DC Chopper Commutation Techniques

DC Chopper Commutation Techniques
Commutation : The process of turning off conducting SCR is called commutation.

Depending on nature of source(AC or DC) the commutation can be natural or forced
In chopper only forced commutation is used.

Forced Commutation : L,C or auxiliary SCR
When SCRs operate on pure DC input voltage, their forward current cannot be reduced below holding current naturally.

Therefore the SCRs are commutated forcibly by using additional commutation circuit

This external commutation circuit will turn off the SCR by either current or voltage commutation

Normally forced commutation is used in thyristorised inverters or chopper circuits

Voltage Commutation : if SCR is turned off by using large reverse voltage across it.

Current Commutation : if SCR is turned off by reducing anode current below holding current .

An externally produced reverse current is forced through the conducting SCR to bring down its forward current below holding current value.

In both techniques commutation is initiated by gating a auxiliary SCR

Conditions for successful commutation :
Forward current of SCR must be reduced to zero

Reverse voltage must be applied to across the thyristor for duration more than the turn off time of the device

Critical rate of rise of voltage of device should never be exceeded, to avoid retriggering of the thyristor(dv/dt of reapplied voltage should not be too large)




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