Effect of source impedance on converter operation: For single-phase fully controlled bridge converter, the SCRs are triggered in pairs alternately. During positive half-cycle of input, SCRs T1 and T2 are triggered whereas during negative half-cycle, SCRs T3 and T4 are triggered. When T1 and T2 are conducting, T3 and T4 are off. On the reversal of supply voltage, firing of T3 and T4 causes application of reverse bias across T1 and T2 and they are turned off. The current shifts from T1 T2 to T3 T4. The instantaneous current shift is possible only when the voltage source has no internal impedance. In practice, the source always possesses some internal impedance may be due to the transformer on supply side. 1) If the source impedance is purely resistive, then voltage drop across it causes reduction in input voltage and ultimately in the output voltage of converter. 2) If the source impedance is largely inductive, then source current cannot change instantly. The current cannot get transferred immediately from outgoing SCRs to incoming SCRs. The commutation of SCRs is delayed. During current transfer, both pairs of SCRs conduct simultaneously and load voltage appears zero. As both pairs of SCRs conduct simultaneously, this commutation period is called “overlap period (μ)”. The output dc voltage is given by,
As source inductance (Ls) increases, the commutation period (overlap angle μ) increases and as a consequence, the output dc voltage decreases.