Gujarat Technological University 2009 Diploma Electrical Engineering (DEE) Power electronics - Question Paper
GUJARAT TECHNOLOGICAL UNIVERSITY
Seat No.
B.E. Sem-Vth Examination December 2010 Subject code: 150903 Subject Name: Power Electronics-I
Date: 16 /12 /2010 Time: 03.00 pm - 05.30 pm
Instructions: Total Marks: 70
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
Q.1 (a) Describe construction and working of a SCR. Analyze its performance using 07 two transistor Analogy. Derive its expression for its anode current in terms of current gain P and leakage Current ICo
(b) What is snubber circuit? Why is it needed? Draw such circuit for a SCR and 07 give guidelines for selecting its components.
Q.2 (a) Explain the working of UJT relaxation oscillator circuit. Derive the expression for 07 frequency of triggering and firing angle delay in terms of eta, charging resistance etc.
(b) Design a UJT relaxation oscillator using UJT 2N2646, for triggering an SCR. 07 The UJT has following characteristics.
n = 0.7, Ip =50|iA, Vv=2V, Iv=6mA, VBB=20V, RBB=7KQ, IEO=2mA, also determine the limits for the output frequency of the oscillator.
OR
(b) List Forced Turnoff methods for SCR and Explain how an auxiliary SCR and a 07 capacitor can be used to turn-off conducting SCR? Draw Necessary waveforms.
Q.3 (a) Draw the circuit of a single phase fully controlled converter with R-L load. Derive 07 necessary equations and sketch output waveforms.
(b) A single phase full wave rectifier has a R-L load having L= 6.5 mH, R=0.5Q and 07 E=10V.The input voltage is Vs=120 V at (rms) 60Hz supply .Determine (a) The load current ILo at wt = a = 60, (b) The average Thyrister current IA, (c) The rms Thyrister current IR (d) the rms output current Irms (e) the average current Idc and (f) the critical delay angle ac.
OR
Q.3 (a) With a neat circuit diagram and wave forms describe the Morgans Chopper circuit 07 states its applications & limitations..
(i) Buck convector (ii) Cuk converter
Q.4 (a) Draw the neat circuit diagram of a Jones chopper controlling the speed of a D.C series 07 motor. Explain its working with the help of various wave forms. Obtain expression for (i) capacitor voltage (ii) Toff period (iii) Relation between battery voltage & capacitor voltage (iv) Value of Capacitor.
(b) Describe basic chopper circuit and explain various methods of load voltage 07 variations by chopper.
OR
Q.4 (a) With neat schematic Block diagram describe the Micro-computer control of 4 07 quadrant DC drives.
Specify their field of applications.
Q.5 (a) Describe the uses of Pulse Transformer and Opto-Coupler in the Gate triggering 07 circuits of SCRS with necessary sketches.
converters. Explain PWM techniques in detail with necessary sketch and waveforms.
OR
Q.5 (a) Describe IGBT-construction and working characteristic with neat sketches. 07
(b) What are the different methods of braking DC motor? Explain regenerative braking 07 scheme with schematic diagram.
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| Earning: Approval pending. |
