AMIETE – ET (OLD SCHEME)

Code: AE26                                                                             Subject: POWER ELECTRONICS

Time: 3 Hours                                                                                                     Max. Marks: 100

NOTE: There are 9 Questions in all.

· Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

· Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1       Choose the correct or the best alternative in the following:                                  (210)

a. GTO can be turned off by

(A)  Applying a negative gate signal.    

                   (B)  Applying a positive gate signal.

(C)   Reducing the current below holding current.

(D) Both (A) and (C).

             b.   DIACs are primarily used as

(A)  Power thyristors.                         (B)  Triggering devices.

(C)  Surge protection devices.              (D)  Both (B) and (C).

             c.   Latching current of thyristor is associated with:

(A)  Turn on process.                         (B)  turn-off process.

(C)  Turn-on and turn off process.       (D)  None of these.           

             d.   A dual converter used for the speed control of dc motors, will have two bridges, they are

(A) Two rectifiers.                              (B) Two inverters.

(C)  One rectifier and one inverter.       (D)            None of above.

             e.   A free wheeling diode is used in a controlled rectifier circuit in case of

(A) Resistive load.                              (B)  Inductive load.

(C) Capacitive load.                            (D)  None of above.

             f.    When energy is returned to the system by a motor, it is referred as

(A) Plugging.                                      (B) Dynamic braking.

(C) Regenerative braking.                    (D) Reciprocating.

g.    If the duty cycle of a chopper circuit is exactly 50%, the pulse is considered to be a:

(A)  sine wave.                                    (B)   low duty cycle.

(C)  high duty cycle.                             (D)  square wave.

             h.   In a Single pulse width modulation technique the dominant harmonic is:

(A) 3^rd.                                               (B)  5^th.

(C)  11^th.                                             (D)  2^nd.

             i.    Three phase to three phase cycloconverter gives better output as compared to single phase to single phase cycloconverter.

(A)   True                                            (B)  False

             j.    The main difference between thyristor and PUT is that

(A)  The gate of PUT is connected in n-type material near cathode.

(B)  The gate of PUT is connected in n-type material near anode.

(C)  PUT has no gate.

(D) PUT is used for firing of thyristor.

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   Explain the methods of turning on and turning off the thyristor.                             (8)

             b.   Write short notes on:-

                   (i)  Cooling of thyristor.

                   (ii) Power MOSFET.                                                                                         (8)

Q.3     a.   Explain Extinction angle control for the power factor improvement for three phase dual converter.                                                              (8)

             b.   Discuss effect of impedance on

                   (i)  Single phase

                   (ii) Three phase converter operation.                                                                  (8)

  Q.4     a.   Classify and compare different types of switched mode regulators.                    (10)

b.   A step up chopper has supply voltage of 250 V while the output voltage is 500 V. If the off period of chopper be 100 msec, determine the pulse width of the output voltage.                               (6)

Q.5     a.   What is the difference between forced and natural commutation? Describe an impulse commutation circuit.                                                             (10)

     b.   Discuss di/dt and dv/dt feature of a thyristor.                                                       (6)

  Q.6     a. List and draw the various configurations of three phase ac voltage regulators.                      (8)

             b.   Discuss the operation of a single phase ac voltage controller for an inductive load.               (8)

Q.7     a.   What are cycloconverters? Explain single phase to single phase cycloconverter with relevant diagrams.                                                           (8)

b.   Write short notes on:-   

                   (i)  Reduction of harmonics in output.    

                   (ii) Circulating current mode operation of cycloconverter.                                    (8)

Q.8     a.   State and explain different methods of voltage control in single phase inverters.                    (8)

             b.   A 3 phase bridge inverter is fed from a 400 V battery. The load is star connected and has a resistance of 10 ohms per phase. Find the rms load current, power output, peak current of thyristor. Assume 180^o mode of operation.                                                                                                          (8)

Q.9     a.   Draw and explain in brief the circuits for single phase and three phase converter drives.                   (8)

             b.   A 230V, 1500 rpm separately excited dc motor has an armature resistance of 1Ω and rated armature current of 10 A. It is fed from 230 V single phase AC, 50 Hz through a fully controlled bridge converter. Compute speed if torque is 5 N-m and firing angle is 30^o.                                   (8)