We know that in converters the commutation of thyristors takes place instantaneously once there is a phase reversal. But in practice, there exists an inductance and resistance in the supply source. This inductance is known as source inductance L_S and due to this the line commutation process of the thyristors and the turning ON process of the thyristors are delayed for a small duration, which is termed as the Overlapping Period.

In order words, due to the presence of source inductance, the current in the outgoing thyristor takes a finite time to reduce to zero. This causes a delay in the commutation of that thyristor and at the same time, the current will rise in the incoming thyristor. This results in the conduction of both incoming and outgoing thyristor simultaneously for a finite time period known as the Overlapping Period.

During the overlapping period, the rate of rise of load current through the thyristors being turned ON will be equal to the rate of fall of current in the thyristors being turned OFF, this results in the short circuit across the source terminals. Hence, the output voltage becomes zero during the overlap period and this period remains for a duration μ called as overlap angle or commutation angle.

The overlapping period in converters due to source inductance affects the performance characteristics of the converter, it reduces the average output voltage and displacement factors. Let us see the effect of source inductance on a single-phase converter.

Effect of Source Inductance on Single Phase Converter :

The phenomenon of the overlapping period is more severe in fully-controlled converters compared to the semi-converters. The below shows the single-phase fully-controlled bridge converter circuit with source inductance L_S.

During the positive half of the supply cycle, thyristors T₁ and T₂ are triggered at firing angle α. The positive cycle will reverse bias the already conducting SCRs T₃ and T₄. The positive group SCRs T₁ and T₂ now act as incoming SCRs whereas the negative group SCRs T₃ and T₄ will act as outgoing SCRs.

Since the load current is assumed to be constant, it should immediately transfer from outgoing SCRs to incoming SCRs, i.e., from T₃ T₄ to T₁ T₂. The current, i₂ due to T₃ and T₄ should go to zero instantaneously and the current, i₁ due to T₁ and T₂ should reach the load current I_dc instantaneously.

We know that an inductor does not allow sudden changes in current. Thus due to source inductance L_S, the fall of i₂ and rise of i₁ will not be instantaneous rather it is gradual. Hence, there will be a small period during which both the currents i₁ and i₂ will flow in the circuit, resulting in a short circuit across the source terminals. This period is called as Overlapping Period.

During this period, the output voltage will be zero. The above shows the typical waveform of voltage and current of the converter with source inductance L_S. At ωt = π, as the supply voltage is negative the thyristors T₁ and T₂ should be turned off, and hence i₁ should be zero.

But due to the presence of source inductance, the current i₁ will not go to zero at ωt = π, rather starts decreasing gradually towards zero. Due to this, the output voltage V_dc will become negative for a small period till the incoming SCRs are triggered.

Derivation of Output Voltage :

Below shows the equivalent circuit for a the above bridge converter circuit.

Applying KVL to the loop LMNOL,

Since the load current is constant,

At the instant ωt = α, load current i₁ through T₁ and T₂ builds up from 0 to I_dc during the overlap angle μ.

i₁ = 0, at ωt = (α + μ)

i₁ = I_dc

Integrating equation (3) we get,

From the waveform, we have,

By using equation (7), a dc equivalent circuit for a single-phase full converter is given as,

This reveals the effect of source inductance L_S is to present an equivalent inductance ωL_S/π which is in series with internal voltage (ωV_m cos α)/π. If I_dc is the load current, the presents of source inductance cause a drop of (ωL_S I_dc)/π in the load or output voltage. From the above variation curve of load voltage with load current, we can conclude that the average output voltage decreases with the increase in source inductance.

Therefore, the effect of source inductance on a single-phase full converter is such that, until the overlap angle lies below π, the output voltage will be equal to a value shown in equation (6). If μ = π, both incoming and outgoing thyristors will remain in the conduction state, thereby making the output voltage zero.