# Electronics and Communication Engineering - Power Electronics

### Exercise :: Power Electronics - Section 2

6.

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

Explanation:

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

7.

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

Explanation:

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.

8.

A single phase step up cycloconverter changes 50 Hz to 100 Hz. Then one half wave of input will give rise to

 A. one half wave of output B. one full wave of output C. two full waves of output D. either (b) or (c)

Explanation:

Since output frequency is 2f, one half wave of input must equal one full wave of output.

9.

Which of these triggering methods for series connected SCRs use LASCR in the gate circuit?

 A. Resistance triggering B. Simultaneous triggering C. Sequential triggering D. Optical triggering

Explanation:

Optical triggering uses LASCR.

10.

A 3 phase ac regulator uses

 A. 3 thyristors B. 6 thyristors C. 9 thyristors D. 12 thyristors