TABLE OF CONTENTS This chapter is devoted to the study of special-purpose electric machines. Although all electric machines have the same basic principle of operation, special-purpose machines have some features that distinguish them from conventional machines.
It is not our intention to discuss all kinds of special-purpose machines in one chapter; rather, an attempt is made to introduce the basic operating principles of some special-purpose machines that are being used extensively in home, recreational, and industrial applications.
With the proliferation of power electronic circuits and digital control systems, precise speed and position control can be achieved in conjunction with special-purpose electric machines such as permanent-magnet (PM) motors, step motors, switched-reluctance motors, brushless direct-current (dc) motors, hysteresis motors, and linear motors. Some of these devices find applications in computer peripheral equipment or in process-control systems whereas others can be used in devices such as home appliances. For example, step motors are employed extensively in computers where precise positioning is required, as in the case of a magnetic head for a disk drive. For applications that demand constant-speed drives, brushless dc motors offer excellent characteristics. Switched-reluctance motors, on the other hand, find applications where we traditionally use dc or induction motors.
In the following sections we discuss the construction, operating principles, and characteristics of each of the above-mentioned special-purpose electric machines.
The development of new permanent-magnet materials has made PM motors a viable substitute for a shunt (dc) motor. In a PM motor the poles are made of permanent magnets,...
