FAQ: What are hybrid stepper motors?

Hybrid stepper motors combine aspects of both permanent magnet (PM) and variable reluctance (VR) stepper motors. Like PM motors, they contain a permanent magnet in the rotor teeth. Two sets of teeth called cups ring the rotor. One ring is all south poles, and the other ring is all north poles.

Like VR motors, hybrid stepper motors have stator poles. Note that stator poles in hybrid motors are sometimes called teeth. (For more information, read FAQ: What is pole count and why does it matter?) More after the jump.

Shown here are the rotor and stator of a hybrid stepper motor. Two soft iron cups sport teeth to guide the flux from a permanent magnet to the rotor-stator air gap.

To illustrate how these motors work, consider a 1.8° hybrid stepper. The teeth of one ring are offset from those on the other ring by 3.6°. The stator poles are offset by 90° and these are opposite polarity, so that every 180° the stator poles are in identical polarity. The rotor poles exist in sets of four, with two being north poles and two being south poles. The rotor poles align alternately with the rotor’s teeth and cups, so that one set of rotor poles attracts fully while the other attracts and repels ¾, ½ or ¼ of a tooth. If there are 50 teeth, the pitch angle is 7.2° … so at the next step, the motor rotates to the next closest step, which is 7.2° × ¼ = 1.8°.

When the current changes, the rotor can turn a very small amount—an improvement over basic PM motors and VR motors. Newer control techniques such as half-stepping and microstepping let designers get even finer movements of rotation, which make for more exact output than that from VR stepper motors (which can’t usually be microstepped). Hybrid stepper motors also have higher torque-to-size ratios and higher output speeds than other stepper-motor types. They are also quieter than VR stepper motors.

One caveat is that hybrid stepper motors are more expensive than other step motors. So, designers should weigh the higher cost against the advantages of quiet operation, smaller steps, and torque output before making a final step-motor selection.