GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER-V (Old) EXAMINATION — WINTER 2019
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Subject Code: 150903 Date: 04/12/2019
Subject Name: Power Electronics - I
Time: 10:30 AM TO 01:00 PM Total Marks: 70
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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Q.1 (a) Define and explain the need of snubber circuit. Draw such circuit for SCR and give guidelines for selecting its components. 07
(b) Explain voltage ratings of SCR. 07
Q.2 (a) What do you mean by freewheeling action? Explain How diode can provide freewheeling action? 07
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(b) State and explain the various turn-on methods for SCR. 07
OR
(b) Draw and explain two transistor analogy of a Thyristor. 07
Q.3 (a) Draw the circuit of a single phase fully controlled converter with R-L load. Derive necessary equations and sketch output waveforms. 07
(b) Explain V-I characteristic & working of Triac. 07
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OR
Q.3 (a) Using relevant circuit diagram and waveforms, explain Complimentary forced commutation (class C) method. 07
(b) For a single phase fully controlled converter with RLE load, draw the circuit diagram and waveforms of input & output voltages and currents, and voltage across SCR. Derive the mathematical expressions of output voltage. 07
Q.4 (a) Discuss the various techniques of improving power factor in phase controlled converters. Explain PWM techniques-in detail with necessary waveforms. 07
(b) Write a note on single phase dual converter showing the circuit configuration and details of operation. What are the applications of such a dual converter? 07
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OR
Q.4 (a) Explain the operation of three phase half wave controlled converter with RL load. Sketch the associated waveforms for a = 60°. 07
(b) With a neat circuit diagram and waveforms describe the Morgan’s Chopper circuit. State its applications & limitations. 07
Q.5 (a) State the principles of DC motor speed control. Explain the single phase semi-converter DC motor drive. 07
(b) Explain the working of step-down and step-up chopper. Give the comparison between step-down and step-up chopper. 07
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OR
Q.5 (a) What are the control strategies for chopper? Explain in brief. 07
(b) Discuss constant H.P and constant Torque operation of speed control of motors. Specify their field of applications. 07
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