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 conven- tional 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 diVerent 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 Wrst example, where the consequences are explored in some depth. We will see that switching is essential in order to achieve high-eYciency power conver- sion, 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 diVerent 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.
General arrangement of drives
A complete drive system is shown in block diagram form in Figure 2.1.
The job of the converter is to draw electrical energy from the mains (at motor at whatever voltage and frequency necessary to achieve the desired mechanical output.
Except in the simplest converter (such as a simple diode rectiWer), there are usually two distinct parts to the converter. The Wrst is the power stage, through which the energy Xows to the motor, and the second is the control section, which regulates the power Xow. Control signals, in the form of low-power analogue or digital voltages, tell the converter what it is supposed to be doing, while other low-power feed- back signals are used to measure what is actually happening. By com- paring the demand and feedback signals, and adjusting the output accordingly, the target output is maintained. The simple arrangement shown in Figure 2.1 has only one input representing the desired speed, and one feedback signal indicating actual speed, but most drives will have extra feedback signals as we will see later. Almost all drives employ closed-loop (feedback) control, so readers who are unfamiliar with the basic principles might Wnd it helpful to read the Appendix at this stage.
A characteristic of power electronic converters which is shared with most electrical systems is that they have very little capacity for storing energy. This means that any sudden change in the power supplied by the converter to the motor must be reXected in a sudden increase in the power drawn from the supply. In most cases this is not a serious problem, but it does have two drawbacks. Firstly, a sudden increase in the current drawn from the supply will cause a momentary drop in the supply voltage, because of the eVect of the supply impedance. These voltage ‘spikes’ will appear as unwelcome distortion to other users on the same supply. And secondly, there may be an enforced delay before the supply can furnish extra power. With a single-phase mains supply, for example, there can be no sudden increase in the power supply from the mains at the instant where the mains voltage is zero, because in-
stantaneous power is necessarily zero at this point in the cycle because the voltage is itself zero.
It would be better if a signiWcant amount of energy could be stored within the converter itself: short-term energy demands could then be met instantly, thereby reducing rapid Xuctuations in the power drawn from the mains. But unfortunately this is just not economic: most converters do have a small store of energy in their smoothing inductors and capacitors, but the amount is not suYcient to buVer the supply suYciently to shield it from anything more than very short-term Xuctuations.