Class A Commutation
In the Class A commutation, the commutating components L and C are used 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.
Why class a commutation is called as self-commutation & current commutation?
The class A commutation is also called as
self-commutation as it automatically turned off at natural zero current. The class
a commutation is also called as current commutation because it turns off due to
anode current below holding current.
Power circuit diagram of class a commutation
Working of class a commutation
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.
Waveform of class a commutation
The
waveform shows gate pulse, scr current, voltage across scr and voltage across capacitor.
The voltage across scr is 1.0 to 1.5 V when it is turned on.
Application of class A commutation
- 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.
- Series inverter
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