
An electric motor that does not need expensive rare-earth magnets - JumpCrisscross
http://www.economist.com/news/science-and-technology/21566613-electric-motor-does-not-need-expensive-rare-earth-magnets-reluctant-heroes
======
jessriedel
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.

~~~
snarfy
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.

~~~
jessriedel
Yep, thanks.

------
neya
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.

~~~
Ygg2
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.

~~~
reportingsjr
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/...](http://electronicdesign.com/article/analog-and-mixed-
signal/induction-motors-pm-synchronous-motors-operate-74650)

------
lispm
'rare-earth' does not mean 'rare'

Wikipedia:

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

~~~
ck2
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?

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

------
jobu
"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.

------
yardie
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?

------
Aardwolf
"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?

~~~
SpacemanSpiff
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.

------
sve
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.

------
ck2
The AC induction motor used in the Tesla doesn't use permanent magnets, so no
rare earth metals either.

------
tradotto
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>

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

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

------
stcredzero
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...](http://www.forbes.com/sites/energysource/2011/03/28/rare-earth-
elements-and-thorium-power/)

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

~~~
stcredzero
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.

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

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

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

