# T.E. (Electrical) (Semester – II),POWER ELECTRONICS,University Of Pune Question Paper,2010 Question Paper

University Of Pune Question Paper
T.E. (Electrical) (Semester – II) Examination, 2010
POWER ELECTRONICS
(2003 Course)
Time : 3 Hours Max. Marks : 100
Instructions : 1) Answer any three questions from each Section.
2) Answers to the two Sections should be written in separate
books.
3) Neat diagrams must be drawn wherever necessary.
4) Black figures to the right indicate full marks.
5) You are advised to attempt not more than six questions.
6) Use of logarithmic tables, slide rule, Mollier charts, electronic
pocket calculator and steam tables is allowed.
7) Assume suitable data, if necessary.
SECTION – I
1. a) Draw V-I characteristic of SCR. Define and explain importance of holding
current and latching current. 6
b) State the difference between line commutation and forced commutation of
SCRS. Compare class B with class D commutation method of SCR. 6
c) For an SCR, gate cathode characteristic has a straight-line slope of 130. For
trigger source voltage of 15 V and allowable gate power dissipation of
0.5 Watts, compute gate source resistance. 4
OR
2. a) With a neat diagram explain R-triggering method of turning ON SCR. Compare
R-triggering with RC triggering method. 6
b) Explain different modes of operation of TRIAC. 6
c) State applications of SCR, GTO and Triac. Compare GTO with SCR. 4
3. a) Draw symbol of MOSFET. With a neat diagram explain switching characteristics
of MOSFET. 8
b) Draw a neat circuit diagram to obtain
i) Static V-I characteristic
ii) Transfer characteristic of IGBT.
Draw these characteristics and explain. 8
OR
4. a) With the help of a neat equivalent circuit diagram explain turn-on and turn-off
process of MCT. 6
b) Compare :
i) MOSFET with BJT
ii) MOSFET with SCR. 6
c) State applications of MOSFET, IGBT and MCT. 4
5. a) Draw circuit diagram of 1-ph fully controlled SCR bridge converter with
R-L-E load. Explain its working. Draw waveforms of i) input voltage
ii) Triggering pulses iii) output voltage iv) output current. 8
b) Draw circuit diagram of 1-ph SCR semi-converter with discontinuous current.
Explain its working. Draw waveforms of i) Input voltage ii) Triggering
pulses iii) Output voltage iv) Output current. Assume R-L-E load. 8
c) Draw graph of o/p voltage v/s firing angle for 1-ph semi-converter. 2
OR
6. a) With a neat circuit diagram explain working of 1-ph SCR DUAL CONVERTER in
i) Non-circulating current mode
ii) Circulating current mode. 8
b) Derive the relation 21 180°=α+α as applied to Dual converter. Explain why
this condition should be satisfied. 4
c) What is meant by inverter operation of converter ? Draw necessary circuit
diagram and output voltage waveform. 6
SECTION – II
7. a) With a neat circuit diagram explain in detail four quadrant operation of a
b) A step-up chopper has input voltage of 220 V and output voltage of 660 V. If
the non-conducting time of SCR-chopper is 100 μs, compute the pulsewidth
of output voltage. In case pulse-width is halved for constant frequency
operation find the new output voltage. 6
OR
8. a) With neat diagram and waveforms explain principle of operation of step down
chopper. With waveforms explain effect of change in frequency and change in
duty cycle on o/p voltage. 8
b) Explain TRC, CLC and PWM control techniques of chopper. 8
9. a) With neat circuit diagram and all necessary waveforms explain in detail working
of 3 PHASE VSI in 180° mode. Draw equivalent circuits for step I and
step II. 14
b) Compare 180° mode with 120° mode of operation of 3 ph VSI. 4
OR
10. a) With neat circuit diagram and necessary waveforms explain operation of 1-ph
b) State applications of 1 ph and 3 ph VSI and CSI. 4
c) With a neat circuit diagram and waveforms explain working of 1-ph series
inverter. State its applications. 6
11. a) Explain Multiple pulse modulation in inverters. Draw necessary waveforms
and explain how four pulses per half cycle can be obtained. 8
b) Explain Sinusoidal pulse modulation in inverters. Draw waveforms and explain
how five PWM pulses per half cycle can be obtained. 8
OR
12. a) Write detailed note on :
i) dt di protection
ii) dt dv protection
iii) Snubber circuit design as applied to SCRs
. 8
b) Write a note on ‘selection of transformers and semi conductor devices for
converters’. 8
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