Exercise: Power Electronics

Questions for: Power Electronics

Assertion (A): Snubber circuit has an RC series circuit in parallel with SCR

Reason (R): Voltage across a capacitor can not change instantaneously.

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

Since voltage across capacitor cannot change instantaneously it protects SCR against high .

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A thyristor has its gate current zero. To start conduction the forward voltage must exceed
A:
about 0.5 V
B:
forward breakover voltage
C:
on state voltage
D:
peak working off state forward voltage
Answer: B

With zero gate current, conduction can be started only if forward voltage exceeds breakover voltage.

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A single phase full converter gives maximum and minimum output voltage at firing angles of
A:
0 and 180° respectively
B:
180° and 0° respectively
C:
0 and 90° respectively
D:
90° and 0° respectively
Answer: A

Output voltage is maximum if a = 0 and is minimum if a = 180°.

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Present day HVDC converters are all
A:
3 pulse converters
B:
6 pulse converters
C:
12 pulse converters
D:
either 6 pulse or 12 converters
Answer: C

To reduce ripple 12 pulse converters are used.

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In a 3 phase semiconverter, the firing frequency is
A:
equal to line frequency
B:
3 times the line frequency
C:
6 times the line frequency
D:
12 times the line frequency
Answer: B

Since the circuit has 3 thyristors, firing frequency is three times the line frequency.

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