INTRODUCTION
In this chapter the common feature which links the motors is that they are all a.c. motors in which the electrical power that is converted to mechanical power is fed into the stator, so, as with the induction motor, there are no sliding contacts in the main power circuits. All except the switched reluctance motor also have stators that are identical (or very similar) to the induction motor.
We begin by looking at motors that are intended to be operated directly oV the mains supply, usually at either 50 or 60 Hz. These motors are known as ‘synchronous’ or ‘reluctance’ motors, and they provide a precise, speciWc and constant speed for a wide range of loads, and are therefore used in preference to induction motors when constant speed operation is essential. Such machines are available over a very wide range from tiny single-phase versions in domestic timers to multi-megawatt machines in large industrial applications such as gas compressors. Their principal disadvantage is that if the load torque becomes too high, the motor will suddenly lose synchronism and stall.
To overcome the Wxed-speed limitation that results from the constant frequency of the mains, controlled-speed synchronous motor drives simply use a variable-frequency inverter to provide for variation of the synchronous speed. These ‘open-loop’ drives are dealt with next.
We then look at what are perhaps best referred to as ‘self-synchronous’ drives, which potentially oVer competition for d.c. and induction motor drives. In these drives the motor is basically a synchronous motor with the stator fed from a variable-frequency inverter; but the frequency is determined by a speed signal from a transducer mounted on the rotor.
This closed-loop arrangement ensures that the motor can never lose synchronism, hence the name ‘self-synchronous’. Amongst this category is the so-called ‘brushless d.c.’ drive, where the motor is speciWcally designed to operate from its own converter, and cannot be supplied directly from conventional mains supplies.
Finally, the most recent addition to the family of industrial drives – the switched reluctance drive – is brieXy discussed. The switched reluctance motor is perhaps the simplest of all electrical machines, but it was only with the advent of power-electronic switching and sophisticated digital control that its potential could be fully demonstrated.