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JE – 1081

*JE1081*

II Semester M.E. (Power Electronics) Degree Examination, July/August 2013 2K8 PE 213 : ELECTRIC DRIVES Time : 3 Hours

Max. Marks : 100

Instructions: 1) Answer any five full questions. 2) Any missing data may be assumed suitably. 1. a) With the help of a block diagram, explain the major components of an electric drive.

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b) A three-phase full converter supplying a d.c. motor has the data : line voltage VL = 220 V (rms), K eλp = 10 Wb, R a = 0 .2Ω, and neglect the ripples in the motor current. Calculate the output average voltage for α = 30°, determine the motor speed for Ia = 50 A. Draw the waveform of output voltage for α = 30 ° . (5+5+5) 2. a) With the help of a circuit diagram, explain the principle of operation of a four quadrant chopper drive of a d.c. motor using MOSFETs.

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b) A d.c. chopper is used in rheostatic braking of a d.c. separately excited motor. The armature resistance is Ra = 0.05 Ω . The braking resistor Rb = 5 Ω . The back emf constant is K v = 1.527 V/A-rad/sec. The average armature current is maintained constant at Ia = 150 A. The armature current is continuous and has negligible ripple. The field current is continuous and has negligible ripple. The field current is If = 1.5 A. If the duty cycle of the chopper is 40%, determine : i) The average voltage across the chopper, Vch ; ii) The power dissipated in the braking resistor, Pg ; iii) The equivalent load resistance of the motor acting as a generator, Req; iv) The motor speed, w; and v) The peak chopper voltage, Vp.

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*JE1081*

JE – 1081

3. a) With the help of a block diagram, explain the closed loop speed control of W( s) separately excited DC motor and obtain the transfer function V (s ) . r b) A 60 hp, 230 V, 1750 rpm separately excited DC motor fed from a chopper, is controlled by a proportional type speed controller. The field current is maintained constant at 1.25 A and the machine back emf constant is 0.81 V/A-rad/sec. The armature resistance is 0.02 Ω and the viscous friction constant is 0.3 N.m/rad/sec. The amplification of speed sensor is 96 mV/rad/sec. The gain of the power controller is 150. i) Determine the reference voltage to drive the motor at rated speed. ii) If the reference voltage is kept unchanged, determine the speed when motor develops the rated torque.

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4. a) Explain the stator voltage control of 3 phase induction motor. What are its limitations ? 10 b) A 2.8 KW, 400 V, 50 Hz, 4 pole, 1410 rpm, delta connected squirrel cage induction motor has the following parameters referred to the stator : Rs = 2 Ω , Xs = Xr = 5 Ω , Rr = 5 Ω , Xm = 80 Ω . The motor speed is controlled by stator voltage control. When driving a fan load it runs at rated speed at rated voltage. Calculate motor terminal voltage, current and torque at 1200 rpm. 10 5. a) Explain V/f control of 3 phase induction motor. Compare its performance with stator voltage control.

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b) Explain the sine-triangle comparison method used in PWM single phase inverter. Draw the waveforms for mf = 9.

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6. a) With the help of circuit diagram and waveforms, explain the operation of a 3-phase bridge inverter for its 120° mode operation. Draw the phase and line voltages for a Y connected balanced load. 10 b) Explain the field oriented control of a 3 phase induction motor with relevant diagram and equations. 10 7. a) With a neat circuit diagram explain static Kramer drive.

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b) Explain the construction and principle of operation of brushless d.c. motor with necessary diagram. 10 _______________