Permanent Magnets and
Multiple Horses

Synchronous AC motor eliminates gearboxes

Many of the advances in motor technology over the last 10 years have occurred in the small motor market; that is, those of 1 hp and smaller. How about motors with high accuracy and reliability for industrial applications that require high torque at low speed?

Building on the technology of smaller-scale applications such as those in the computer industry, where a favorable weight-to-performance ratio is an advantage, ABB Inc, New Berlin, WI, has introduced a line of low-base-speed permanent magnet AC motors at the heart of a system they call the DriveIT Direct Drive Solution. The motor design is a radial-flux construction, air- or water-cooled, with a permanent magnet rotor. Powers range from 22 to 670 hp (17 to 500 kW) and include base speeds from 220 to 600 rpm; motor voltages range from 380 to 690 VAC. The line is available in standard IEC-frame sizes from 280 to 400. All of the usual options and modifications available for the traditional cast iron induction motor frames are also available for these motors. 

Designed for variable-speed operation only, the motor must be controlled by an AC drive specifically developed for permanent magnet flux control. The DriveIT low-voltage AC drive, based on the company’s ACS 600/ACS 800 AC drives, has an enhanced Direct Torque Control algorithm and connects directly to a motor/load, without gearboxes or pulse encoders. “The DTC algorithm enables each machine’s motor drive to calculate the state (torque and flux) of the motor 40,000 times per second, which makes the drive virtually tripless,” notes Chuck Hollis, manager of ABB’s ACS 600 drives line. Elimination of the encoder further reduces maintenance requirements, he noted, and encoderless operation dramatically decreases downtime/stoppage, thus increasing a production facility’s uptime and throughput. Optimal motor operation, based on load conditions, also saves energy consumption, yielding energy savings, he said.

With a permanent magnet motor, there is no rotor slip, thus providing better accuracy than standard, asynchronous (or induction), motors. In an asynchronous motor, the slip varies according to speed and load. With a synchronous motor, it is simpler to optimize the speed, and the elimination of slip compensation improves the dynamic motor control. In addition, the elimination of slip losses in the rotor increases motor efficiency by 1 to 3%. The rotor also stays cooler, making higher power density possible. Water-cooling the motor’s stator can increase power density even more. The AC motor delivers high-torque at low-speed — a benefit traditionally associated with DC motors.

The motor’s NdFeB magnets simplify construction by creating a constant flux in the air gap that eliminates the need for the rotor windings and brushes normally used for excitation in synchronous motors. “The result is a motor that combines the high-quality performance of the synchronous motor with the robust design of the asynchronous induction motor,” notes Jan-Anders Bergman, vice president and general manager of electrical machines, for ABB Inc Automation Technologies. “And the motor is energized directly on the stator by the variable speed drive.”

Elimination of the gearbox via these new permanent magnet motors/drive configurations saves space and installation costs, energy and maintenance, and provides more flexibility in production line and facility design. 

—RM