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Cutting the price tag on electric vehicles will be key to their widespread adoption.
But the Chinese, who supply 97 percent of the rare earth elements like niobium and samarium that are used to make the powerful permanent magnets that are used in many automotive electric motors (like that in the Toyota Prius) are holding back supply and have raised prices by as much as a factor of four in the past year. China’s move has prompted a search for alternatives.
One that has emerged is what’s known as an AC induction motor. This design, which was used by Chevrolet in its EV-1 electric car in the late 1990s, does away with costly permanent magnets, instead inducing a magnetic field in coils in the spinning rotor through stationary field coils that are located in the motor’s housing. Most of the rotors in AC induction motors are made from cast aluminum but Bob Weed, Vice President of the Copper Development Association, thinks that copper could be a better choice.
“We looked at the AC induction motor. If you replace the aluminum rotor with a copper rotor, which has much better conductivity, you’ll avoid a lot of the losses,” said Weed. “You’ll also avoid the losses of a permanent magnet motor by not having a drag on the motor when it is in a high speed coasting situation,” he added.
The high-priced Tesla Roadster electric sports car uses a hand-fabricated copper and steel rotor to build a motor that weighs just 100 pounds and that can generate 300 horsepower while taking up no more space than a bread box. But building the rotor by hand is expensive, so the key is to develop the same kind of casting technology for copper rotors as is used to make the aluminum versions. The much higher melting point of copper when compared to aluminum makes this difficult, but the CDA has been working with companies in the US, India and China and initial production motors are in production.
Although aluminum is lighter, copper has higher conductance, which means the rotor can be made smaller and thus less expensive while producing a more compact motor with the same power, or can be made the same size, but with 25-30% more power, according to Weed.
“Using a cast copper rotor instead of a fabricated rotor then should be able to get much better performance for much less money.”