Class C Commutation

The class C commutation is also called as voltage commutation because the SCR is turned off by applying reverse voltage across it. The SCR T1 is main SCR which is connected in series with load. The commutating capacitor C is connected between anode terminal of SCR T1 and SCR T2.

Power circuit diagram of class C commutation

Working of class c commutation

SCR T1 and SCR T2 OFF

The SCR T1 and SCR T2 are in off condition during initial condition.

SCR T1 – OFF

SCR T2 – OFF

SCR T1 - ON and SCR T2 - OFF

When SCR T1 is turned on by gate pulse, the charging of capacitor is done through (+)V_dc– R – (+) C – C (-) – SCR T1 – V_dc (-).
The charging of the capacitor is done up to V_dc voltage. As soon as the SCR T1 is turned on, the load current passes through path(+)V_dc – R_L – SCR T1 – V_dc(-).

SCR T1 – ON

SCR T2 – OFF

SCR T1 - OFF and SCR T2 - ON

When SCR T2 is turned on by gate pulse, the SCR T1 gets reverse voltage through capacitor C therefor the SCR T1 is turned off.
The charging of capacitor is done through load resistance by path (+)V_dc – R_L – (+) C – C (-) – SCR T2 – V_dc (-). The charging of capacitor is done in the reverse direction.

SCR T1 – OFF

SCR T2 – ON

SCR T1 - ON and SCR T2 - OFF

The SCR T1 is again turned on by gate pulse. As soon as the SCR T1 is turned on, the SCR T2 gets reverse voltage through capacitor and turned it off.
The capacitor again charge through path (+)V_dc – R – (+) C – C (-) – SCR T1 – V_dc (-).

SCR T1 – ON

SCR T2 – OFF

Waveform of class c commutation

The waveform of gate pulse G1, gate pulse G2, voltage across SCR T1, voltage across SCR T2, current passes through SCR T1, current passes through SCR T2 and load current is given.