All BEVs today use radial-flux motors. In a radial-flux motor, a doughnut-shaped wound-copper stator sits around a central rotor often embedded with a series of permanent magnets. They are extremely efficient and well-suited to the task, but they have limitations.
The automaker has published dozens of patents related to the emerging motor topology, but one in particular caught our eye. It describes a system of mechanical field-weakening, which increases the speed of an electric motor at the cost of torque, just like a conventional transmission does.
It could mean smaller, lighter motors with a more useful powerband if it works. The biggest winners would be compact, inexpensive electric vehicles.
Field weakening is typically achieved through complex inverter controls. It creates the “constant power” region of an electric motor’s output curve. If you didn’t know, almost all permanent magnet motors have the same horsepower and torque curve, just at various magnitudes. This is presented in the chart above.
Mechanical field-weakening systems make it much easier to visualize how it works. Here’s an example of a radial flux motor utilizing mechanical field weakening. This is an “outrunner” style radial flux motor where the rotor sits outside the stator. They’re more popular in direct-drive applications.
As the stator is drawn further away from the rotor, the magnetism emanating from the stator gets weaker in relation to the rotor, which means less torque but also higher motor speeds. Just like shifting into second gear, it’s a compromise to get the engine—electric in this case—into a different operating region that works better for a particular situation.
Why go mechanical if field-weakening software works with no moving parts? There are losses associated with doing it electronically. In Japan, for instance, people compete against each other in so-called “Econo Power” races, and many of the vehicles utilize mechanical field weakening to eke out high efficiency.
Mechanical field-weakening is conceptually simpler to achieve with an axial-flux motor, although there are still serious challenges. Axial flux motors are basically two disks opposed to each other with an air gap between them. When turned on, they spin, sure, but they also try to pull each other together. They’re magnets, that’s what they do. It’d be weird if they didn’t.
The force trying to bring these two plates together at high torque ratings is extreme. This isn’t usually a huge problem because you can use a fixed bearing to keep them apart. When you want to do mechanical field weakening, though, the rotor or stator has to move axially to adjust the air gap, but both must also stay totally rigid…