# JNTU Kakinada B-Tech 3-1 RT31025 I POWER ELECTRONICS R13 April 2018 Question Paper

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Code No: RT31025
III B. Tech I Semester Supplementary Examinations, May -2018
POWER ELECTRONICS (Electrical and Electronics Engineering)Time: 3 hours Max. Marks: 70
Note: 1. Question Paper consists of two parts (Part-A and Part-B) 2. Answering the question in Part-A is compulsory
3. Answer any THREE Questions from Part-B
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PART ?A
1 a) How the secondary breakdown occurs in Power BJT? Show it on I-V characteristics
of Power BJT.
[3M]
b) How the freewheeling diode effects the performance of single phase half wave
converters?
[3M]
c) Explain why the firing angle is restricted in single phase fully controlled converter
[4 M]
d) What is six pulse converter? Write its advantages. [4M]
e) What are the control strategies used in DC-DC converters? [4M]
f) What are the methods used for control the output voltage of inverter? [4M]
PART ?B
2 a) Describe the turn-on methods of SCR. [4M]
b) Explain the dynamic characteristics of power IGBT. [8M]
c) What is the importance of snubber circuit in protection of SCR? [4M]
3 a) Explain the operation of half wave converter with RL load and freewheeling diode
and also reduce the expression for average load current.
[8M]
b) A single phase full-wave ac voltage controller feeds a load of R=30 ? with an input
voltage of 230V, 50Hz. Firing angle for both the thyristors is 65?. Calculate (i) rms
value of output voltage. (iii) Average and rms current of thyristors.
[8M]
4 a) A single phase full converter, connected from 230 V, 50 Hz source, is feeding a load
R= 25 ? in series with a large inductance that makes the load current ripple free. For a
firing angle 30?, calculate the input and output performance parameters of this
converter.
[8M]
b) Explain the operation of single phase semiconverter feeding RLE load with neat
circuit diagram and waveforms also deduce the rms output voltage.
[8 M]
5 a) Explain the effect of source inductance on three phase full converter in detail. [8 M]
b) Design a three phase dual converter to achieve at four quadrant operation for I
d
= 10 A
at 200 V. The converter is supplied from 400 V, three phase and 50 Hz supply. I
ripple
=
2A.
[8 M]
6 a) Discuss the working of a single phase bridge type cycloconverter with RL loads and
for continuous waveform operation with neat circuit diagram and output rms voltage
and current wave form for fo = 4 fs.
[8 M]
b) Explain the operation of Boost chopper with relevant waveforms and derive the
expression for average output voltage.
[8 M]
7 a) Explain the operation of unipolar switching in full bridge inverter. [6 M]
b) Explain the operation of three phase bridge inverter for 120? mode of operation with
aid of relevant phase and line voltage waveforms.
[10M]
*****
R13
SET – 1

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