Natural and Forced Commutation in SCRs

Commutation in dc machines refers to the changing of current flow from one circuit to another.  In SCRs ( Silicon controlled Rectifiers) and thyristors, it refers switching off a conducting electronic component. 

In AC circuits, SCRs are commutated by the negative half cycle which reverse biases the anode and cathode terminals.  This is known as natural commutation.

However, in DC circuits, special circuits should be designed to switch off the SCRs once, they have been switched on.  The current is reduced to zero by means of external circuits.  This is known as Forced Commutation