Transport: The car industry’s effort to reduce its dependence on rare-earth elements has prompted a revival in the fortunes of an old-fashioned sort of electric motor

ONCE again, worrywarts are wringing their hands over possible shortages of so-called “critical materials” crucial for high-tech industries. In America the Department of Energy is fretting about materials used to manufacture wind turbines, electric vehicles, solar cells and energy-efficient lighting. The substances in question include a bunch of rare-earth metals and a few other elements which—used a pinch here, a pinch there—enhance the way many industrial materials function.

It is not as though the rare-earth elements—scandium, yttrium and lanthanum plus the 14 so-called lanthanides—are all that rare. Some are as abundant as nickel, copper or zinc. Even the two rarest (thulium and lutetium) are more abundant in the Earth's crust than gold or platinum.

A decade ago America was the world's largest producer of rare-earth metals. But its huge open-cast mine at Mountain Pass, California, closed in 2002—a victim mainly of China's drastically lower labour costs. Today, China produces 95% of the world's supply of rare-earth metals, and has started limiting exports to keep the country's own high-tech industries supplied.

The rare-earth element that other industrial countries worry about most is neodymium. It is the key ingredient of super-strong permanent magnets. Over the past year the price of neodymium has quadrupled as electric motors that use permanent magnets instead of electromagnetic windings have gained even wider acceptance. Cheaper, smaller and more powerful, permanent-magnet motors and generators have made modern wind turbines and electric vehicles viable.

That said, not all makers of electric cars have rushed to embrace permanent-magnet motors. The Tesla Roadster, an electric sports car based on a Lotus Elise, uses no rare-earth metals whatsoever. Nor does the Mini-E, an electric version of BMW's reinvention of the iconic 1960s car. Meanwhile, the company that pioneered much of today's electric-vehicle technology, AC Propulsion of San Dimas, California, has steered clear of permanent-magnet motors. Clearly, a number of manufacturers think the risk of relying on a single source of rare-earth metals is too high.

The latest carmaker to seek a rare-earth alternative is Toyota. The world's largest carmaker is reported to be developing a neodymium-free electric motor for its expanding range of hybrid cars. Following in AC Propulsion's tyre tracks, Toyota is believed to have based its new design on that electromotive industrial mainstay, the cheap and rugged alternating-current (AC) induction motor patented by Nikola Tesla, a Serbian-American inventor, back in 1888.

Think of it as a rotating transformer, with the primary windings residing in a stationary casing (stator) and the secondary conductors attached to an inner shaft (rotor). The stator surrounds—but does not touch—the rotor, which is free to rotate on its axis. An alternating current applied to the stator's windings creates a rotating magnetic field, while simultaneously inducing a current in the separate conductors surrounding the rotor. With an alternating current now circulating within it, the rotor creates a rotating magnetic field of its own, which proceeds to chase the stator's rotating field—causing the rotor to spin in the process and generate torque.

Modern AC induction motors usually have three (or more) sets of stator windings, which smooths things out and allows more torque to be generated. Such machines are known as “asynchronous” motors, because the rotor's magnetic field never catches up with the stator's field. That distinguishes them from “synchronous” motors that use a permanent magnet in their rotors instead of a set of conductors. In a synchronous motor, the stator's rotating magnetic field imposes an electromagnetic torque directly on the fixed magnetic field of the rotor, causing the latter assembly to spin on its axis in sync with the stator field. Hence the name.

In the past the main problem with asynchronous induction motors was the difficulty of varying their speed. That is no longer an issue, thanks to modern semiconductor controls. Meanwhile, the induction motor's big advantage—apart from its simplicity and ruggedness—has always been its ability to tolerate a wide range of temperatures. Providing adequate cooling for the Toyota Prius's permanent-magnet motor adds significantly to the vehicle's weight. An induction motor, by contrast, can be cooled passively—and thereby dispense with the hefty radiator, cooling fan, water pump and associated plumbing.

Who needs a gearbox?

Better still, by being able to tolerate temperatures that cause permanent magnets to break down, an induction motor can be pushed (albeit briefly) to far higher levels of performance—for, say, accelerating while overtaking or climbing a steep hill. Hybrid vehicles like the Toyota Prius or the Chevrolet Volt have to rely on their petrol engines and gearboxes for extra zip. By contrast, the Tesla Roadster uses just one gear—such is the flexibility of its three-phase induction motor.

In moving to a pure induction design, Toyota will be taking a page out of Tesla's book, in both senses of the name. Weighing in at 115lb (52kg), the Roadster's tiny three-phase induction motor is no bigger than a watermelon. Yet it packs a hefty 288 horsepower (215 kilowatt) punch. More impressively, the motor's 295lb-ft (400 newton-metres) of torque is available from rest to nearly 6,000 revolutions per minute, which eliminates the need for a conventional gearbox. The result is a motor that is light, compact and remarkably efficient.

Overall, the Tesla Roadster achieves a battery-to-wheels efficiency of 88%. That is three times better than a conventional car. With its vast engineering resources, Toyota could well do even better. And somewhere, Nikola Tesla must be smiling.