Lingaya’s
University
M.Tech.
1st Year (Term - II) (Part Time)
Examination
– Feb 2011
Power
Electronics Devices and DC Converter (EL - 505)
[Time: 3 Hours]
[Max. Marks: 100]
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Before
answering the question, candidate should ensure that they have been supplied
the correct and complete question paper. No complaint in this regard, will be
entertained after examination.
 |
Note:
– All questions carry equal marks. Attempt five questions in all. Question no.
1 is compulsory. Select two questions from Section B and two questions from
Section C.
Section – A
Q-1. Answer the following questions:
(i) What is the difference between latching current and holding current of a thyristor. [2]
(ii) What is the effect of leakage reactance in a rectifier circuit? [4]
(iii) Describe transient characteristics of a thyristor. [2]
(iv) What is UJT oscillator triggering? [2]
(v) What is self commutation by an L-C circuit? [2]
(vi) What role free wheeling diode performs in controlled rectifier? [2]
(vii) What are the factors that affect the choice of converter circuit? [2]
(viii) What is Morgan chopper? [4]
Section – B
Q-2. Discuss pulse transformer trigger circuit. Draw its equivalent circuit. What are two functional modes, write and derive equations for e, e0, eT.
A relaxation oscillator using an UJT is to be designed for triggering an SCR.
The UJT has the following data:
h=0.72, IP=0.6mA, VP=18.0V, VV=1.0V, IV=2.5mA, RBB=5kW, normal leakage current with emitter open = 4.2mA.
The firing frequency is 2 KHz. For c=0.04mF, compute the values of R,R1 and R2. [20]
Q-3. Discuss series and parallel operations of thyristors. Write equations for string efficiency and derating. A 3KV, 750A power electronics circuit has thyristors with 800V and 175A rating. Use a derating of 25%. Find number of thyristors in series and parallel. [20]
Q-4. Discuss single-phase Half-wave circuit with RL Load and free wheeling diode (controller rectifier). Derive equations for i0 in conduction mode and free wheeling mode. Draw voltage and current waveforms. [20]
Q-5. Explain resonant – pulse commutation. Draw and discuss waveform. For a constant load, current, derive equations for
(a) Conduction time for auxiliary thyristor
(b) Voltage across the main thyristor when it gets commutated
(c) The circuit turn-off time for the main thyristor. [20]
Q-6. What is a multi-phase chopper? Draw current waveform and bring out clearly the difference between the in-phase and phase-shifted operation of a multiphase chopper. What is Jones chopper? [20]
Q-7. Compare the performance of three-phase rectifier with single phase rectifier in terms of magnitude of average output voltage and ripple content. What are the various filter circuits? [20]
Q-8. What is the difference between current commutated chopper and voltage commuted chopper? Describe AC line commutation. Discuss the performance of SCR with different loads. [20]
h=0.72, IP=0.6mA, VP=18.0V, VV=1.0V, IV=2.5mA, RBB=5kW, normal leakage current with emitter open = 4.2mA.
The firing frequency is 2 KHz. For c=0.04mF, compute the values of R,R1 and R2. [20]
Q-3. Discuss series and parallel operations of thyristors. Write equations for string efficiency and derating. A 3KV, 750A power electronics circuit has thyristors with 800V and 175A rating. Use a derating of 25%. Find number of thyristors in series and parallel. [20]
Q-4. Discuss single-phase Half-wave circuit with RL Load and free wheeling diode (controller rectifier). Derive equations for i0 in conduction mode and free wheeling mode. Draw voltage and current waveforms. [20]
Section – C
Q-5. Explain resonant – pulse commutation. Draw and discuss waveform. For a constant load, current, derive equations for
(a) Conduction time for auxiliary thyristor
(b) Voltage across the main thyristor when it gets commutated
(c) The circuit turn-off time for the main thyristor. [20]
Q-6. What is a multi-phase chopper? Draw current waveform and bring out clearly the difference between the in-phase and phase-shifted operation of a multiphase chopper. What is Jones chopper? [20]
Q-7. Compare the performance of three-phase rectifier with single phase rectifier in terms of magnitude of average output voltage and ripple content. What are the various filter circuits? [20]
Q-8. What is the difference between current commutated chopper and voltage commuted chopper? Describe AC line commutation. Discuss the performance of SCR with different loads. [20]
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