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Electric Motor Breaks New Ground
A patented electric motor design from NovaTorque improves efficiency and design flexibility by shoehorning an axial motor design into a rotary motor package
Joseph Ogando, Senior Editor -- Design News, July 14, 2008
Of all the ways to divvy up brushless DC motor designs, one important one boils down to their fundamental layout. Axial models offer a simple construction and can be designed to produce lots of torque, while rotary motors have a familiar form factor working for them. Now, a truly new motor design from NovaTorque Inc. reconciles these mutually exclusive design approaches.
“When I tell people we’ve come up with a new kind of electric motor, they react with disbelief, but we have. It’s an axial motor with the absolute form factor of a rotary motor,” says John Petro, NovaTorque’s president.
NovaTorque pulled off that feat with what Petro describes as a “simple trick.” Rather than the usual two-dimensional interface between the rotors and field poles, the new motor design uses an opposing pair of magnetic conical rotors, which it creates by mounting magnets on steel cone structures produced in a low-cost powder metallurgy process. The cones, in turn, are attached to the motor shaft.
This strategy addresses one of the key shortcomings associated with axial designs while preserving their intrinsic efficiency. As Petro explains, a traditional axial motor’s steel won’t saturate at the flux levels put out by weak, low-cost magnets. That forces motor designers to either upgrade to powerful but pricey magnets or accept a reduction in performance.
In NovaTorque motors, however, the conical structures act as flux concentrators by increasing the surface area between the rotor and the pole face. This increase allows the motor designer to optimize the magnetic flux density in the field poles and bump up the air gap area for gains in efficiency and torque output, according to Robert Rogenmoser, NovaTorque chief operating officer.
Angles on the cone can vary as NovaTorque tailors the designs to different applications. A 30 percent angle, for example, would double the surface area and the flux density. “It’s pure geometry,” says Rogenmoser.
At the same time, the cones' shape still preserves the straight flux path that helps axial motors avoid efficiency-threatening flux leakage. And the conical sections also allow the motors to be wound with no end-turn copper, a significant source of both I2R losses and added copper cost in rotary motors. “All the resistance in the end-turn copper is waste,” says Rogenmoser. The design also does away with the back irons that also contribute to further losses in rotary designs.
NovaTorque’s models and measured benchmarking studies show significant efficiency gains from going conical. Figures vary depending on the point of comparison on different torque-speed curves, but Petro says overall efficiency improvements can exceed 10 percent, a huge figure in the motor world and one that has implications for motor sizing, thermal performance and even design flexibility. “Efficiency is the root of all motor trade-offs,” Petro says. The motors also shine in the power density department. Petro says they have at least a 100 percent power-density advantage over induction motors and a 50 percent edge over conventional permanent magnet motors.
NovaTorque functions as a development company and plans to license its motor designs. The company does, however, have a pilot production plant capable of producing motors up to about 60 mm, or about 1 hp, for a range of applications in HVAC, appliances and more. The company is also modeling much larger motors, up to a couple hundred hp, for use in electric vehicles.
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