Synchronous motor needs low starting current

DN Staff

January 23, 1995

1 Min Read
Synchronous motor needs low starting current

Bradenton, FL--Ten horsepower is about the practical limit for single-phase motors. This barrier develops because starting inrush current of six-to-nine times running current exceeds the capability of a single-phase supply. Thus, motors larger than ten hp are usually three-phase.

Developed by Precise Power Corp., the Written-Pole motor does not employ the fixed poles used on conventional synchronous motors. Instead, it has a continuous layer of ceramic magnet material on the surface of its rotor. On the stator, the main windings resemble those of conventional motors, with an additional concentrated winding around an exciter pole. While the machine operates, the exciter pole magnetizes or "writes" the magnetic layer into any desired magnetic pole pattern that suits your specific needs.

At speeds significantly below even-pole synchronous speeds, the Written-Pole motor develops a large synchronous torque. A hysteresis induction starting mode brings the motor to partial speed. The exciter coil, energized at line frequency, continuously "writes" a pole pattern in the rotor surface. The pattern is magnetically synchronous with the stator rotating field.

Now the rotor will accelerate with maximum synchronous torque to a predetermined even-pole synchronous speed. At this predetermined synchronous speed, the exciter turns off and the motor begins to operate as a permanent magnet synchronous motor.

This ability to reach synchronous speed using a high-slip starting method requires starting current one-third that of a conventional motor. As a result, practical single-phase Written-Pole motors come with ratings to 60 hp or more. As an option, three-phase motors are also available.

Additional details...Contact Richard T. Morash, P.E., Precise Power Corp., Box 9547, Bradenton, FL 34206, (813) 745-3515.

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