A 3 phase ac regulator uses 3 thyristors 6 thyristors 9 thyristors 12 thyristors

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Correct Answer

6 thyristors

Explanation

Two thyristors for each phase.

Power Electronics problems

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1. In a single phase full wave ac regulator the rms value of load current is 20 A load is purely resistive. Then
Options
A. average thyristor current will be less than 20 A but rms thyristor current will be more than 20 A
B. both the average and rms thyristor currents will be less than 20 A
C. both the average and rms thyristor currents will be more than 20 A
D. rms thyristor current will be 20 A and average thyristor current will be 10 A

2. An RC snubber circuit is used to protect a thyristor against :
Options
A. false triggering
B. failure to turn on
C. switching transients
D. failure to commutate

3. In a series inverter supplying a load resistance R, the commutating elements L and C should be such that
Options
A.
B.
C.
D.

4. Which of these triggering methods for series connected SCRs use LASCR in the gate circuit?
Options
A. Resistance triggering
B. Simultaneous triggering
C. Sequential triggering
D. Optical triggering

5. A 750 A circuit uses thyristor of current rating 175 A. If derating is 25%, the number of thyristors in parallel is
Options
A. 3
B. 4
C. 6
D. 10

6. HVDC transmission systems generally use
Options
A. 12 pulse converters
B. 6 pulse converters
C. 3 pulse converters
D. either 12 or 6 pulse converters

7. A dc separately excited motor has a constant field current. The armature is fed from a single phase supply through a semi-converter. When a = 0, speed is 600 rpm. If a = 90�, the speed is likely to be
Options
A. about 300 rpm
B. about 150 rpm
C. about 75 rpm
D. about zero

8. Assertion (A): An inverter does not require forced commutation
Reason (R): A series inverter is a forced commutation inverter.
Options
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

9. A single phase series converter is used for
Options
A. high voltage output
B. high current output
C. high voltage and high current output
D. improving the circuit efficiency

10. The average load current supplied by a thyristor depends on
Options
A. firing angle
B. firing frequency
C. magnitude of gate current
D. all of the above

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