Permanent Magnet Motors Scale Up

DC torque at AC costs is coming to higher-horsepower applications

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Industrial motor users have had two main options for producing high torque at low speeds: a DC motor or an AC motor with a gearbox. DC motors perform well at low speeds, but they require extensive maintenance for brush replacement. An AC motor with a gearbox eliminates much of the motor maintenance, but the gearbox adds costs, takes up space, and requires extensive maintenance.

Permanent magnet motors have long been used to provide high torque at low speeds, but these motors were only available in small frame sizes with low horsepower ratings. Small permanent magnet motors can be controlled to very precise speeds, making them especially useful in servo applications such as computer hard drives.

Recent technology advances are now allowing motor manufacturers to produce permanent magnet motors in much larger sizes. ABB (www.abb.com/motors&drives) now makes AC synchronous permanent magnet motors up to 670 HP (500 kW) at 600 rpm. The company also has plans to increase power ratings in the next 6-12 months. It claims no other motor manufacturer makes general-purpose permanent magnet motors in this size range.

Increased power should allow industrial users to apply permanent magnet motors in a wide range of applications. "With separate forced air or water cooling, these motors can be run with 100% torque down to zero speed, making them ideal for applications where high continuous torque at relatively low speeds is needed," says Lars-Erik Thand, area sales manager, ABB Drives and Motors group.

The differences in torque between synchronous permanent magnet motors and squirrel cage induction motors become more pronounced as motor size increases. The accompanying torque comparison chart shows ft.-lbs. of torque on the vertical axis vs. IEC frame sizes/weights on the horizontal axis. These torque values are for motors operating at speed ranges of 100-900 rpm.

Torque differences are insignificant at the low end but, as frame sizes increase, torque difference becomes substantial. The smallest IEC frame size shown (280SM) corresponds to a rating of 15-110 hp depending on speed and torque requirements. The largest IEC frame size shown (400LKC) corresponds to a rating of 150-820 hp.

When a synchronous permanent magnet AC motor is used to replace an asynchronous AC induction motor and a gearbox, there are additional advantages along with the increase in torque. Asynchronous motors have rotor slip that varies according to speed and load. This decreases motor efficiency and makes precise speed control more difficult.

A synchronous permanent magnet motor will typically operate at an efficiency about 1-3% higher than an asynchronous induction motor. Efficiency is further increased because the permanent magnet motor installation does not require a gearbox. The gearbox introduces inefficiencies of its own as it reduces speed and increases torque delivered to the load.

Eliminating slip also allows synchronous permanent magnet motors to be used in variable-speed applications without encoder feedback. Omitting the encoder reduces maintenance and can also decrease downtime.

For a given torque rating, synchronous permanent magnet motors are more compact than asynchronous induction motors. This is due in part to the high values of flux density available with modern permanent magnet materials such as neodymium iron boron (NdFeB).

NdFeB is the most powerful magnetic material available for operation at ambient temperatures, and it is less costly and less brittle than samarium cobalt, another rare earth material widely used in earlier permanent magnet motors.

Synchronous motors cannot be started without an inverter, so it makes sense to design synchronous permanent magnet motors strictly for variable-speed applications. Permanent magnet motors are more expensive than asynchronous induction motors of similar horsepower ratings, but cost per torque is comparable if the expense of the gearbox is added to the cost of the asynchronous induction motor.

Eliminating the encoder can bring the cost of a permanent magnet motor installation below that of a comparable installation with an asynchronous induction motor, gearbox, and encoder. Aside from the initial cost savings, users can also expect lower operating costs due to increased efficiency along with reduced maintenance.

E-mail Dan at dhebert@putman.net.

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