Driving Bipolar Stepper Motors

Stepper motors are devices, which convert electrical impulses into discrete mechanical rotational movements. In a typical stepper motor, power is applied to two coils. Two stator cups formed around each of these coils, with pole pairs mechanically offset by ½ a pole pitch, become alternately energized North and South magnetic poles. Between the two stator-coil pairs the offset is ¼ of a pole pitch.

The permanent magnet rotor has the same number of pole pairs as the stator coil section. Interaction between the rotor and stator (opposite poles attracting and like poles repelling) causes the rotor to move ¼ of a pole pitch per winding polarity change. Motors are available in either 2 coil (bipolar) or 4 coil (unipolar) windings. In the unipolar version, the coils are bifilar (two side by side wires) wound on each stator half and opposite ends of each pair are connected together to form a center tapped coil. With this method, the flux is reversed by powering either one end or the other of the bifilar coil pair with the center connected to common. For a bipolar motor, an external device referred to as an H-Bridge can be used to reverse the polarity of the winding and thus the flux. An H-Bridge can also drive a unipolar motor by not connecting the center tap (common) lead or using only one of the windings in the pair.