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Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
PDDC SEMESTER VII- EXAMINATION SUMMER 2017
Subject Code: X71102 Date: 01/05/2017
Subject Name: Power Electronics
Time: 02.30PM to 05.00PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
Q.1

What is the difference between latching current and holding current? Explain the two transistor analogy of thyristor and derive the equation for anode current.
07

Explain the full wave bridge controlled rectifier (inductive load) with necessary waveform and derive the equations for average current and rms current.
07
Q.2

Describe the construction and working of IGBT and also give the advantages of IGBT over BJT and MOSFET.
07

A three-phase half-wave controlled rectifier is connected to a 230 50Hz input supply with 100Ω load. If the desired average output voltage is 50% of the maximum possible average output voltage, calculate the delay angel α.
07
OR

What is the difference between natural and force commutation? Explain class A force commutation with necessary circuit and waveforms.
07
Q.3

What do you mean by chopper? List the different chopping devices. Explain the working principal of step down chopper.
07

Explain three phase half wave controlled rectifier (resistive load) with necessary waveform.
07
OR
Q.3

Explain buck-boost convertor with necessary waveforms.
07

A step-down chopper is powered with source voltage of 100V.The load voltage waveform consist of rectangular pulses across a 1 KΩ resistor of duration 1 ms in one cycle of 2.5 ms. Calculate average output voltage, rms output voltage, ripple factor and output power of the given circuit.
07
Q.4

Why free-wheeling diode is required in inductive load? Explain speed control of dc motor using controlled rectifier.
07

Define the term inverter. Give the principle of operation of PWM inverter with necessary waveforms and circuit.
07
OR
Q.4

What do you mean by performance parameters of inverter? List and explain different performance parameters of inverter.
07

Describe the working of a single phase full bridge inverter with waveforms.
07
Q.5

Draw and explain the operation of 180º conduction mode of three phase
inverter with necessary waveform and circuit.
07

List the different heating techniques. Explain RF heating in brief.
07
OR
Q.5

Classify the different types of chopper. Explain Class B and Class D chopper.
07

Give the difference between online and offline UPS. Explain working of UPS with detailed block diagram.
07