Types of Stepper Motors
Permanent Magnet (PM): Also called “tin can" or “can stock," these are a cheap, low resolution type of stepper motors having step angles of typically 7.5 to 15 degrees (48 to 24 steps per revolution.)
The working concept of a PM stepper motor can be understood through the illustration provided below. (Please click to enlarge.) The type utilizes permanent magnets and eliminates the employment of rotor teeth or steps normally associated with the other more sophisticated types of stepper motors. Instead the rotational effect is produced through alternate magnetization of the rotor plate. The electrical pulses applied to the windings generate induced magnetic flux alternately which influence the permanent magnets of the rotor. The effect of these interacting magnetic fluxes produces the high torque levels responsible for the required rotations.
The above mechanism with PM stepper motors make them comparatively more efficient than the variable reluctance types.
Variable reluctance (VR): This type of stepper motor primarily consists of a central rotor cylinder/shaft assembly made of multiple toothed elevations over its entire radial surface. The stator assembly, which consists of several layers of super enameled copper wire wound over laminated iron stampings, is kept closely attached with the rotor. The electrical pulses applied to the stator windings produce alternately sequencing magnetic fluxes over the rotor teeth creating a twisting effect. These alternate push/pulls and twisting of the rotor give rise to the required revolutions and the torque to the rotor/shaft assembly.
Hybrid (HB): The stepper motors included in this type are more expensive than the above types due to the following interesting features:
- Hybrid stepper motors have improved step resolution with increment angles ranging from 3.6 to 1 degree for 100 to 400 steps per revolution respectively.
- The system incorporates the advantages of both PM and VR types and utilizes a special arrangement of permanent magnet poles over the entire concentric multi-toothed rotor/shaft assembly.
- The multi-toothed rotor assembly exhibits excellent coordination with the magnetic fluxes generated by the stator windings, which guides the rotor movement in a well-controlled manner across the intermediate air gaps.
The above operations of the hybrid type produce more dynamic torque resolutions and high efficiency compared to the PM and the VR types.