### Lingaya’s University - Power Electronics Devices and DC Converter - Feb 2011 Question Paper

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] 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

(i) What is the difference between latching current and holding current of a thyristor. 

(ii) What is the effect of leakage reactance in a rectifier circuit? 

(iii) Describe transient characteristics of a thyristor. 

(iv) What is UJT oscillator triggering? 

(v) What is self commutation by an L-C circuit? 

(vi) What role free wheeling diode performs in controlled rectifier? 

(vii) What are the factors that affect the choice of converter circuit? 

(viii) What is Morgan chopper? 

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. 

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. 

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. 

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. 

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? 

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? 

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.