Question Paper: Power Electronics : Question Paper Dec 2015 - Electronics and Comm. Engg. (Semester 7) | Gujarat Technological University (GTU)
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## Power Electronics - Dec 2015

### Electronics and Comm. Engg. (Semester 7)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS
(1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.

### Answer the following in brief

1(a)i What are the requirements to make the SCR on? Explain the various Turn On methods in brief.(4 marks) 1(a)ii Make the comparison between DIAC and TRIAC.(4 marks)

1(b)i A 200 A SCR is to be used in parallel with 150 A SCR. The on state voltage drops of the SCRs are 2 and 1.8 V, respectively. Calculate the series resistance that should be connected with each SCR if two SCRs have to share the total current 250 A in proportion to their rating(3 marks) 1(b)ii Explain the snubber circuits for SCR.(3 marks) 2(a) Explain the operation of single phase semi controlled bridge converter with resistive and inductive load. Draw the associated waveforms.(7 marks)

### Solve any one question from Q.2(b) & Q.2(c)

2(b) Calculate the average output voltage of a three phase half controlled bridge operating with triggering angle π/2 and connected to three phase ac supply of 400V and 50 Hz. The load current is assumed to be continuous.(7 marks) 2(c) A single phase half wave converter is used to supply power to a load of impedance 10 Ω from 230 V, 50 Hz ac supply at the firing angle of 30°. Calculate average load voltage, load current and RMS load voltage.(7 marks)

### Solve any one question from Q.3 & Q.4

3(a) With help of neat circuit diagram and waveforms, explain the operation of three phase bridge inverter with resistive load in 120° conduction mode.(7 marks) 3(b) With help of neat circuit diagram and waveforms describe the working of a two quadrant chopper. List out its applications.(7 marks) 4(a) Explain the single phase PWM, transformer connection, multiple commutation and filter methods used for reduction of harmonics in inverter output.(7 marks) 4(b) Compare the step up and step down chopper.(7 marks)

### Solve any one question from Q.5 & Q.6

5(a) What is commutation? With help of circuit diagram and necessary waveform explain the complementary (Class C) forced commutation.(7 marks) 5(b) Explain the construction and characteristics of UJT and working of UJT as an oscillator for generation of triggering pulses.(7 marks) 6(a) With help of circuit diagram explain the half wave and full wave RC firing circuits of SCR.(7 marks) 6(b) Draw and explain the structure of power MOSFET. Also explain principle of operation and I-V characteristics of power MOSFET.(7 marks)

### Solve any one question from Q.7 & Q.8

7(a) Write short note on isolation circuits and pulse transformer.(7 marks) 7(b) Write short note on On-line UPS.(7 marks) 8(a) Write short note on Boost converter.(7 marks) 8(b) Write short notes on RF heating.(7 marks)