Presentation. The convertors in dc drive systems are designed to control and supply separately-excited dc machines in applications that demand rapid response, high accuracy and great reliability.

Fig. 1:
a - single convertor; b - double convertor

The convertors are available with single-conyertor arrangement and in double-convertor arrangement.

Single converters. In a single convertor the thyristors are arranged in a six-pulse two-way circuit (see Fig. 1,a).

A single convertor can supply current in one direction only, but its output voltage can change polarity. This determines the rotation and torque direction characteristics of the machine, as illustrated in Fig. 1,a.

The single convertor can operate in the first and fourth quadrants. The direction of torque can only be changed by reversing the field current of the machine.

Double convertors. In a double convertor there are two sets of thyristors connected in antiparallel (see Fig. 1,b). Both current and voltage can change sign, and the machine can be braked and reversed without additional equipment; this is known as four-quadrant drive.