In practice, inductance and resistance must be present in the supply source, and time is required for a current change to take place. The net result is that the current commutation is delayed, as it takes a finite time for the current to decay to zero in the outgoing thyristor, while the current will rise at the same rate in the incoming thyristor. Thus, in practice, the commutation process may occupy a quite significant period of time, during which both the "incoming" and "outgoing" thyristors are simultaneous in conduction. This period, during which both the outgoing and incoming thyristors are conducting, is known as the overlap period and the angle for which both devices share conduction is known as the overlap angle $(\mu)$ or commutation angle.

During this "commutation overlap" period, the waveforms of the voltage at the output terminals of the converter, as well as the current and voltage at the input terminal are different from those obtained with zero-source inductance. This has the modifying effect upon the external performance characteristics of the converter. At the outputs, the effect of the input source inductance is to cause a loss of mean voltage, as well as modification to the harmonic distortion terms, while at the input terminals, a slight reduction of displacement factor, with respect to the a.c. source voltage, as well as the modification to the distortion-terms in the current waveform, takes place.

The inductive reactance of the a.c. supply is normally much greater than its resistance and, as it is the inductance which delays to current change, it is reasonable to neglect the supply resistance. However, if the source impedance is resistive, then there will be a drop across this resistance and the average output voltage of a converter gets reduced by an amount equivalent to the average drop. Since the source resistance is usually small, the commutation angle during which the load current is transferred from the outgoing to the incoming thyristors is generally neglected.