Stepping Motors
Disc Motors
Stored energy in the inductance of an iron-core rotor produces arcing at the brushes during commutation. Arcing causes more brush wear than friction. But because a disc motor contains no rotor iron, it does not arc. As a result, brushes in a disc motor may last as long as the bearings.
The lack of inductance in disc rotors also gives a low electrical time constant. Typical time constants are less than 1 msec, allowing disc motors to reach full torque and acceleration much faster than conventional motors.
Most disc motors are rated for peak currents of 10 times the continuous rating to overcome load inertias during acceleration and deceleration. Conventional motors are usually limited to peak currents of only two to three times continuous rating. Otherwise, higher currents produce a large magnetic field in the armature that can demagnetize the magnets.
Another type of flat servomotor for microstep positioning has a slightly different construction. Although it does not contain an iron-core armature, it does have a disc-magnet rotor. The motor stator has two phases that are driven with appropriate drives to obtain stepping. It has more rotor inertia than the previous disc motor, and less than constant torque over its speed range.
Disc motor makers are trying to improve their designs through development of an integral-velocity damper. The damper is a small motor attached to the servomotor shaft that is driven as a generator. It contains the same number of poles as the host servomotor (for example 1.8° steps and 50 pole pairs), but its output is amplified and fed back to the servomotor -- out of phase. Such out-of-phase feedback reduces servo error. Primarily beneficial for incremental motion control, the feedback signal eliminates overshoot or ringing.
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