Much ado about “rare earths”, elements that aren’t actually rare but are sometimes difficult to extract from the ores and have some odd properties. China, the last remaining source of rare earths in quantity, has announced limitations on their export, and this has large implications in electric and electronic technology. Transistors (including the microscopic ones in integrated circuits), LEDs, and lasers require rare earth elements to “warp” the properties of semiconductors so that they do what is wanted; anything that uses magnets needs rare earths to make them smaller and more powerful; and there are many other places where rare earths are needed for efficiency, miniaturization, or some special function. Nowhere are large quantities required, but the small amounts needed are vital.

One of the principal users of magnets is electric motors, and a shortage of the rare earth neodymium (among others) would put a large crimp in electric motor production. Brian Foster at Chicago Boys notes that one of the things that happens when a shortage occurs is that people find ways to do without the item in short supply, including finding substitutes. His focus is the larger point that many people make the error called “static analysis”, assuming that the market follows simple supply-demand laws instead of accommodating the new conditions. To illustrate his point, he cites General Electric’s efforts to conserve or minimize the use of rhenium in jet engines and an announcement from Toyota regarding electric motors for cars:

The Japanese auto maker believes it is near a breakthrough in developing electric motors for hybrid cars that eliminates the use of rare earth metals, whose prices have risen sharply in the past year as China restricted supply. The minerals are found in the magnets used in the motors.

The examples are on point, but are flawed. Rhenium is not a rare earth; it is a platinum-group metal. Toyota’s announcement is an opportunistic attempt to seize a current issue and use it to garner favorable publicity, with only secondary connections to the supply of neodymium for magnets.

One of Foster’s commenters suggests that mining the works of Nikolai Tesla might spur useful innovation, but that would be redundant. One of Tesla’s main inventions, in terms of impact on later technology, was the polyphase motor — and much recent technology focuses on precisely that, although in ways Tesla might not have been able to anticipate. There has been a quiet revolution in electric motor technology in the past quarter-century or so, and a designer of electric motors brought forward by time machine from, say, 1980 would recognize the principles but be flabbergasted by the details.

An electric motor is an example of converting linear force or motion to rotary. In mechanics this function is provided by the crank, such as is found in every reciprocating engine and many other places (including Arizona, although the current example is a metaphor). The two are not precisely equivalent, because (to a first approximation) electromagnetism can only pull. When a crank is “top dead center” the force of the piston is equal in both directions, but the builder can always anticipate (or arrange) a disturbing force, and as soon as the metastable equilibrium is disturbed in the slightest the crank goes around. Once the electromagnetic force is  at top dead center a disturbance results in pushing it back to the starting point, and it holds firmly in place instead of going around. There are two basic ways to overcome this.

A “brush and commutator” motor, more commonly called simply “brush”, “DC”, or “universal”, has multiple coils and a set of switches. Each coil is offset somewhat from the line of force, so it is pulled toward that line. As soon as it gets close to equilibrium that coil is switched off and another engaged; the newly-engaged coil is again offset, so the force turns the motor. The switched fields exert their force on a stationary field, which is provided either by a coil with constant current or a permanent magnet. Coils dissipate power, making the motor less efficient, and have to be built, making it more expensive, so permanent magnets are preferred. The more powerful the stationary field is, the more compact the motor can be for a given power level. Powerful permanent magnets require additives to the basic material, and the best additive known is the rare earth neodymium.

The other way to do it is to have something outside the motor turn the magnetic fields on and off in sequence. Alternating current, pioneered by Tesla, turns itself “on” and “off” many times per second, which is almost there. If there is only a single “phase” — two wires, each taking turns to be positive and negative — it doesn’t help, though. First it pulls to the left, then to the right, but both pulls are at equilibrium, and the motor doesn’t turn. If you have three (or more) wires, again taking turns in sequence, and each is connected to its own set of coils, the motor is pulled to equilibrium at one point, then that phase goes off and another further along pulls it to the next point. That’s the polyphase motor.

The big advantage of the polyphase motor is something that seems almost like magic. A brush motor needs a constantly-existing magnetic field for the switches and coils to work with. Alternating current sent through a coil creates an alternating magnetic field, and an alternating magnetic field creates a current in any nearby conductive object. That induced current causes a magnetic field of its own, and the designer can arrange it so that the induced field is pulled by the magnetism created by the incoming electric current, which is what created the field it’s pulling — as I said, magic. That is the principle of the induction motor, the cheapest and most efficient type of electric motor. No switches, no magnets, no power-robbing “field coils”, no moving mechanical parts except the one that’s supposed to move.

The big disadvantage of the polyphase induction motor is speed control. A brush motor changes speed according to how much current it’s fed, and that can be varied almost infinitely. A polyphase induction motor turns at a rate that is controlled by the rate at which the incoming current alternates, and that is controlled by the people running the generator(s). It can be built to run at half, a quarter, and so on of the generator’s speed, at the expense of additional complexity (and therefore cost), but to smoothly vary the speed you’d have to call the power plant and ask them to change the speed of the generator — and they won’t do it, not least because your neighbor, running off the same current, wants a different speed.

And that’s where the revolution (!) starts.

Transistors and electronic controls enable us to turn current on and off without clunky mechanical switches. The first application of that to motors was to replace the commutator-and-brush system with transistors, to produce the brushless motor. Incoming electricity is divided up among three or more coils, each with a transistor that turns the coils on and off in sequence, and the motor turns. The classic brush motor has the constant field outside, on the stationary part, and the switches and changing field on the part that turns. It was quickly recognized that supplying current to something that’s turning is almost as complex as the commutator is, so the designers turned it inside out. The constant field is on the turning part, and the variable coils are on the outside — which is how an induction motor is built! That’s how computer fans and similar small motors are made nowadays. They’re really polyphase motors with a constant field rather than an induced one.

The limitation of brushless motors is that magnetic materials are relatively fragile. In the inside-out configuration the permanent magnet has to turn, and if it’s turning fast or exerting lots of force it’s likely to explode. If you want a powerful motor with easily-variable speed the magnets have to be on the outside, on the stationary part, and that means you need mechanical monkey-motion — a commutator or “slip rings” — to get the current past the interface between stationary and turning. That’s where we are with electric cars. A car needs a powerful, variable-speed motor that’s compact, and that calls for a motor with permanent magnets on the outside; whether the field-switching is done by transistors or a commutator is a design tradeoff based on expense.

Meanwhile the induction motor fans have been busy, too. Electronic controls also mean you can take the incoming power and convert it to almost any frequency you like, and electromagnetic induction works better at higher alternating rates. Divide up the incoming power among three or more phases, as with the brushless motor, but use an induced field instead of a permanent one, and presto! — a motor that turns at a precise speed controlled by the frequency at which the power is divided up, which can be varied if desired, and it needs neither heavy, expensive magnets nor any connection between the turning part and the stationary one. That’s how the motor in your disk drive works. It has many individual phases (always an odd number) which are turned on and off in sequence at a precisely controlled rate, and the disk platter turns at exactly the speed required to make the data read out at the proper rate.

As electronic controls became more robust, applicable to higher power levels, the next step was variable speed AC drives. Manufacturing plants are full of three-phase induction motors because that’s the least expensive way to get high power, but because the motors were locked in to the speed of the generators they needed expensive, complex gearboxes to get variable speed. An AC drive takes the incoming power, whatever it may be, and converts it to three-phase power at variable frequency. All those millions of old-fashioned three-phase motors can now be recycled, giving near-infinite speed variation without gears, pulleys, or other mechanical devices, and people needing high power at variable speed jumped on that like a pack of rats. As with the disk platter, the speed can be precisely controlled by varying the frequency. That’s what got the Iranians in trouble. Isotope-separation centrifuges require controlling speed very, very precisely, so they are turned by polyphase induction motors driven by AC drives; the “Stuxnet worm” gets into the computer-controlled AC drives and messes them up.

Variable AC gives another advantage: there’s almost no limit on how big and powerful the motor can be. Cajun Dale installs twenty-two thousand horsepower motors to run pipeline pumps, and the next generation of Navy ships will almost certainly have motors that size or bigger turning the screws instead of connecting the turbines to the propellers with big, heavy, expensive reduction gears — they need the electricity anyway for Space-age weapons like lasers and railguns. The only remaining disadvantage is that AC induction motors are somewhat larger than permanent-magnet motors of the same power — and that’s where Toyota’s announcement comes in.

Toyota are saying that they believe they can build an AC induction motor that’s almost as small and compact as a permanent magnet one. That’s big news, because it means the next generation of electric cars can be cheaper and more efficient, but the connection to the supply of rare earths in general and neodymium in particular is indirect. They aren’t finding new sources of neodymium or ways to use it more efficiently; they are doing away with neodymium-alloy magnets altogether. That will certainly affect the neodymium market, and it will certainly make electric motors better, so it isn’t surprising that they would leverage the announcement to get good PR.

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