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Class A Load Commutation
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Class A: Load Commutation: The class A or load commutation (also called self-commutation or resonant commutation) is employed in thyristor circuits supplied from DC source. The commutating components include inductor (L) and capacitor (C). The load resistance (R) and these commutating elements L and C are so chosen that there is a natural tendency for the load current that flows through the thyristor to become zero. When the load resistance (R) is very low, the elements L, C and R are connected in series. However, if load resistance (R) is high, then capacitor (C) is connected across it and then this parallel combination is connected in series with inductor (L) and thyristor as shown in the figure.



The load resistance R in series with L and C forms a series R-L-C circuit connected across DC source through the thyristor as shown. Initially the thyristor is off, hence entire supply voltage E appears across it and therefore it is forward biased. If a gate pulse is applied, the device can be turned on. Once turned on, it acts as short-circuit, thereby connecting series RLC circuit across DC source. When series RLC circuit (Load circuit) is underdamped, the current is oscillating having natural zero values even though the supply is DC voltage. Referring to waveform of current, when the thyristor is turned on at t = 0, the current starts to flow, then attains peak and finally falls to zero. During this, the capacitor voltage rises towards 2E. When current reaches to natural zero value, the capacitor voltage is higher than the supply voltage E and hence the thyristor gets reverse biased. Thus the zero-current and reverse-bias cause the thyristor to turn-off. 

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