TURN-OFF METHODS

The reverse voltage which causes to commutate the SCR is called commutation voltage. Depending on the commutation voltage located, the commutation methods are classified into two major types.

1) Forced commutation and 2) Natural commutation.

• In natural commutation, the source of commutation voltage is the supply source itself. If the SCR is connected to an AC supply, at every end of the positive half cycle the anode current goes through the natural current zero and also immediately a reverse voltage is applied across the SCR. These are the conditions to turn OFF the SCR.

• This method of commutation is also called as source commutation, or line commutation, or class F commutation. This commutation is possible with line commutated inverters, controlled rectifiers, cyclo converters and AC voltage regulators because the supply is the AC source in all these converters

• In case of DC circuits, there is no natural current zero to turn OFF the SCR. In such circuits, forward current must be forced to zero with an external circuit to commutate the SCR hence named as forced commutation.

• This commutating circuit consist of components like inductors and capacitors called as commutating components. These commutating components cause to apply a reverse voltage across the SCR that immediately bring the current in the SCR to zero.
• Based on the manner in which the zero current achieved and arrangement of the commutating components, forced commutation is classified into different types such as class A, B, C, D, and E. This commutation is mainly used in chopper and inverter circuits

Forced commutation

• When the SCR is operated on the DC supply, it is not turned off naturally. The SCR is turned off by some external circuit is called as forced commutation.
• This external circuit is called as commutation circuit.
• There are 5 type of force commutation these are:

Class A: Self commutated or resonating commutation

Class B: Self commutated or current commutation

Class C: C or L-C switched by another load-carrying SCR

Class D: C or L-C switched by an auxiliary SCR

Class E: An external pulse source for commutation