Electrical Revolution: Class A Commutation ( Current or self or Resonance Commutation )

The commutating components L and C are used in order 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.
As the SCR T1 is turned on, the charging of capacitor is done through supply voltage V_dc.
The polarity of capacitor is shown in the figure A.
When a capacitor charges up to V_dc voltage, the current passing through inductor L reverses.
As the inductor current becomes less than the holding current of the SCR T1, the SCR T1 is turned off.
The turned off time of SCR depends upon resonance frequency of the circuit which depends upon commutating components L and C.
The class A commutation is also called as self - commutation and it is used in the series inverter.
The class A commutation is used for up to 1 kHz frequency because the cost of circuit increases due to higher rating of inductor L and capacitor C at low frequency.