INTRODUCTION

In this chapter we look at examples of the power converter circuits which are used with motor drives, providing either d.c. or a.c. outputs, and working from either a d.c. (battery) supply, or from the conventional a.c. mains. The treatment is not intended to be exhaustive, but should serve to highlight the most important aspects which are common to all types of drive converters.

Although there are many different types of converters, all except very low-power ones are based on some form of electronic switching. The need to adopt a switching strategy is emphasised in the first example, where the consequences are explored in some depth. We will see that switching is essential in order to achieve high-eficiency power conversion, but that the resulting waveforms are inevitably less than ideal from the point of view of the motor.

The examples have been chosen to illustrate typical practice, so for each converter the most commonly used switching devices (e.g. thyris-tor, transistor) are shown. In many cases, several different switching devices may be suitable (see later), so we should not identify a particular circuit as being the exclusive preserve of a particular device.

Before discussing particular circuits, it will be useful to take an overall look at a typical drive system, so that the role of the converter can be seen in its proper context.