# RTMNU B.E ECE Power Electronics 2011 Question Paper

RTM Nagpur University Question Paper
Faculty of Engineering and Technology
B.E ECE Examination
Power Electronics
Time: Three Hours
Maximum Marks: 80
Question Paper Year: 2011

Section A
1. (a) Draw and explain dynamic characteristics of a thyristor during its turn on and turn off processes. Show the variation of voltage across the thyristor and current through it during these dynamic processes. Indicate clearly the various intervals into which turn-on and turn-off times can be subdivided.
(b) Explain the characteristics of the following devices:
(i) Silicon unilateral switch.
(ii) Triac.
(iii) GTO.
2. (a) What do you mean by commutation of SCR? What are the different methods of forced commutation? Explain class C commutation technique.
(b) Explain the working of PUT relaxation oscillator. What is the advantage of PUT relaxation oscillator over UJT relaxation oscillator?
3. (a) A single phase full wave diode bridge rectifier is supplied with 230V, AC supply. Resistive load of 60O is connected across output of rectifier. Calculate DC and RMS power consumed by load. Calculate peak current flowing through load.
(b) Discuss the effect of source inductance on the performance of a single phase fully controlled converter indicating clearly the conductance of various SCR during one cycle.
4. (a) Explain the operation of three phase bridge rectifier supplying lightly inductive load with suitable waveforms.
(b) What are the purposes of filters in rectifier circuits? Differentiate between a.c. and d.c. filters.
5. Write short notes on:
(i) di/dt rating of SCR.
(b) Integral cycle control.
(c) Thermal runaway.

Section B
6. (a) Show that for the mid-point single phase converter, the average reduction in load voltage due to commutation is directly proportional to load current. Draw near output voltage and output current waveform.
(b) A single phase converter employs two SCR and centre tapped transformer with turn ratio2:1. The load is inductive (R=10O and L=1mH) and delay angle is 40°. Determine the average load voltage, average load current. Supply voltage is 230V at 50Hz. The load current can be assumed to be ripple free.
7. How are the drawbacks of basic series inverter overcome? Explain in detail the working of modified improved series inverter.
(b) What are the various techniques for voltage and harmonic control in inverter? Explain single pulse width modulation scheme in brief.
8. What is the different control techniques used in chopper? Explain.
(b) Draw and explain the operation of four quadrant chopper.
9. (a) Explain the operation of 3-phase bridge inverter in 180° conduction mode. Draw output phase and line voltage waveforms.
(b) Explain sinusoidal PWM as used in PWM inverter.
10. Write short notes on any Two:
(a) UPS.
(b) Jones Chopper.
(c) AC voltage regulator.