Stepping Motors
Disc Motors
Disc motors are permanent-magnet stepping motors that exhibit performance comparable to that of hybrid motors. Rotors in disc motors are thin (typically less than 1-mm) discs, unlike the cylindrical rotors in hybrids and conventional permanent-magnet motors. Conventional permanent-magnet motors generally are limited to a minimum step angle of 30° for a maximum of 12 steps/rev.
Until recently, stepping motors provided 200 steps/rev at most, resolution previously available only through hybrid stepping motors. The latest version, which has a 0.9° step angle, provides 400 steps/rev. Due to very low distortion of the output torque sinusoid, microstepping accuracy is ±3% maximum.
The disc motors are generally a little over half as big as hybrid motors of equal power output and weigh 60% less. The rotors and magnetic circuits in the stators are quite light. And the stators are molded plastic rather than metal.
While a variety of disc motors have been available for about 20 years, they are still frequently used for their low inertia and high torque. Recent improvements include neodymium magnets and power ratings up to 250 W for industrial-grade motors.
A conventional dc servomotor uses an iron-core armature with coils of wire wound in slots. However, some servomotors contain a flat armature constructed of several layers of copper conductors bonded to a rigid insulating disc. Since no iron is needed in the disc armature, it has a very low inertia. The thin armature design provides a high torque-to-inertia ratio. High ratios produce accelerations from 0 to 3,000 rpm in only 60° of rotation.
Conventional motors also produce cogging when the permanent magnets in the stator try to line up with the iron in the rotor. This happens even if the motor is not powered and may not be tolerable in some applications. In contrast, disc armatures contain no iron and, thus, do not cog. The result is a servomotor with very smooth rotation at any speed.
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