Class A is one of frequently used thyristor commutation techniques. If thyristor is triggered or turned on, then anode current will flow by charging capacitor C with dot as positive. The second order under-damped circuit is formed by the inductor or AC resistor, capacitor and resistor.

 

Load commutation of SCR is a forced commutation technique in which SCR is forced to turn off. Turning off an SCR essentially requires its anode current to fall below holding current and a reversed biased voltage must be present across SCR for sufficient time so that SCR may regain its blocking state. Load Commutation of SCR is also known as Class-A commutation.

 

When the above circuit is energized from DC source, current waveform as shown in figure is obtained. As can be seen, the circuit current is under-damped in nature. The current is rising initially up to maximum and then begins to fall. When the current decays to zero (Capacitor is fully charges at instant A and hence will not allow further current flow) and tends to reverse. During the time when SCR conducts, voltage across SCR (VT) is zero. But as soon as current reaches zero and tend to reverse, a reverse voltage (since capacitor C is fully charged up to source voltage Vs, this capacitor voltage is applied across SCR) equal to the source voltage is applied across the terminals of SCR. Thus after time A, anode current of SCR is zero and it is reversed biased. The two conditions of SCR turn-off are met and hence SCR / thyristor is turned off on its own at instant A.

Class A Commutation of Thyristor | LC Load Resonance | MATLAB Simulation

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