A Constant Turn-Off Time Control for Variable Frequency Thyristor Inverters

A new type of control system is described for thyristor inverters that are used to supply high-frequency power loads where the exact value of the frequency is not critical, such as for induction beating. Commutation is provided by a parallel LC tank circuit which is operated above its resonant frequency. The amount of reactive power is controlled by adjusting the actual operating frequency of the inverter. The control system is designed to maintain a constant turn-off time for each commutation every half-cycle, independent of the frequency. Also, it has a very rapid response to changes in the load conditions, thereby decreasing the chance of a commutation failure and reducing the need for additional safety margin.