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Appendix B. Stepper Motors
PRINTER STEPPER MOTORS
Stepper Motor Characteristics
• The stepper motor is a sealed unit.
• The stepper motor is phase controlled by
direct current.
• The sequence of the phase pulses from the
logic circuits controls the direction of
rotation.
The dc stepper motor consists of a permanent-
magnet rotor (armature) and a pair of two-
phase stator windings. The motor is a sealed
unit having no gears or commutators and re-
quiring no maintenance.
Shaft rotation is not continuous unless the
stepper motor is continually pulsed. When
current flows through the stator windings, a
magnetic field set up in the stator pole acts
on the permanent-magnet rotor to produce
torque in the rotor shaft. This torque turns
the rotor shaft only part of a revolution; then
locks it in an electrically detented position.
Electrical detenting is due to direct current in
the stator windings action on the permanent-
magnet rotor. The motors on the belt printer
cannot be easily turned manually with the
system power on because they are electrically
detented. Serial printer stepper motors are
not detented (after they have been stopped
for 50 ms) and can be turned manually.
Note: When the system power is off, the
residual detenting due to the permanent
magnet can be felt as a slight drag or
roughness, and heard as a clicking sound
when the shaft is turned. If a motor
is removed from either printer, ensure
that none of the wires in the cable touch
each other. When the wires touch, a back
electromotive force is created that retards
shaft movement ..
Stepper Motor Operation
For ease of understanding, the motor used in
this example turns nine degrees per step.
This simplified stepper motor consists of eight
coil-wound stator poles and a ten-pole perma-
nent-magnet rotor.
I nput A
--tI~----""
.----t ....... -
Common Return
Input A
Stator-Pole Winding
CAUTION
The permanent-magnet rotor is magnetized after
assembly at the plant. Do not open or disas-
semble the motor. Disassembling the stepper
motor reduces the magnetic flux of the rotor,
which reduces the torque of the motor. If a
motor is disassembled, do not reinstall it in
the machine.
If we physically rotate the stator (in either direc-
tion), the rotor maintains its detented position
and follows the stator as shown. (Both the
stator poles and the rotor rotated clockwise 45
degrees.)
Note: Current flows only in one-half of the
winding at a time. Polarity of the stator pole
is determined by which half of the winding has
current.
Stepper Motors
8-1
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