Rare-earth elements have been in the news because the US hardly produces any these days, and one rare-earth, neodymium, makes super-strong permanent magnets, which go into permanent-magnet motors and generators. But the induction motor designed by Nikola Tesla doesn’t use permanent magnets — and it has other advantages, too:
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.
[...]
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.
AC induction motors have been used on diesel-electric locomotives for quite a while now. I wonder why they weren’t initially applied to electric cars?
I feel like there’s been very little cross-over between locomotive technology and automotive technology. Has anyone tried making a diesel-electric tractor-trailer rig?
That “very little cross-over between locomotive technology and automotive technology” is ironic, since GM was for a long time the owner of Electromotive, the main diesel-electric locomotive manufacturer until GE ate their lunch.
I would absolutely love the opportunity to purchase a diesel/electric prime mover with no transmission, dynamic/regenerative braking, and a torque band of nearly 100% from 0 to 6000 RPMs. I would venture to say that a setup like this generating only, say, 150 horsepower would out-pull my current diesel powerplant in my pickup which is nearing 400 horsepower, just by way of the broad torque band, high efficency, and durability. Not to mention that it would likely get better fuel economy, too.
By the way, very interesting post on induction motors. Learned something. Thanks!