Mechanical Systems 367
of the stator-coil sets. Adding more stator-coil pairs to a motor of this type
improves its rotation and makes the stepping action very accurate.
Figure 14-13 shows an electrical diagram of a DC stepping motor.
The stator coils of this motor are wound in a special type of construction
called bifilar. Two separate wires are wound into the coil slots at the same
time. The two wires are small in size, which permits twice as many turns
as can be achieved with a larger-sized wire. Construction of this type sim-
plifies the control circuitry and DC energy-source requirements.
Operation of the stepping motor illustrated in Figure 14-13 is achieved
in a four-step switching sequence. Any of the four combinations of switch-
es 1 or 2 will produce an appropriate rotor position location. After the four
switch combinations have been achieved, the switching cycle repeats it-
self. Each switching combination causes the motor to move one-fourth of
a step.
A rotor, similar to the one shown in Figure 14-13 normally has 50
teeth. Using a 50-tooth rotor in the circuit of Figure 14-13 would permit
four steps per tooth, or 200 steps per revolution. The amount of displace-
ment, or step angle, of this motor is, therefore, determined by the number
of teeth on the rotor, and by the switching sequence.
A stepping motor that takes 200 steps to produce one revolution willFigure 14-13.
Circuit diagram
and switching se-
quence of a DC
stepping motor
(Courtesy Superior
Electric Co.)