Power Electronics II - Dec 2012

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) Discuss the effect of source inductance on the performance of a single phase fully controlled converter.(5 marks) 1 (b) Explain continuous conduction mode and discontinuous conduction mode of class A chopper.(5 marks) 1 (c) Compare between current source and voltage source inverters (5 marks) 1 (d) List the advantages offered by the D.C chopper drives over the line commutated converter controlled D.C drives.(5 marks) 2 (a) Explain sinusoidal pulse width modulation as used in PWM inverters.(10 marks) 2 (b) A capacitor commutated single phase bridge inverter is operated at 50 Hz with load resistance of 5Ω. Thyristor turn off time is 62μs.
Determine :
1. Commutating capacitor C for successful commutation of SCR.
2. Load current I_L
3. F critical for reliable commutation.
4. R critical.
(10 marks) 3 (a) Draw and explain the operation of single phase capacitor commutated current source inverter with resistive load. Also draw the voltage and current waveforms.(10 marks) 3 (b) What are the disadvantages of Harmonics present in the inverter circuit. List the methods present in harmonic reduction of inverters and explain any one method in detail.(10 marks) 4 (a) Draw and explain the operation of a speed control of a D.C series motor by a single phase semiconverter for a continuous motor current. Draw the corresponding voltage and current waveforms.(10 marks) 4 (b) A D.C shunt motor takes a current of 80A on a 480V supply and runs at 960 Rpm, the armature resistance is 0.25Ω and the field resistance is 120 Ω. A chopper is used to control the speed of the motor in the range of 400 -750 rpm having constant torque. The on period of the chopper is 3ms. The field is supplied directly from 480V supply. Determine the range of frequencies for the chopper.(10 marks) 5 (a) Explain the operation of induction motor for two different cases when fed with a current source inverter.
(i) Operation at and below rated frequency
(ii) Operation above rated frequency
(1 marks) 5 (b) Exlain the stator voltage control scheme of induction Motor. Also draw and explain the speed ? torque curves.(10 marks) 6 (a) Describe the operation of On-line UPS system with the help of neat block diagram.(10 marks) 6 (b) With the help of neat circuit diagram explain the working of SMPS.(10 marks)

Write a short notes

7 (a) Dual converter (7 marks) 7 (b) McMurray inverters(7 marks) 7 (c) Series inverter(7 marks)