Question

A 3-\(\phi\) semi-converter drives a dc motor which runs at a rated speed of 1000 rpm at \( 0^\circ \) firing angle. Find the speed of the motor when firing angle is \( 60^\circ \).

• A. 866 rpm
• B. 933 rpm
• C. 1333 rpm
• D. 750 rpm

Hint:

In semi-converters, speed \( \propto \cos(\alpha) \) for a constant load.

Answer 1

The Correct Option is A

Speed is proportional to average output voltage in a semi-converter. At \( \alpha = 0^\circ \), speed = 1000 rpm At \( \alpha = 60^\circ \), voltage reduces by \( \cos(60^\circ) = 0.5 \) So, \[ \text{Speed}_{60^\circ} = 1000 \times \cos(60^\circ) = 1000 \times 0.866 = 866 \, \text{rpm} \]