03 May 2011

TURN OFF methods of SCR or Thyristor?

we have already seen different Turn On methods for SCR. Lets see different turn off methods for scr
The term commutation means the transfer of current from one path to other. In thyristor or SCR circuit  the term is used to describe the transferring current from one thyristor to other. SCR cannot turn off on its own, the circuit to which it is connected must reduce the thyristor current to zero to enable its turn off. Commutation is the term to describe this process.
Thyristor can be only operated in two modes, either in ON state or in OFF state. However thyristor cannot itself control the current and voltage flowing in the circuit. Control of voltage and current in the circuit can be controlled by control circuits which defines the time of ON and OFF state of the thyristor. This switching on and off process of the thyristors can be done by commutation. So all thyristor circuits will involve the cyclic or sequential switching of the thyristors. There are two methods by which a thyristor or SCR can be commutated.

Natural Commutation:

Natural commutation is the simplest and most widely used method. This method makes use of the alternating and reverse nature of the ac voltages to effect the current transfer. In an ac circuits the current always passes through the zero for every half cycle. As the current passes through the natural zero, reverse voltage appears across the device simultaneously . This immediately turns off the device. This process is called 'natural commutation' since no external circuit is required for the purpose o turning off the device. The line commutated converters and inverters comes under this category 

Forced Commutation:

Once thyristor or SCR is operating in ON state ( forward conduction mode) carrying forward current they can only be turned off by reducing the current flowing through them to zero for sufficient times to remove the charge carriers. In case of dc circuits for switching off the scr the forward current should be forced to zero by means of some external circuits. This process is called 'forced commutation' and the external circuit require for it are known as commutation circuits. The components ( inductance and capacitance) which constitute the commutating components. A reverse voltage is applied across the scr by means of commutating circuits that immediately brings the forward current in the device to zero, thus turning the scr off. 

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