A dc separately excited motor has a constant field current. The armature is fed

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Question
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

Correct Answer

about 300 rpm

Explanation

Average output voltage is reduced to 50% if a = 90�. Hence speed is also reduced to 50%.

Power Electronics problems

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1. 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

3. 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

4. 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

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

6. 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

7. 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

8. 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

10. A high frequency series inverter consists of
Options
A. a number of series inverters in series
B. a number of series inverters in parallel
C. a number of parallel inverters in parallel
D. a number of parallel inverters in series

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