Figure 1.59 shows the basic form of the variable reluctance type of
stepper motor. With this form the rotor is made of soft steel and is not a
permanent magnet. The rotor has a number of teeth, the number being
less than the number of poles on the stator. When an opposite pair of
windings on stator poles has current switched to them, a magnetic field
is produced with lines of force which pass from the stator poles through
the nearest set of teeth on the rotor. Since lines of force can be
considered to be rather like elastic thread and always trying to shorten
themselves, the rotor will move until the rotor teeth and stator poles line
up. This is termed the position of minimum reluctance. Thus by
switching the current to successive pairs of stator poles, the rotor can be
made to rotate in steps. With the number of poles and rotor teeth shown
in Figure 1.59, the angle between each successive step will be 30 ~ . The
angle can be made smaller by increasing the number of teeth on the
rotor.
To drive a stepper motor, so that it proceeds step-by-step to provide
rotation, requires each pair of stator coils to be switched on and off in the
required sequence when the input is a sequence of pulses. Driver circuits
are available to give the correct sequencing and Figure 1.60 shows an
example. The stepper motor will rotate through one step each time the
trigger input goes from low to high. The motor runs clock~se when the
rotation input is low and anticlock~se when high. When the set pin is
made low the output resets. In a control system, these input pulses might
be supplied by a microprocessor.