Stepper Motors: Types
above: Cut-away diagram of a permanent-magnet
There are three basic types of stepper
- permanent magnet motor
- variable-reluctance motor
- hybrid motor -- a "mix" of both permanent and variable reluctance
The first image (left) shows a cut-away diagram of a typical permanent-magnet
stepper motor; its rotor is called a canstack rotor is illustrated
in the next image. The canstack rotor reveals
that the permanent-magnet motor can have multiple rotor winding,
meaning the shaft will turn fewer degrees as each pulse of current
is received at the stator. E.g., if the rotor has 60 teeth and the
stator has eight poles with five teeth each (total = 40 teeth),
the stepper motor is able to move 200 distinct steps to make one
complete revolution. Hence, the shaft of the motor will turn 1.8°
per step. The primary feature of the permanent-magnet stepper motor
is that a permanent magnet is used for the rotor -- hence, no brushes
are needed. A problem this type of stepper motor is that it has
limited torque and may only be used for low-speed applications.
above: Canstack rotor used in permanent magnet
The variable-reluctance motor doesn't utilize permanent magnets,
so the field strength can be varied. The amount of torque for this type
of motor is still quite low so it's normally used for small positioning
tables and other small positioning loads. Because this type of motor doesn't use permanent magnets, it can't use the same type of stepper controller
as other varieties of stepper motors.
The hybrid stepper motor, shown below, is the most widely used and combines the principles of both the permanent magnet and the variable-reluctance
motors. Virtually all hybrid stepper motors have two phases and operate
on the principle used to previously descirbe the 12-step motor.
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Motor: Theory of Operation
Another helpful resource on this topic.