The class F commutation is also called as line or natural commutation because the SCR is turned off automatically during negative cycle of alternating supply. There are no commutating components used in this method therefore it is not called as forced commutation method.
The class E commutation is also called as external pulse commutation because the SCR is turned off by external source. The SCR is turned off by applying reverse voltage across it.
The class D commutation is also called as impulse commutation. The SCR T1 is main SCR whereas SCR T2 is auxiliary SCR. The auxiliary SCR T2 is used to turn off main SCR T1 therefore this method is also called as auxiliary commutation.
The commutating components L and C are connected in parallel with SCR T1 in the class B commutation. The class b commutation is also called as resonant commutation because L and C makes resonance circuit. It is also called as self-commutation because it turns off automatically and its time depends upon L and C parameter.
In the Class A commutation, the commutating components L and C are used to turn off the SCR. When the load resistance of very small value is used, the inductor L and capacitor C are connected in series with load. Similarly the load resistance of high value is used; the load resistance is connected across capacitor. The value of L and C are selected that the circuit becomes under damped. The current attains zero value at point A as shown in the figure B.
