separately-excited dc motor from a singlephase source and explain it. 10

Q. Tlie efficiency of a 20 kVA, 2500/250 single-phase transformer at unity power factor is 98% both at rated load a nd half rated load. Determine
the core loss and ohmic losses and
the p.u. value of the equivalent resistance of the transformer. 20

Q. Discuss phase and frequency modulation. Explain, why in practice phase modulation is not favoured. 20

Q. An RLE load is operating in chopper circuit from a 500 V dc source as shown in Figure. For R L 0.064 and constant the duty cycle is 0.2. Find the chopping frequency to limit the amplitude of load current excursion to 10 A. 10

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Q. A 3-phase, 12 pole alternator is coupled to an engine running at 500 r.p.m. The alternator supplies power to an induction motor which has a fuli-load speed of 1455 r.p.m. Find the slip and number of poles of the motor. 20

Q. Evaluate the maximum and minimum values of of the amplitude-modulated voltage signal defined as

10(1 +0.5 cos wt 0.2 cos 2 wt) cos w0t. 20


Q. A separately excited dc motor, operating from a single-phase half controlled bridge (semiconverter) at a speed of 1440 rpm, has an input voltage of 330 sin 314 t and back emf 80 V. The SCRs are fired symmetrically at a 30° in every half cycle and the armature has a resistance of 4 Q(oms). Calculate the average armature current and the motor torque. 10