Exercise: Power Electronics

Questions for: Power Electronics

A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When a = 0, speed is 500 rpm. If a = 45°, the speed is likely to be
A:
about 350 rpm
B:
about 250 rpm
C:
about 175 rpm
D:
about 125 rpm
Answer: D

Average output voltage ∝ cos a and therefore reduced to 0.707 of the value for a = 0. Speed is nearly proportional to voltage.

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In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be
A:
400 V
B:
800 V
C:
200 V
D:
2002 V
Answer: C

In bridge connection PIV is half of that in M-2 connection.

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A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is
A:
4
B:
5
C:
6
D:
8
Answer: B

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Assertion (A): Power transistor has lower switching time than SCR

Reason (R): Power transistor has greater switching time than SCR

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: D

Power transistor does not have high rating.

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For a BJT a = 0.98, then β =
A:
0.02
B:
0.5
C:
4.9
D:
49
Answer: D

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