8.1 Introduction

In the normal operation of a full-wave three-phase converter with inductive load the output dc current is diverted from thyristor to thyristor and from phase to phase in a regular manner. Two thyristors are normally conducting at a time: one from the upper group and one from the lower group, Fig. 7.1. There are six thyristor commutations per mains cycle where the current is diverted through a thyristor to another phase. The output current is usually smoothed to its dc value by the load inductance. At the point of commutation the current does not become zero immediately in the conducting thyristor and it does not rise immediately from zero to the dc value in the thyristor which is taking the current because of the source inductance; it will take a finite time dictated by the value of the dc current, the magnitude of the line voltage and the value of the source inductance. During this finite time two thyristors are conducting from the same group and one from the other group. The two thyristors from the same group in effect constitute a short circuit of two phases via the source impedance; this is called overlap. No surge of current is observed but the fact that two phases are shorted reduces the voltage available to the load. More importantly 'notches' distort the line voltage.

The modes are increased from 6 to 12 to account for the commutation instances. Three new groups of switching functions are introduced in this chapter...