Class A commutation is also called as load commutation or resonant commutation or self-commutation. This type of commutation is prevalent in thyristor circuits supplied from DC source. This load commutation is possible only in DC circuits and not in AC circuits.
Description of Class A Commutation
For load commutation of a thyristor, the commutating components i.e. Inductance L and capacitance C are connected
in series with the load R as shown in below figure. This circuit is employed for low values of load resistance R.
in series with the load R as shown in below figure. This circuit is employed for low values of load resistance R.
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| Class A commutation circuit for low value of load resistance |
For high value of load resistance the capacitance C is connected across the load resistance as shown in the below figure.
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| Class A commutation circuit for high value of load resistance |
The above circuit should be designed in such a way that the overall circuit is under-damped. When the circuits are energised with a DC source, they give the below current waveform as shown.
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| commutation waveform |
It can be seen that the current first rises to a maximum and then falls to zero at instant T. After zero value of current it tends to become negative but the thyristor is turned off at instant T.
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