Commutation of Thyristor or Silicon Controlled Rectifier:Commutation is the process by which we can turn OFF a thyristor. So the process of switching OFF a thyristor or SCR is known as Commutation.
As we know that once a thyristor starts conducting then it continue to conduct till the current flowing through it reduced below the holding current. In commutation we mainly reduce thyristor's current from holding current.
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There are two methods of commutation of thyristors :Natural Commutation
1. Natural Commutation:
The process of turning off a thyristor without using any external circuits is known as Natural commutation. This type of commutation is only possible in AC Applications.
When using A.C. Supply, the current passing through the device is alternating. This alternating current go to peak positive value , passes through its zero and then go through peak negative value. When the alternating quantity at zero then the current passing through the device also zero and at same time a reverse polarity develops across the thyristor. This will quickly Turn OFF the thyristor. This process of commutation is known as Natural Commutation as no external circuits or components are used.
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This method is only applicable when using A.C. Supply. This is not possible to get this work on D.C. Because D.C. is a unidirectional quantity and doesn't change its direction as a result there is no change in polarity voltage that is required to Turn OFF the thyristor.
2. Forced Commutation :
The process of turning OFF a thyristor or SCR by using external circuits is known as Forced Commutation. This method of commutation is used for D.C. Commutation.
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When using D.C. supply, we make use of external circuit and other active/passive components to reduce the passing current's value below holding current. That means we forces the forward current to come to zero value. Therefore it is called forced commutation. The circuit used for this commutation method is known as commutation circuit and the components that are used in the circuitry , are known as commutating components.