Exercise: Power Electronics

Questions for: Power Electronics

A static induction thyristor
A:
is a majority carrier device
B:
is a minority carrier device
C:
either a majority carrier or minority carrier device
D:
has a total number of 2 leads
Answer: B

It can be turned on by positive gate pulse and turned off by negative gate pulse. It is minority carrier device. It can be manufactured in high current and high voltage ratings and its on state voltage drops is low.

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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
A:
about 300 rpm
B:
about 150 rpm
C:
about 75 rpm
D:
about zero
Answer: A

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

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A single phase semiconverter has a freewheeling diode. If the firing angle is a and the load is purely resistive, the periods of conduction and freewheeling respectively are
A:
p - a and 0
B:
p - a and a
C:
p + a and 0
D:
p + a and a
Answer: A

Freewheeling diode conducts only when load is inductive. If load is purely resistive free wheeling diode does not come into operation.

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A DIAC has two terminals, anode and cathode.
A:
True
B:
False
C:
D:
Answer: B

They are designated as T1 and T2 because diac can conduct in both directions.

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

Especially high during switching.

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