Question

In case of a three phase full controlled converter with 6 SCRs, commutation occurs every

(a) 120°

(b) 60°

(c) 180°

(d) 30°

I got this question in an international level competition.

My query is from Three Phase Converters-2 in division Converters of Power Electronics

Answer 1

Correct answer is (b) 60°

Explanation: Every SCR conducts for 120°. This means that the SCRs from the positive group are fired 120° among themselves, same is true for SCRs from negative group. For example, if T1 starts conducting at 90° it will conduct till 90+120 = 210°. But while T1 is conducting, half of the time i.e. from 90 to 150, T6 is conducting and another half of the time T2 is conducting. Hence, commutation (change in the SCR which is conducting) takes place every 60 degrees irrespective of the firing angle. Construct the firing sequence table for better understanding.