
236 Electric Motors and Drives
of being progressively slowed. The stored kinetic energy of
the motor (and load) will cause it to overrun, so that the
number of motor steps will be greater than the number of
command pulses. Failures of this sort are prevented by the
use of closed-loop control, as discussed later.
PRINCIPLE OF MOTOR OPERA TION
The principle on which stepping motors are based is very
simple. When a bar of iron or steel is suspended so that it is
free to rotate in a magnetic field, it will align itself with the
field. If the direction of the field is changed, the bar will turn
until it is again aligned, by the action of the so-calle ...