Electric Motors and Drives, 4th Edition

Chapter Four

D.C. Motor Drives

1. Introduction

Until the 1960s, the only satisfactory way of obtaining the variable-voltage d.c. supply needed for speed control of an industrial d.c. motor was to generate it with a d.c. generator. The generator was driven at fixed speed by an induction motor, and the field of the generator was varied in order to vary the generated voltage. For a brief period in the 1950s these ‘Ward Leonard’ sets were superseded by grid-controlled mercury arc rectifiers, but these were soon replaced by thyristor converters which offered cheaper first cost, higher efficiency (typically over 95%), smaller size, reduced maintenance and faster response to changes in set speed. The disadvantages of rectified supplies are that the waveforms ...