Current drive for stepper motors

95 Stepper motors have either unipolar or bipolar stators. In unipolar models, each stator winding has a centre tap, which enables the magnetic field to be inverted by switching from one to the other half of the winding. Bipolar types have a single stator winding, so that the direction of the current through it must be changed to attain inversion of the magnetic field. From this, it is clear that, given that the two motors are of similar size, the bipolar type will provide a larger couple than the unipolar model. There is, however, a price to be paid for this larger couple: the drive of a bipolar motor is more complex than that of a unipolar type. The drive for bipolar motors may, in current drive for stepper motors principle, be obtained by means of a • full bridge circuit. i.e. four tran- sistors per stator winding; • half bridge circuit and dual power supply, i.e. two transistors per stator winding; • half bridge circuit with large out- put capacitor. The last method is t...