An electric motor that does not need expensive rare-earth magnets • Hacker News Board

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

#21

Post by pjkundert » Fri, Nov 16, 2012, 3:45 PM UTC

neya wrote: post #2

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.

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.

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

#22

Post by mistercow » Fri, Nov 16, 2012, 3:48 PM UTC

jessriedel wrote: post #17

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…

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.

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

#23

Post by jessriedel » Fri, Nov 16, 2012, 4:28 PM UTC

mistercow wrote: post #22

jessriedel wrote: post #17

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…

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 r…

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.

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

#24

Post by akiselev » Fri, Nov 16, 2012, 4:30 PM UTC

ck2 wrote: post #13

lispm wrote: post #3

'rare-earth' does not mean 'rare' Wikipedia: > Although it belongs to the rare earth metals, neodymium is not rare at all.

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.

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

#25

Post by reportingsjr » Fri, Nov 16, 2012, 5:04 PM UTC

neya wrote: post #2

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.

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/...

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

#26

Post by reportingsjr » Fri, Nov 16, 2012, 5:05 PM UTC

Ygg2 wrote: post #6

neya wrote: post #2

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/...

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

#27

Post by stcredzero » Fri, Nov 16, 2012, 5:08 PM UTC

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...

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

#28

Post by neya » Fri, Nov 16, 2012, 5:31 PM UTC

pjkundert wrote: post #21

neya wrote: post #2

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.

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…

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!

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

#29

Post by neya » Fri, Nov 16, 2012, 5:32 PM UTC

reportingsjr wrote: post #25

neya wrote: post #2

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.

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…

Thank you! That link is very informative.

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

#30

Post by [deleted] » Fri, Nov 16, 2012, 5:45 PM UTC

pjkundert wrote: post #14

neya wrote: post #2

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.

Full rated torque at 0 rpm.

[deleted]