TABLE OF CONTENTS There is a bewildering number of circuits for switching current between the motor phases, but in this chapter discussion is confined to the basic circuits, since the potential advantages of more advanced drives are examined at a later stage (Chapter 5).
The variable-reluctance stepping motor has at least three phases, but the phase currents need only be switched on or off; the current polarity is irrelevant to torque production. A simple unipolar drive circuit so-called because it produces unidirectional currents suitable for use with a variable-reluctance type motor is discussed in Section 2.2. For the hybrid motor, or any type of motor incorporating a permanent magnet, the current polarity is important and a bipolar drive is required to give bidirectional phase currents. The transistor bridge drive introduced in Section 2.3 uses more semiconductor devices per phase than the unipolar drive, but by placing additional windings in the hybrid motor it is possible to simplify the drive; this technique is described in Section 2.4.
As well as choosing a circuit configuration, the designer must also choose between one of three types of switching device: the bipolar junction transistor (BJT), including Darlington drives; the MOSFET; and the IGBT. When operating as a switch the junction transistor must be driven firmly into its saturation region, where the dc current gain is very low, so a significant amount of base drive power is required. However, the on-state collector-emitter voltage drop is low (0.2 V), so that comparatively little power is...
