Hi,

In Drives having Current source inverter,
when the machine is in slow speed --> forced commutation of thyristors.

When the machine is in high speed --> natural commutation of thyristors.

Why..what exactly happening in these modes...how machine stator voltage is helpful for natural commutation....i knew that at low speed, machine voltage will not be high..but how it is linked with the commutation of thyristors..?

 

At slow speed, stator currents and voltages seems like DC from the point of view of the inverter switching frequency. In this sense the inverter acts like a chopper and then force conmutation is the only way of turning off the thyristors. But when you are operating at synchronous frequency, the stator provides the appropriate voltage (magnitude and frequency) to use natural or "line" commutation of the devices.