1 GENERAL

All the motors described earlier have the characteristic that they can be operated, albeit at essentially fixed speed, when fed by an appropriate D.C. or A.C. supply. In this section we consider a group of drive systems where the motor can only be used in conjunction with its specific power converter and control, and consequently only overall characteristics are relevant.

Switched-reluctance drive systems are of growing importance notably in applications where high low-speed torque is required, and less importance is placed on smoothness of rotation. Although considerable advances have been made in improving the noise characteristic of this drive, it can still be a limiting factor where a broad operating speed range is required.

Stepper-motor systems are somewhat in decline. Their operating characteristic of being controlled by a computer pulse train is now a common feature of many modern servo-drives. Also, where rapid settling times are required, stepper drives, which are inherently open loop, are not ideal in both fundamental performance and the fact that varying mechanical friction has a significant impact. This variability can make stepper drives unacceptable in applications where the transient performance is important.

2 SWITCHED-RELUCTANCE MOTORS AND CONTROLLERS