EV Motors and Permanent Magnets
11/23/2021 Kevin C. Kurtz

Rare-Earth magnets are higher in cost than copper windings, making the option of using a magnet-less AC induction or brushed DC motor attractive. Many applications (vacuum cleaners, mixers, hair dryers, most home fans, etc) will opt for the less expensive and more common (simpler to control) AC induction motor. You hook up an alternating current source and it spins. Even the original 21st century electric cars didn’t run on PM (permanent magnet) motors. Why is this changing?

There’s one obvious answer, efficiency. Battery life is going to be a big contributor to a lot of decisions as a designer in any portable application. Battery technology has come a long way, but power density is still a concern. We need to utilize the power available efficiently to get as much range out of the vehicle as possible. If you look at most of the battery dominated industries (portable power tools, consumer electronics, auxiliary vehicle 12V accessories, but not limited to DC power sources) it will be evident that there has always been a need for small permanent magnet motors. Justifying the controller design complications and material expense when faced with waning battery life is an easy sell.

Permanent magnet motors use current more efficiently than AC induction motors (less, but not none, loss in heat, stray eddy currents, core losses, etc). They also have their full torque available instantaneously, can be built much smaller in size, don’t require any maintenance (as opposed to magnet-less brushed DC motors), and have lowered in expense as the design efficiency has increased and alternative magnet chemistries and processes have been introduced (for example, we use less dysprosium, a VERY expensive element, in modern high coercivity sintered NdFeB magnets than we did 10 years ago).

The need for a large permanent magnet motor in a power efficient application is what has changed. Typically, these designs were fractional horsepower for portable use. There is a great deal of rare-earth material in the rotor assembly of these motors, but year over year we’ve been able to cut down on these materials (especially the heavy rare-earth materials with the greatest expense and rarity) with the same or similar motor characteristics. By more efficiently designing the permanent magnets in the rotor assembly, experimenting with new soft magnetic support materials, lowering the clearance between the rotor and stator, and designing more sophisticated controllers for speed or more poles in the motor, as well as advancements in balancing and manufacturing (advanced automation, high yield, and very small tolerances) the power efficiency of these PM motors continues to increase as the expense decreases making them the clear choice in modern electric vehicle propulsion.

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