The race is on for magnet-less electric machines for EVs

Image: Chris Mi UofM.JPG

Dr. Chris Mi returned to his native China for the SAE E-motors event. (Lindsay Brooke)

Blog from the 2011 SAE International E-Motors Symposium

SHANGHAI, CHINA, Thursday afternoon, Nov. 17, 2011—In the race to improve EV traction motors, engineers are looking beyond the two popular incumbent technologies. But until new motor architectures are ready to leave the laboratory—and that’s not likely to occur soon—ongoing refinements are expected to keep interior-permanent magnet (PM) and induction machines (IM) spinning the wheels of electric mobility.

E-motor experts here at the 2011 SAE Powertrain Electric Motors Symposium this week discussed the performance, efficiency, cost benefits, and trade-offs of the two machines amid heightened concerns over the cost and future availability of rare-earth metals required for the magnets in PM motors.

Despite their superior torque density, low audible noise, and high peak efficiency, the PM machines’ magnets have motor designers reevaluating IM types as well as switched-reluctance motors (SRM), said David Fulton, Engineering Director at Remy Inc.

“Besides having no magnets, induction machines are robust, have lower material and sensor costs than PM, while providing high power density and low torque ripple,” Fulton told the audience.

Cooling IMs can be more difficult, he noted, due to heat generated by the rotor—“but they’re well suited to spray-oil cooling so you must design IMs for the vehicle cooling system.”

General Motors' new E-Assist system, used to date in 2012 Buick models and the 2013 Chevrolet Malibu, is the world’s first mild-hybrid setup to use IM motors. Their cost delta compared with similar PM types is $300 per system, according to Martin Murray, Director of GM China Vehicle Electrification.

“Traditional magnet-less machines are high-torque and should be reviewed again,” asserted Prof. Zi-Quang Zhu, head of the Electrical Machines and Drives Research Group at the U.K.’s University of Sheffield.

Concurring with many of his colleagues at the SAE event, Zhu noted that “the soaring price for NdFeB (neodymium-iron boron) PM magnets is causing a new look at both synchronous-reluctance and PM-assist and PM-assist synchronous-reluctance machines.”

Unlike IM machines, synchronous reluctance motors do not rely on slip under normal operating conditions. As a result, they produce torque at synchronous speed, Zhu explained. The PM-assist variant features a ferrite magnet or “a small amount of rare-earth magnet,” although there is a reduction of rare-earth material compared with the PM machines, he said.

Continental Automotive is developing a synchronous reluctance drive axle system for EVs.

In a panel discussion, a number of the experts cited the potential of switched-reluctance machines (SRM). While this type of motor has not yet been a choice for electrified vehicles, its flatter peak-efficiency curve (approximately 90%) than that of PM motors, over a broad load range, appeals to engineers. Comparatively simple in design, SRMs do require relatively sophisticated switching control for optimum efficiency. They also exhibit windage losses due to rotor saliency unless the rotor spaces are filled in, which can add cost, according to Remy’s Fulton.

Dr. Chris Mi, Associate Professor of Electrical and Computer Engineering at the University of Michigan-Dearborn, noted that an overall performance delta of only “about 10%” exists between IPM (interior permanent magnet), IM, SRM, and DC-type motors. He offered the following efficiency comparisons:

At 1500 rpm:

IPM motors = 91.3%

SRMs = 85.2%

IMs = 83.1%

At 6000 rpm:

IPMs = 96.1%

SRMs = 95.2%

IMs = 88.2%

“If you’re looking for efficiency, PM is definitely the choice,” Dr. Mi said. “But they’re also significantly more expensive, and heavier than both SRMs and IMs.”

A tear-down analysis of the motor types of comparable specification by Dr. Mi’s university team noted that the IPM had $242 in material cost (primarily laminated steel, copper, and NdFeB magnets) vs. $144 for the IM motor and $74 for the SRM. In terms of overall mass, the IM motor was the heaviest, at 36.25 kg/80 lb, compared with 31 kg/68 lb for the IM machine and 26.7 kg/58.9 lb for the SRM motor.

The E-motor experts also discussed new motor technologies that are currently being investigated. They included flux-switching PM (its magnets are sandwiched within each winding); double-salient PMs; and various hybrid-PM and coil-excited machines. All remain in the laboratory, but the experts left open the possibility that some of them might emerge to disrupt the PM and IM status quo.

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