Question

In the circuit shown below, SCR T1 conducts first. If T1 is fired at an angle of α > 30°, then T1 would conduct from

(a) α to 180°

(b) 30 + α to 180°

(c) 30 + α to 150°

(d) 30 + α to 120°

I have been asked this question in my homework.

The question is from Three Phase Converters-1 topic in chapter Converters of Power Electronics

Answer 1

Right answer is (b) 30 + α to 180°

The explanation: When firing angle is more than 30°, T1 would conduct from 30 + α to 180°. Irrespective of the firing angle, T1 will be turned on at 180° because it conducts first which means it is connected to the R phase and the phase sequence is R-Y-B. As R starts at 0° its value is 0 at 180° which reverse biases the SCR T1.