ONCE again, worrywarts in Washington are wringing their hands over possible shortages of so-called “critical materials” for America's high-tech industries. In particular, the Department of Energy frets about certain metals used in manufacturing wind turbines, electric vehicles, solar cells and energy-efficient lighting. The substances in question include a bunch of rare-earth metals plus a handful of other elements which—used a pinch here, a pinch there—enhance the way many industrial materials perform.
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 widely spread throughout the Earth's crust than gold or platinum. But because they have similar chemical properties, and tend to be lumped together in rocks along with radioactive thorium and uranium, extracting and refining them can be difficult, expensive and messy. Disposing of the toxic waste is one of the biggest headaches.
A decade ago, America was the world's leading producer of the rare-earth metals. But its huge open-cast mine at Mountain Pass, California, closed in 2002—a victim of China's much lower labour costs, America's increasingly stringent environment rules, and delays in renewing the mine's operating licence. Today, China produces 97% of the world's supply of rare-earth metals—a by-product of the country's vast iron-ore mining operations in Inner Mongolia. Over the past year, the Chinese authorities have cut back drastically on exports of rare-earths, as China's own high-tech industries absorb more of the output (see “More precious than gold”, September 17th 2010).
The rare-earth the Department of Energy seems particularly paranoid about is neodymium. This is widely used for making super-strong permanent magnets. Over the past year, the price of neodymium has quadrupled, as electric motors and generators that use permanent magnets instead of electromagnetic windings in their rotors have proliferated. Cheaper, smaller and more powerful, permanent-magnet machines have been one of the main factors behind the increasing popularity of wind turbines and electric vehicles.
That said, not all makers of electric vehicles have rushed to embrace permanent-magnet motors. For one, the Tesla Roadster, an electric sportscar based on the Lotus Elise, uses no rare-earth metals whatsoever. Nor does the Mini-E, an electric version of BMW's recreation of the iconic 1960s car. Meanwhile, the company that pioneered much of today's electric-vehicle knowhow, AC Propulsion of San Dimas, California, has steered clear of permanent-magnet technology. More recently, Continental AG, a German car-components firm, has developed an electric motor for a forthcoming European electric vehicle that likewise uses no rare-earths. Clearly, a growing number of car companies think the risk of depending on a single (and not so reliable) 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 developing a neodymium-free electric motor for its expanding range of hybrid cars. Following in AC Propulsion's footsteps, Toyota has based its new design on industry's electromotive mainstay, the cheap and rugged alternating-current induction motor patented by Nikola Tesla, an American inventor, back in 1888.
Tesla's invention is, in essence, a rotating transformer. Its primary windings reside in a stationary steel casing (the stator) and and secondary conductors are attached to an inner shaft (the rotor). The stator surrounds—but does not touch—the rotor, which is free to rotate about 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 attached to the rotor. With an alternating current now circulating within it, the rotor creates a rotating magnetic field of its own, which then proceeds to chase the stator's rotating field—causing the rotor to spin in the process and thereby generate torque.
Modern induction motors usually have three (or more) sets of stator windings, each using a different phase of the alternating current being applied. Having three “waves” of magnetism induced in the rotor with every revolution, instead of just one, smooths out the induction process 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 aluminium or copper conductors. In a synchronous motor, the stator's rotating magnetic field imposes an electromagnetic torque directly on the fixed magnetic field generated by the rotor's permanent magnet, causing the rotor-magnet assembly to spin on its axis in sync with the stator field. Hence the name.
In the past, the main disadvantage of 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.
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 hard while overtaking, or when climbing a steep hill. Hybrid vehicles like the Toyota Prius or the Chevrolet Volt have to use their petrol engines to get extra zip. Pure electric vehicles such as the Nissan Leaf depend on gearboxes to generate the extra torque for arduous tasks. By contrast, the Tesla Roadster uses just one gear—such is the flexibility of its three-phase induction motor.
So far, Toyota has remained mum about its neodymium-free electric motor-generator. The design used in the current version of the Toyota Prius (the car actually has two such units, one for propulsion and regenerative braking, and the other to run all the on-board accessories) combines both conductors and a permanent magnet in its rotor core. On light loads, the unit works more like a permanent-magnet motor. On heavier loads, the induction features predominate.
In moving to a pure induction design, Toyota could do worse than take a page out of the Tesla car company's manual. Weighing in at 52kg (115lb), the Tesla Roadster's tiny three-phase induction motor is no bigger than a watermelon. Yet it packs a hefty 288 horsepower punch. More impressively, the motor's 400 Newton-metres (295 lb-ft) of torque is available from rest to nearly 6,000 revolutions per minute. Having access to such a wide torque band eliminates the need for a second or third gear in the transmission. The result is a power unit that is light, compact and remarkably efficient.
Overall, the Tesla Roadster is said to achieve a battery-to-wheels efficiency of 88%—three times better than a conventional car. With Nikola Tesla's robust and reliable induction motor making such a successful comeback, it is puzzling to see why anyone should worry about potential shortages of neodymium and other rare-earths for alternative power and transport.