VARDHAMAN COLLEGE OF ENGINEERING
(AUTONOMOUS)
B. Tech VI Semester Regular Examinations, May 2018
(Regulations: VCE-R15)
POWER ELECTRONICS
(Electrical and Electronics Engineering)
Date: 17 May, 2018 FN
Time: 3 hours
Max Marks: 75
Answer ONE question from each Unit
All Questions Carry Equal Marks
Unit I
1.

Discuss switching characteristics of power MOSFETS.
7M

Explain two transistor analogy of a thyristor. Derive an equation for anode current. Explain function of gate in a thyristor.
8M
2.

What is meant by commutation of SCR? How is commutation methods classified? Explain any one method in detail.
7M

Three thyristors are connected in series to form a string. The string must withstand an of state voltage of 8KV.The static equalizing resistance is 20KΩ. The dynamic equalizing circuit has Rc=25Ω and C=0.1μF. The leakage current of the thyristors are 22mA, 20mA and 18mA respectively. Find:
i. Voltage across each SCR during off state
ii. Discharge current through each SCR at the time of turn on
8M
Unit II
3.

With a neat circuit diagram and relevant waveforms, explain the working of a single-phase fully controlled bridge rectifier with R load.
7M

Derive an expression for the average load voltage and current for a single phase fully controlled converter with source inductance.
8M
4.

With a neat circuit diagram and relevant waveforms, explain the working of a single-phase fully controlled bridge rectifier with RLE load.
7M

A single-phase half-wave controlled rectifier is operated from 230V, 50Hz supply. A resistive load of 10 ohm is connected to the rectifier. If the average output voltage is 75% of the maximum possible average output voltage, calculate:
i. Delay angle of the thyristor
ii. rms and average values of the output voltage
iii. rms and average values of the thyristor current
8M
Unit III
5.

Derive an expression for average current through a load in a single phase full wave AC voltage controller with R-L load.
7M

With a neat circuit diagram and relevant waveforms, explain the working of a three-phase fully controlled bridge rectifier with R load for continuous conduction mode.
8M
6.

Derive an expression for rms and average value of output voltage for a single phase full wave AC voltage controller with R load.
7M

A three phase six pulse converter is operated from a three-phase star connected 440V, 50Hz supply. The load is purely resistive with R 15Ω. If the average voltage is 50% of maximum possible average output voltage, calculate:
i. Firing angle
ii. rms and average values of output current
iii. rms and average values of thyristor current
8M
Cont…2
2
Unit IV
7.

With a neat circuit explain single-phase cycloconverter with resistive loads.
7M

Explain difference between AC voltage controller and cycloconverter.
8M
8.

Which commutation technique is used in Morgan chopper? With a neat circuit discuss its disadvantages.
8M

A step-down dc chopper has a resistive load of R=15Ω and input voltage =240V. When the chopper remains ON, its voltage drop is 1.5V. The chopper frequency is 1.2kHz. If the duty cycle is determine:
i. Average output voltage
ii. rms output voltage
iii. Chopper efficiency
7M
Unit V
9.

With a neat circuit and associated waveforms explain operation of series inverter.
7M

A series inverter has R=80Ω, L=8mH and C=1.2μF. Check whether the circuit will work as a series inverter. Find the maximum output frequency.
8M
10.

What is a PWM inverter? What are the various techniques to control output voltage of inverter?
7M

Draw the diagram of parallel inverter and discuss its modes of operation.
8M