There are two basic forms of stepper motor, the permanent magnet
type with a permanent magnet rotor and the variable reluctance type
with a soft steel rotor. Both form of stepper motor have a stator with a
number of diametrically opposite pairs of poles, each wound with a coil.
Figure 1.58 shows the permanent magnet type with two pairs of stator
poles. Each pole is activated by a current being passed through tile
appropriate field winding, the coils being such that opposite poles are
produced on opposite coils. The current is supplied from a d.c. source to
the windings through switches. With the currents switched through the
coils such that the poles are as shown in Figure 1.58, the rotor will move
to line up with the next pair of poles and stop there. This would be, for
Figure 1.58, an angle of 45 ~ . If the current is then switched so that the
polarities are reversed, the rotor will move a step to line up with the next
pair of poles, at angle 135 ~ and stop there. The polarities associated with
each step are: