Power Electronics II - Dec 2011

Electronics Engineering (Semester 7)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks. 1 (a) Explain how semiconverter provides better power factor compared to full converter when both are working as rectifiers with R-L load.(5 marks) 1 (b) Explain why the separately excited D. C motor is used in most of the applications where variable speed is required compared to the ordinary D. C shunt motor.(5 marks) 1 (c) Draw torque ? speed characteristics for the following control circuits for squirrel cage A.C induction motor.
i. Variable stator voltage control
ii. V/F control
Why the second control circuit is used in most of the industrial applications?(5 marks) 1 (d) Give the advantages of flyback converter compared to forward converter.(5 marks) 2 (a) Explain how full converter can be operated as
i. Rectifier
ii. Inverter
Using appropriate waveforms. What are the conditions for successful inverter operation?
(10 marks) 2 (b) Draw the circuit diagram of dual converter and explain its working. Give the relation between firing delay angle of two converters when the circuit is operating in circulating current mode. Give the advantage of this mode of operation in D.C motor control.(10 marks) 3 (a) Explain the working of any voltage commutated chopper circuit using SCR?s. Draw relevant current / voltage waveforms.(10 marks) 3 (b) A D.C separately excited motor is driven by a class A chopper circuit fed from 220V d.c supply. If motor rating is 110V, 75A, 750 RPM with R_a = 0.1? and motor speed required is 500 RPM at the half the full load torque determine duty cycle of chopper. If the on time of chopper is 1 millisecond determine the frequency of the chopper circuit. Assume that field winding is fed from fixed 110V d.c supply.(10 marks) 4 (a) Explain working of McMurray bridge inverter using circuit diagram and appropriate waveforms.(10 marks) 4 (b) Explain working of multiple pulse PWM with sinewave reference signal. Discuss the application of the circuit in A.C motor control.(10 marks) 5 (a) Explain constant torque and constant power operation of a separately excited DC motor. Give schematic diagram of control circuit.(10 marks) 5 (b) A separately excited d.c motor is driven by full converter bridge operating on 250V single phase 50 Hz supply. The motor ratings are 110V, 950 RPM, 25 Amps with R_a=0.1?.
Find the delay angle ? in the following two cases:
i. Motoring mode 700 RPM , half the full load torque
ii. Braking mode, 650 RPM , half the full load torque.
(10 marks) 6 (a) Explain using schematic diagram how slip power recovery technique can be used to control A.C motor speed above and below synchronous speed. What is the advantage of this technique?(10 marks) 6 (b) Using block diagram explain the different configurations of UPS.(10 marks)

write a short notes (any two)

7 (a) Converter types used in SMPS.(10 marks) 7 (b) Selection of battery in UPS(10 marks) 7 (c) Effect of source inductance on converter(10 marks)