The soaring cost of rare earth oxides (REOs) has forced the price of magnets to rise, driving component manufacturers, machine builders, and OEMs to seek out ways to reduce the impact of the effect. The problem is that, when it comes to permanent-magnet motors, we'd come to take the benefits of REOs for granted.

For example, until very recently, the combination of the enhanced magnetic field strength introduced by neodymium and the thermal resilience contributed by dysprosium enabled the economical production of compact, high-torque motors that can survive harsh conditions. With the REO price jump, engineers find themselves considering alternative designs to minimize the amount of rare earth magnets in a system without hurting performance.

This post, the second in a five-part Design News series on rare earth materials, looks at engineering approaches to mitigate the effects of rare earth element (REE) prices on motor costs. In part 1, we set the context and outlined the four basic design approaches engineers can use to tackle the problem. Here, we'll focus on the first method to reduce the amount of rare earth magnets in permanent-magnet motors.

Faraday's Law, which relates EMF to magnetic flux, describes the basic physics behind motor operation. In turn, we can express flux through a loop as a function of magnetic field and area. As a result, adjusting the geometry of a motor, particularly the windings, can give designers degrees of freedom for reducing magnet content while maintaining torque levels. "If done carefully, shortening the axial length of a motor and increasing its diameter to meet the same performance specifications can lead to less magnet material," said Duane Hanselman, associate professor of electrical and computer engineering at the University of Maine.

Making tradeoffs
Motors can be magnet-oriented (to minimize copper windings) or copper-oriented (to minimize magnet material). Back when neodymium was cheap and copper was expensive, motor designers let the REO magnets do the work. Adjusting to the new cost basis just requires redistributing the active materials within a given volume essentially to compensate for the reduced magnet material with copper.