TABLE OF CONTENTS The essential property of the stepping motor is its ability to translate switched excitation changes into precisely defined increments of rotor position ( steps ). Stepping motors are categorised as doubly salient machines, which means that they have teeth of magnetically permeable material on both the stationary part (the stator ) and the rotating part (the rotor ). A cross-section of a small part of a stepping motor is shown schematically in Fig. 1.1. Magnetic flux crosses the small airgap between teeth on the two parts of the motor. According to the type of motor, the source of flux may be a permanent-magnet or a current-carrying winding or a combination of the two. However, the effect is the same: the teeth experience equal and opposite forces, which attempt to pull them together and minimise the airgap between them. As the diagram shows, the major component of these forces, the normal force ( n), is attempting to close the airgap, but for electric motors the more useful force component is the smaller tangential force ( t), which is attempting to move the teeth sideways with respect to each other. As soon as the flux passing between the teeth is removed, or diverted to other sets of teeth, the forces of attraction decrease to zero.
The following sections explain how this very simple principle is put to work in practical stepping motor devices. Most stepping motors can be identified as variations on the...
