An electric motor that does not need expensive rare-earth magnets | Hacker News Reader

An electric motor that does not need expensive rare-earth magnets

economist.com

I went to the trouble of learning this just now, so I might as well share:

The most basic electric motor is to have two electromagnets which can rotate with respect to each other. You can make one fixed, but the other has to rotate while still being powered. This requires an electrical connection for a spinning part, and the easiest way was "brushes", apparently. These are literally just metal wires with some spring to them so they can maintain some contact with a rotating part. As you can imagine, this leads to frictional losses and wear, so low reliability.

Another option is to replace the spinning electromagnet with a spinning permanent magnet. That way you don't need the electrical connection, but you do need expensive rare-earth metals to form the permanent magnet.

And then there is the option mentioned in the article, where the spinning permanent magnet is not made of expensive rare-earth metals, but rather of a metal in which a magnetic field is induced by stationary windings.

Yep, thanks.

A lot of modern motors are brushless, and instead of using brushes, they use a controller that monitors the position of the rotor and flips the polarity at the appropriate times.

The methods for monitoring the rotor vary, so you can essentially you can think of brushed motors as a special case of brushless motors. In a brushed motor, the "controller" is essentially a physical switch that flips back and forth as the rotor spins. But physical switches wear out, so less wear-prone methods like measuring back-EMF can make it last a lot longer.

Well, what you say is definitely true but is actually tangential to what I was getting at.

In an old-fashioned brushed motor, the brushes serve two engineering purposes: (1) they allow an electrical connection to a moving part and (2) they automatically accomplish the electrical switching necessary to drive the motor through a full cycle. By "automatically", I mean that they do it by virtue of their physical placement and construction, without any need for computer control.

In distinguishing between brushed and brushless motors, I was concentrating on (1). Brushed motors are simpler, but have flexing metal strands which rub, causing friction and wear. In my mind, this is the primary reason to move from brushed to brushless motors. Now, in addition, when you remove brushes (and make the central spinning magnet a permanent magnet rather than electromagnet) you also get a chance to manually control the switching of the power--per (2)--to the fixed, exterior electromagnet. And this has efficiency advantages which you mention.

I'm sure you know all that, I just wanted to explain my reasoning and be pedagogical for others. Please let me know if I messed up.

Induction motors have long been in existence and they achieve the same goals too. In fact, many popular automotive companies still use Induction motors in their cars (Eg.Tesla). What makes these switch reluctance motors attractive over the induction motors , then? Could someone please clarify??

Thanks.

IIRC, Induction motors are great, but they have one major fault, without some complex circuitry, they can't be instantly switched on/off. Although a polyphase induction motor should not have that problem.

They are not as good as permanent magnet motors except on price/material availability.

See this article: http://electronicdesign.com/article/analog-and-mixed-signal/...

Induction motors used in vehicles are basically always three-phase machines driven by a variable-frequency inverter.

Full rated torque at 0 rpm.

Induction motors are good for some things, bad for others

It has all to do with speed x torque (x power) behaviour, and how it's controlled.

To change the speed on an induction motor is complicated, as opposed to changing the speed in a DC motor, where you just change voltage supplied

Simplicity and reliability. The rotor is completely inert (no windings).

The big drawback has historically been "torque ripple". However, with modern high-speed power electronics, the current to the windings can be modulated to counteract this.

Full torque at 0 RPM. An SR machine combined with efficient backlash-free reducing gearing (torque amplification) may be able to deliver the reliability and torque density to replace frictional brakes. The "holy grail" of a combined traction motor and braking system may be within reach.

Thank you very much. Exactly what I wanted to know. But, just a doubt, even Induction motors offer full torque at 0 RPM, right? Please correct me if I'm wrong, because that's what a reviewer said while comparing a BMW and the Tesla Model S. Thanks!

Depending on the control system, yes. If you run an induction motor off a fixed-frequency supply, it produces very little torque until it is spinning near it's synchronous speed (typically 3600 RPM, but depends on the supply frequency and pole multiplicity). That's why, for example, bench grinders can take forever to come up to speed.

But modern control systems (which, I might add, are still much simpler than those for switched reluctance motors) can achieve full torque from a standstill (basically by bringing the synchronous speed to near zero).

Thank you very much!

Induction motors are not as power efficient as permanent magnet (PM) motors. Induction motors also weigh more compared to an equivalent PM motor and have more torque at lower RPMs. The reason induction motors are used is that they are cheaper than PM motors due to only using materials like aluminum and copper. PM motors use rare earth metals which are pricey and not great for us since China currently controls most of the world's production supplies.

For more detail read this article: http://electronicdesign.com/article/analog-and-mixed-signal/...

Thank you! That link is very informative.

'rare-earth' does not mean 'rare'

Wikipedia:

> Although it belongs to the rare earth metals, neodymium is not rare at all.

That's true for neodymium, but the majority of the world's production comes from China, which can be a concern politically, and if they raise prices, economically too. (Apparently Dysprosium is also mainly mined in China.) The argument usually heard for reducing reliance on rare-earth metals is the political one.

It's not especially rare but it is often difficult to extract. It's certainly not limitless either.

http://web.mit.edu/newsoffice/2012/rare-earth-alternative-en... http://www.rtcc.org/sei-scarcity-of-metals-could-hamper-low-...

It's also "not profitable" enough for corporation to mine in the USA.

But I believe Canadian corporations own the largest deposits in the USA?

That used to be the case but not any longer. Mining companies are working on restoring a few of our rare earth metal mines.

This is very true. If you are interested in learning about Nd magnets, have a read of my May 2012 article for Front of House(FOH) a professional audio trade magazine. Its on page 32 of the following link:

http://digitaleditiononline.com/publication/?i=110979

Apologies for not being able to provide a direct link, it is one of those "virtual issue" magazine sites.

Neodymium-based magnets are very important for the solenoid motor structures used in many types of loudspeakers.

"to deliver a given amount of twisting force—or torque—a reluctance motor has to be larger than an equivalent permanent-magnet motor"

Anyone know how much larger a reluctance motor would need to be? Seems like this would be a problem, since bigger usually means heavier, especially given that the motors are made of iron.

Dyson hand vacuums and their funny-shaped air circulators have been using reluctance motors for a few years. Are they using the same material or do they also rely on neodynium magnets as well?

"Despite that, it can still act as a generator when slowing down, as permanent-magnet motors do in electric cars."

I wonder how this works: The motor requires a control system anticipating how it moves to keep it running. But when used as a dynamo, it's the inverse, something else makes it rotate. Does the control system go to a different mode here?

So assuming we're talking about a synchronous machine aka permanent magnet AC motor, the control of whether the motor acts as a generator or motor is inherent in the physics of the machine. Inside the motor are polyphase AC windings which with the help of an inverter create a rotating electromagnetic field (usually the stator). The rotor position is sensed, and thus the control system can vary the angle between the electromagnetic field and magnets in the rotor. This angle is called the torque angle, and if it it is leading the rotor ("pulling it along") then the machine produces torque. If some external force accelerates the rotor (or if the control changes the phase angle) then the rotor is lagging ("pushing against the magnetic field") then the torque is converted into output current. Practically this means that a synchronous motor spinning at a particular speed (again set by the AC inverter) will consume power if it is under load and produce electric power if externally driven. The AC inverter is designed such that power can flow in either direction. When supplying power to the motor it acts as an inverter, when the motor is supplying power it acts as a rectifier. So basically the control just needs to change the phase angle to control whether the motor produces torque or acts as a generator.

I assume that the operation of the switched reluctance motor covered in the article is similar.

I think the world is getting somewhat bamboozled here. Many of the commenters here correctly noted that induction motors (IMs) are brushless and have no permanent magnets too. Each tooth on the stator can pull the rotor towards it as well as push it away. This is not so for switched reluctance motors (SRM) of this company. The stator teeth in the SRM can only pull the rotor towards it. This is the reason for its worse performance than an induction motor for any given size. Also, SRMs tend to be much louder than IMs or permanent magnet machines. As for any claims of efficiency, the way to think about it is any motor type can be made as efficient as you like so long as you don't care how big it gets. You need to compare motors of the same size to each other before you see differences between the various types. And SRMs that this company is persuing are the worst of the bunch.

The AC induction motor used in the Tesla doesn't use permanent magnets, so no rare earth metals either.

Squirrel Cage Induction Motors have been around for years and have been used in Industry forever.

No brushes, no magnets...

http://en.wikipedia.org/wiki/Squirrel-cage_rotor

And jack all low end torque, something very important to car applications. http://en.wikipedia.org/wiki/File:Variation-couple-uf.svg

With a variable-frequency drive (or better yet, flux vector control) you can achieve full torque at zero speed with a squirrel-cage motor.

If we created demand for Thorium, we could solve the energy and rare-earth problems with one blow. There is a kind of rare earth ore that we could mine profitably, if there was only something to do with the Thorium.

http://www.forbes.com/sites/energysource/2011/03/28/rare-ear...

Thorium power is interesting, but in no way is it a substitute for other rare-earths.

A thorium market would open up development of Monazite sands, which right now are uneconomic because of the cost of thorium disposal. This would give the US a supply of rare earths as well as solving global warming without introducing nuclear proliferation risk. The same technology could also be used to reduce the lifetime of radioactive waste from 10's of thousands of years to under 300.

Here's a nice discussion about Switched Reluctance Motors courtesy of TI (and control thereof): http://www.ti.com/lit/an/spra420a/spra420a.pdf

I know, let's get rare-earth metals from Mars!

Oh wait, that would make them rare-mars metals...

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