Why Axial Flux Motors Are a Big Deal For EVs

The motors are much smaller and lighter than the radial flux machines present in almost all modern EVs.

Mercedes-Benz recently acquired British axial flux motor designer YASA. The company’s high-performance motor technology was on full display in the automaker’s new Vision One-Eleven electric vehicle concept unveiled today. YASA’s motor technology has so far only been seen on low-volume hybrid cars like McLarens and Ferraris, but we’re about to see a whole lot more of them—and for good reason.

In terms of converting electricity into a spinning torque, axial flux motors are about as efficient as standard radial flux machines. The big difference is axial flux’s smaller size and lighter weight for any given power output. As YASA founder and CTO Dr. Tim Woolmer explains in a recent video, those weight and size savings have a waterfall effect in terms of EV efficiency.

Lightweight EVs are more efficient, and if weight can be saved anywhere in the vehicle it means greater range. YASA’s motors are one-sixth the size and half the weight of many comparable radial flux machines. That means less structure is necessary to mount the motor, the battery can be smaller for a given range, and the structure to support the battery can likewise be reduced. Other subsystems like cooling can also be brought down in size and mass.

For a good example of the opposite of this effect, look to the Hummer EV. Its 212-kilowatt-hour battery delivers only around 330 miles of range. The body structure and other systems necessary to support this 3,000-pound behemoth of a pack mean the truck ends up weighing north of 9,000 pounds.

Radial flux machines are hard to beat, though. There were many technical challenges to overcome in making axial flux motors manufacturable and efficient at a large scale. Woolmer has been working on the motors since 2005, and since then he and his team at YASA have solved many of the problems associated with the technology. Radial flux rotors, for instance, are made of a stack of simple laminations that can be manufactured with ease. This technique cannot be used to manufacture axial flux machines, and it takes considerable engineering to get cost parity between the two technologies.

The fact that YASA was bought out by Mercedes seems to indicate that it was able to at least get close. The German automaker plans to allow YASA to continue supplying other OEMs with the motors, however, its first priority is to deploy the technology in its AMG cars, where it will allow for more powerful and efficient EVs as well as more capable hybrids. In the future, it’s likely other manufacturers will hop on the axial flux bandwagon, or perhaps use even more unusual configurations, like transverse flux.

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