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.

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CLASS A COMMUTATION DRAWING.PDF

CLASS A COMMUTATION WAVEFORMS.PDF

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