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How to Build EV Motors Without Rare Earth Elements
Rare earth elements (REEs) have unique properties that make them indispensable to many forms of modern technology. Some of these elements, such as neodymium, samarium, dysprosium, and terbium, can be combined with ferromagnetic elements such as iron and cobalt to produce crystals that are not only highly magnetic but also strongly resist demagnetization. The metric typically used to gauge these important qualities of a magnet is called the maximum energy product, measured in megagauss-oersteds (MGOe). The strongest and most commercially successful permanent magnets yet invented, neodymium iron boron, have energy products in the range of 30 to 55 MGOe.
The most common motor type at the moment combining the two kinds of torque is the interior-mount permanent-magnet motor, in which the permanent magnets embedded within the rotor add to the reluctance torque. Many commercial EV manufacturers, including GM, Tesla, and Toyota, now use this type of rotor design. The design of the motors for the Toyota Prius underscores the effectiveness of this approach. In these motors, the magnet mass decreased significantly over a period of 13 years, from 1.2 kilograms in the 2004 Prius to about 0.5 kg in the 2017 Prius. Much the same occurred with the Chevrolet Bolt motor, which reduced the overall usage of magnet material by 30 percent compared with the motor in its predecessor, the Chevrolet Spark.
Lately, there’s been a lot of attention focused on a new type of permanent-magnet material, iron nitride (FeN). This magnet, produced by Niron Magnetics, has high remanence, equivalent to that of REE-magnets, but like alnico has low coercivity— about a fifth of a comparable neodymium iron boron magnet. Because of these fundamentally different properties, FeN magnets require the development of new rotor designs, which will probably resemble those of past alnico motors. Niron is now developing such designs with automotive partners, including General Motors.
Yet another REE-free permanent-magnet material that comes up in discussions of future motors is manganese bismuth (MnBi), which has been the subject of collaborative research at the University of Pittsburgh, Iowa State University, and Powdermet Inc. Together these engineers designed a surface-mount permanent-magnet synchronous motor using MnBi magnets. The remanence and coercivity of these magnets is higher than ferrite magnets but lower than neodymium iron boron (NdFeB). The researchers found that a MnBi-magnet motor can produce the same torque output as a NdFeB-magnet motor but with substantial compromises: a whopping 60 percent increase in volume and a 65 percent increase in weight. On the bright side, the researchers suggested that replacing NdFeB magnets with MnBi magnets could reduce the overall cost of the motor by 32 percent.