Circuit Operation
The circuit of a three-phase fully-controlled bridge rectifier with source
inductance is presented above. The presence of source inductance introduces
an additional mode of operation when the firing angle is less than a certain
value. Let us assume that SCRS S1 and S2 are in conduction
when SCRS S3 is triggered. Then the current from the source does
not transfer from S1 to S3 instantaneously, and the
transfer of current, called commutation, takes a while. During this commutation
overlap, both S1 and S3 conduct in addition to S2.
SCR S1 continues to conduct till the current through S3 rises
to equal the dc link current.
The effects of commutation overlap are:
i. A slight reduction in output voltage,
ii. A notch in the supply voltage to the circuit during commutation overlap.
When the source has inductance, other loads connected to this source along
with the controlled rectifier are supplied voltages with notches in them and
some of these loads can be sensitive to these notches and they may operate
improperly. Hence in order to reduce the magnitude of notches, it is mandatory
in some countries for the rectifier to be provided with an inductance in series
with each of its three-phase input lines. If these inductors are much larger
than the source inductance, the notch voltages are absorbed by these inductances
and the other loads connected to the same 3-phase source are not supplied
with distorted voltages. The internal inductances connected in series with
the source are sometimes referred to as 4% inductances. If the inductor is
such that the voltage drop across it is about 4% of the phase voltage at rated
current, it is normally sufficient to reduce the notches at the source terminals
to an acceptable level.
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