Overlap period

The commutation period when both incoming and outgoing Silicon Controlled Rectifiers (SCRs) are conducting due to source inductance is called the overlap period. During this period, both the incoming and outgoing SCRs are in conduction simultaneously, leading to a temporary short circuit in the circuit.

Explanation:

In a typical three-phase AC circuit, SCRs are used as power electronic switches to control the flow of current. During normal operation, the SCRs are triggered sequentially to allow the flow of current in a controlled manner.

However, when there is a large source inductance in the circuit, the current cannot change instantaneously. As a result, when an SCR turns off, the inductor tries to maintain the current flow by inducing a voltage in the opposite direction. This voltage can be quite large, and it can cause the SCR to turn on again.

Now, during the overlap period, both the incoming and outgoing SCRs are conducting simultaneously. This occurs when the incoming SCR has turned on due to the induced voltage from the inductor, while the outgoing SCR is still in conduction before turning off. As a result, there is a temporary short circuit in the circuit.

The overlap period can be calculated by considering the time it takes for the inductor current to decay to zero after the incoming SCR turns on. This time can be determined by the inductance value and the resistance of the circuit.

Importance:

The overlap period is an important consideration in power electronic circuits, especially in applications where there is a high source inductance. It can lead to significant current and voltage stresses on the SCRs, affecting their performance and reliability.

By understanding the overlap period, engineers can design appropriate commutation circuits to minimize the duration of the overlap and reduce the stress on the SCRs. This can be achieved by using snubber circuits, freewheeling diodes, or other commutation techniques to control the inductive voltage and ensure smooth switching of the SCRs.

In conclusion, the overlap period refers to the commutation period when both incoming and outgoing SCRs are conducting due to source inductance. It is an important consideration in power electronic circuits and can be minimized through proper design techniques to ensure the reliable operation of the SCRs.