Mechanical Systems 365
cumulative, regardless of the distance moved, or the number of steps tak-
en. Motors of this type are energized by a DC drive amplifier that is con-
trolled by digital logic circuits. The drive-amplifier circuitry is a key factor
in the overall performance of this motor. The stator construction and coil
layout are shown in Figure 14- 11. The rotor of a stepping motor is of a per-
manent-magnet type of construction.Figure 14-11. Stator and coil layout of a DC stepping motor (Courtesy Superior
Electric Co.)A very important principle that applies to the operation of a DC step-
ping motor is that like magnetic poles repel, and unlike magnetic poles at-
tract. If a permanent-magnet rotor is placed between two series-connected
stator coils, it produces the situation shown in Figure 14-12. With power
applied to the stator, the rotor can be repelled in either direction. The di-
rection of rotation in this case is unpredictable. Adding two more stator
coils to a simple motor, as indicated in Figure 14-12B, will make the direc-
tion of rotation predictable. With the stator polarities indicated, the rotor
will align itself midway between the two pairs of stator coils. The direc-
tion of rotation can now be predicted, and is determined by the polarities