Gate pulse duration in cycloconverter with inductive load

With an inductive load, the load current lags the voltage, so the outgoing thyristor is still conducting when the incoming thyristor is fired.

• Suppose the firing angle $\alpha$ is less than the load angle $\phi$. When the incoming SCR (say P2 or T2) is fired, the outgoing SCR is still carrying current until the extinction angle $\beta > \pi + \alpha$.
• The conducting SCR clamps the voltage, so the incoming SCR is reverse biased (or has no forward voltage) at the instant of the gate pulse.
• A short pulse ends before the outgoing SCR stops conducting, so the incoming SCR never turns on. It then waits for the next pulse, one full cycle later.
• Result: only one SCR (one group) conducts, the output becomes unsymmetrical, a DC component appears in the load and transformer, and frequency control is lost.

Remedy: use a long (wide) gate pulse lasting up to $\pi$ (or a high-frequency pulse train) so the incoming thyristor turns on as soon as the outgoing one stops and it becomes forward biased.