Step motors are electromagnetic, rotary, incremental actuators that mechanically convert digital pulse inputs to analog shaft rotation. The number of input pulses sets the angular rotation in degrees, and frequency determines the speed. Logic controls for stepper motors interface easily with other mechanisms and can be made bidirectional and synchronous, and to rapidly accelerate, stop, and reverse direction. Furthermore, they have low rotor moment of inertia and don’t drift.
Generally, steppers operate open loop, and often match the response of dc servosystems without the additional cost of feedback controls. There are three basic motor types: variable reluctance (VR), permanent magnet (PM), and hybrid. VR motors are characterized by a soft iron multipole rotor and a wound stator. They generally operate with step angles from 5 to 15° at relatively high step rates, but lack detent torque. When phase A energizes, four rotor teeth align with the four stator teeth through magnetic attraction. The next step turns off phase A and energizes phase B, turning the rotor counterclockwise 15°. Continuing the sequence, phase C turns on next, and then phase A repeats. The rotor turns clockwise when the sequence reverses.
Permanent-magnet motors usually have step angles of 45 or 90° and step at relatively low rates, but provide high torque and good damping characteristics. PM motors also differ from VR designs by having a permanent-magnet rotor magnetized perpendicular to the axis, but lack teeth. When energizing the four phases in sequence, the rotor turns as it attracts the magnetic poles. In this design, each winding produces a 90° step.
Hybrid motors combine the qualities and desirable features of VR and PM motors. They have high detent, holding, and dynamic torques and run at relatively high speeds. Normally they step through angles of 0.9 to 5° and often come with bifilar windings to work with single-source power supplies. When the phases energize individually in the order shown, the rotor turns in increments of 1.8°. The motor can also be driven with two phases energized simultaneously to generate more torque.
Information for this article was provided by API Controls Inc., 45 Hazelwood Dr., Amherst, NY 14228.