CLASS:

CLASS: M.Sc. ELECTRONICS

14A / 377St. JOSEPHS COLLEGE (AUTONOMOUS) TIRUCHIRAPPALLI 620 002

SEMESTER EXAMINATIONS APRIL 2014TIME: 3 Hrs.

MAXIMUM MARKS: 100

SEMSETPAPER CODETITLE OF THE PAPER

IV201212PEL4114POWER ELECTRONICS

SECTION A

Answer all the questions:

20 x 1 = 20

Choose the correct answer:

1.A freewheeling diode across inductive load will provide _______.

a)quick turn on b)slow turn off

c)reduced utilization factor d)improved power factor

2.An IGBT has three terminals called _______.

a)Collector, emitter and baseb)Drain, Source and base

c)Drain, Source and gate d)collector, emitter and gate

3.The firing circuit using magnetic amplifiers may be designed to supply controlled firing pulses either to a controlled rectifier.

a)single phase b)multiphase

c)single phase or multiphase d)None

4.The _______ method of SCR firing is quite elegant and relatively simple.

a)Firing circuit b)phase locked loop

c)VJTd)Timing wave multiplexing

5.In _______ chopper, both the average output voltage and current can be only positive.

a)class Ab)class B

c)class Cd)class D

6.A chopper can be used on _______.

a)Pulse width modulation only

b)Frequency modulation only

c)Amplitude modulation only

d)Both PWM & FM

7.A single phase CSI with capacitor c as the load. For a constant source current, the voltage across the capacitor is _______.

a)square wave b)Triangular wave

c)sine wave d)pulsed wave

8.Simplest method of eliminating third harmonic from the output voltage waveform of a single-phase bridge inverter is to use _______.

a)inverters in series b)single pulse modulation

c)stepped wave inverters d)multiple-pulse modulation

9.The converter cannot operate in the _______ conversion mode because the power cant flow from the motor to the ac source.

a)synchronous b)Asynchronous

c)both a & bd)pole face

10.In DC motor _______ windings are provided to help commutation.

a)series b)parallel

c)interpole d)pole face

Fill in the blanks:

11.Rectification ratio is also known as _______.

12.The solid-state devices are most useful when _______ operated in a _______ mode.

13.A chopper is a _______ converter.

14.A PWM switching scheme is used in single phase inverters to _______.

15.The DC motor is operated in _______ systems with the shunt field separately.

State True or False:16.The harmonic factor (HF) is equal to the rms value of all the harmonics divided by the rms value of fundamental component of the I/P current.

17.Firing circuits are usually connected from line to neutral for fullwave circuit and line to line for halfwave circuits.

18.Single SCR chopper circuit is simple and used for high power output.

19.Stepdown cycloconverter works on natural commutation.

20.In single-phase SCR drive, the motor current is also positive because the SCR does not allow any reverse current to flow.

SECTION BAnswer all the questions:

5 x 4 = 2021.a.Define the following terms.(i)Rectification ratio

(ii) Crest factor

(iii) Form factor

(iv) Transformer utilization factor

OR

b.With neat diagram, explain the operation of single-phase halfwave with RL load.

22.a.Describe the functions of firing circuits using logic gates.

OR

b.Write a brief note on voltage reset control.

23.a.Explain the principle of operation of DC chopper.

OR

b.Write a brief note on Type a chopper.

24.a.Describe the working of a single phase half-bridge inverter.

OR

b.Write a brief note on current source inverter.

25.a.Describe the functions of three phase half-controlled bridge for motor drive in SCR.

OR

b.Write a brief note on synchronous motor control.

SECTION C

Answer any FOUR questions:

4 x 15 = 6026.(a)What is an IGBT? Describe the working of an IGBT. Explain

switching characteristics of an IGBT.(b)A single phase 230v, 1kw heater is connected across 1-phase 230v, 50Hz supply through an SCR. For firing angle delays of 45 and 90. Calculate the power absorbed in heater element.

27.With neat diagram, explain, how the single phase circuit is controlled by MMF-Reset?

28.With neat diagram, explain the operation of class B and class C quadrant chopper.

29.(i)What is a cycloconverter? Enumerate some of its industrial applications.(ii) Describe the operating principle of single-phase to single phase stepup cycloconverter with the help of mid-point and bridge type configuration.

30.With neat diagram explain the operation of class B and class C quadrant chopper.

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