Concept of commutation

Even as we recognize that a thyristor could be switched on by triggering gate terminal with low voltage short duration pulse. But after switching in, it shall conduct constant before the thyristor is biased that is reverse the stress current falls to zero.

This conduction that is continuous of factors issues in a applications which can be few. The task ideal for switching straight down a thyristor is known as as commutation.

The thyristor mode that is running changed from ahead performing mode to ahead blocking mode by the commutation procedure. Therefore, the thyristor commutation practices or thyristor commutation strategies are used to turn off.

The commutation techniques of thyristors are classified into two types:

1. Natural Commutation

2. Forced Commutation

Natural Vs Forced communication



Natural commutation

Forced commutation


Requires AC voltage at input

Requires DC voltage at input


External components are not required.

External components are required.


Used in controlled rectifiers, AC voltage controller

Used in choppers, inverters etc.


SCR turns off due to negative supply voltage.

SCR turns off due to current & voltage both


No Power loss takes place during commutation

Power loss takes place during commutation


Zero cost

Significant cost