Meaning of Natural Commutation
The commutation methods are divided into two categories based on the location of the commutation voltage namely Natural Commutation and Forced Commutation.
- If the SCR is linked to an AC source, the anode current passes through zero at the conclusion of each positive half cycle and an immediate reverse voltage is supplied across the SCR.
- thereby bringing the current to a stop and giving the thyristor junctions time to recover.
- These are the prerequisites for turning the SCR OFF.
- There is no inherent current zero in DC circuits to turn the SCR off.
- To commutate the SCR in such circuits, the forward current must be forced to zero using an external circuit; therefore, the term "forced commutation."
Turning OFF SCR:
- The anode or forward current of the SCR must be lowered to zero or below the holding current level in order to turn OFF the conducting SCR. Next, a strong enough reverse voltage must be provided across the SCR to restore its forward blocking state.
- The commutation voltage is the reverse voltage that switches the SCR on and off.
Natural commutation, therefore, entails cutting the current to zero and giving the thyristor junctions some time to recuperate.
Natural commutation means (a) reducing the thyristor current to zero (b) reducing the thyristor current below “holding current” (c) reducing the current to zero and allowing time to recover thyristor junctions (d) none of (1), (2) and (3)
Natural commutation entails bringing the current to zero and giving the thyristor junctions some time to recover. When the SCR is linked to an AC source no anode current passes and an immediate reverse voltage is supplied across the SCR.
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