Machine Design

No power? No problem

Technology from TECO-Westinghouse Motor Co., Round Rock, Tex., allows motors to operate more consistently despite weak, intermittent, or unreliable power supplies.

TECO-Westinghouse has teamed with Precise Power Corp., the originator of what's called the Written-Pole motor concept. Motors using Written-Pole technology combine the efficiency and constant-speed characteristics of a synchronous motor with the high-starting torque of an induction motor.

Interestingly, motors with Written-Pole technology have a rotor surface covered with a continuous layer of magnetic material. Its stator windings are similar to those of conventional induction motors but also contains an additional concentrated winding that forms a writer-pole.

In Written-Pole motors, "permanent'' magnet poles are continuously and instantaneously written on a magnetic layer in the rotor by an exciter pole in the stator. The magnetic poles are written to a different spot on the rotor during each revolution when rotor speed changes. This keeps the pole pattern at a constant poles/sec speed.

Most Written-Pole motors feature an external rotor that spins around an internal stator, opposite that of conventional motors. This inverted structure creates a flywheel effect that allows the machine to ride through 99% of power-quality disturbances, according to company officials. At full load, Written-Pole motors can provide full power to driven equipment through power interruptions as long as 15 sec.

Another advantage is apparent at start-up. The rotor's construction with permanent magnets reduces starting current. In fact, written-pole motors need only one-third the amount of starting current as conventional induction motors.

In a conventional generator, the internal rotor must spin at a fixed speed such as 1,800 rpm to give a steady output frequency of 60 Hz. Two seconds after a power interruption, the conventional rotor's speed decreases to about 1,260 rpm. Since the positions of a conventional rotor's magnetic poles are permanent, when the rotor falls behind, the output frequency drops to 42 Hz -- enough to disrupt equipment. A Written-Pole generator's rotor speed also decreases after a power interruption but the generator's pole-writing coils can rewrite the north and south poles on the magnetizable layer of the rotor 60 times/sec. Essentially, the coils position the poles where they need to be to maintain a 60-Hz frequency, avoiding load disruption.

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