Exercise: Power Electronics

Questions for: Power Electronics

A chopper
A:
converts constant voltage dc into ac and then into variable voltage dc
B:
converts constant voltage dc into variable voltage dc directly
C:
converts ac of one frequency into ac of another frequency
D:
converts ac to dc
Answer: B

A chopper is a dc-dc converter. It converts a fixed dc voltage into a variable dc voltage.

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The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by
A:
forward characteristics of thyristors
B:
reverse recovery characteristics of thyristors
C:
both forward and reverse recovery characteristics of thyristors
D:
none of the above
Answer: B

Value of C is proportional to maximum difference between reverse charge of SCRs in series.

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Assertion (A): Inverters and choppers use fast switching thyristors

Reason (R): Fast switching SCR has low turn off time.

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
Answer: A

Low turn off time enables fast switching.

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Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.
A:
True
B:
False
C:
D:
Answer: B

They are suitable for both dc and ac circuit breakers.

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In a step down chopper using pulse width modulation, Ton = 3 x 10-3 and Toff = 1 x 10-3 s. The chopping frequency is
A:
333.33
B:
250
C:
500
D:
1000
Answer: B

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