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## 31/03/2017

### 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 Vdc
• The polarity of capacitor is shown in the figure A.
• When a capacitor charges up to Vdc 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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