Stepper Motor - Permanent magnet: PM stepper motors generally are thought of as low-torque, large step-angle devices. Torque developed by the stepper motors is far below that for equivalent-size hybrid stepper motors, and step angle generally is 90 or 45° . Position resolution, moreover, is on the order of +10% of step angle, a value that generally relegates the stepper motors to unsophisticated motion-control applications. Maximum pulse rates are for 100 steps/sec for large units to 350 steps/sec for small units. Stepper motors offer a rotor inertia, which is moderate between 5 and 75 gm-cm2.
Rare-earth magnets make possible PM stepper motors having a large number of poles. With a suitable number of poles, PM stepper motors develop more torque than either hybrid stepper motors or dc servomotors. Speed range for the stepper motors is less than that for dc types but much higher than that for hybrids.
Position resolution of the PM stepper motors is less than that for hybrids. But unlike hybrids, some PM stepper motors perform well in closed-loop systems.
Both cemf and iron losses are proportional to the number of poles in the stepper motor. Thus, available torque from a PM stepper motor falls off more slowly with speed than in hybrids and more rapidly than in dc motors. The result is that PM stepper motors operate effectively at higher speeds -- up to about 3,500 rpm -- than hybrids but not as high as dc types. The speed range for PM stepper motors, however, suits a wide range of servo applications.
Stepper Motor - Hybrid: Hybrid stepper motors are frequently chosen for a wide variety of motion-control systems because they are easy to use. Stepper motors can maintain accuracy and reliability in open-loop mode, requiring less complex drive electronics than closed-loop servocontrollers. And absolute positioning accuracy for stepper motors is comparable to closed-loop servocontrollers for many applications.
Stepper Motor: Basics of Stepper Motor Design Engineering
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