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Jun 2nd 2011 | from the print edition

Difference engine

Nikola Tesla's revengeTransport: 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 frettingabout 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 usepermanent 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-vehicletechnology, 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.

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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 currentapplied 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 rotatingmagnetic 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 bothsenses 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.