
On Tesla's induction motor - colinprince
http://www.economist.com/blogs/babbage/2011/04/induction_motors
======
kwantam
The article incorrectly implies that the only way to make a synchronous
machine is with a permanent magnet, and thus all synchronous machines have
temperature issues. On the contrary, it is entirely possible to build
synchronous machines with driven rotors (e.g., through slip rings) that have
none of the temperature issues associated with using permanent magnets (it's
the magnets themselves that are temperatures sensitive, not some intrinsic
property of the synchronous machine design). They are more expensive to
manufacture than the good old squirrel cage induction motor, but synchronous
machines in general are more efficient over a wider range of output rotational
rates than induction machines, have somewhat better failure modes, and are
much easier to start up.

In many cases, it's cheaper to (try to) solve these problems with smart drive
electronics, i.e., add additional complication to the cheap part of the system
rather than to the expensive part. But for the highest output machines---
multi-megawatt generators, for example---synchronous machines remain king.

There are some other fun things you can do with motor drive electronics. A lab
mate of mine in grad school wrote a cool thesis on using the iron in an
induction machine as an electric transformer in such a way that the motor
drive and power converter could be controlled independently. In essence, in a
Y-connected three-phase system, the motor is insensitive to 3rd harmonic
content in the drive signal, so you can pass energy through the air gap in the
3rd harmonic and rectify it on the other side to turn your induction machine
into the magnetics for a power converter.

<http://dspace.mit.edu/handle/1721.1/28691>

~~~
beambot
That's a pretty cool inductive motor hack. Personally, my favorite esoteric
motor has to be the wobble motor built using electropermanent magnets
(essentially, programmable permanent magnets):

[http://www.hizook.com/blog/2010/12/07/electropermanent-
magne...](http://www.hizook.com/blog/2010/12/07/electropermanent-magnets-
programmable-magnets-zero-static-power-consumption-enable-s)

Do you know of any other "cool" motor hacks?

~~~
coryrc
Brusa has an induction motor with magnets embedded in the rotor. At high
torque, the induction overrides the permanent magnets. At lighter loads, the
permanent field dominates and turns it into a synchronous motor.

[http://www.diyelectriccar.com/forums/showthread.php/evdl-
syn...](http://www.diyelectriccar.com/forums/showthread.php/evdl-synchronous-
induction-hybrid-motori-5938.html)

~~~
kwantam
Another friend of mine from grad school did his thesis on something like this.

<http://dspace.mit.edu/handle/1721.1/18013>

"We propose a novel variation of a doubly-fed induction generator which aims
to improve power density and simplify construction. Our design is a doubly-
fed, dual-rotor, axial-flux, permanent-magnet machine."

The magnets for that thing were truly scary. Every once in a while I gave him
a hand with the assembly and it took three or more of us to safely install
each magnet in the rotor.

------
ChuckMcM
Ah yes, AC motors, the bane of most of the EE's in my graduating class since
computing electromagnetic fields in a rotating context was such old
technology. At the time the only 'use' for them was machine tools and blower
fans, both of which took advantage of the fact that you could plug them
directly into a 3-phase power line and they would run at 60 RPM all day and
all night.

Variable frequency 3-phase inverters which can support the current capacity to
generate 100HP+ (75KW) was really only invented with iGBTs in the last couple
of decades. The article doesn't mention that the inverters have to be cooled,
as they are a significant source of heat as well.

~~~
kragen
I had no idea about IGBTs. Thanks!

[http://en.wikipedia.org/wiki/Insulated-
gate_bipolar_transist...](http://en.wikipedia.org/wiki/Insulated-
gate_bipolar_transistor)

------
turbofail
Another permanent-magnet-less motor design is the switched-reluctance motor.
Its primary advantage over the induction motor is its dirt-simple rotor
construction - it's essentially just a properly shaped chunk of iron, where an
induction motor would require a copper squirrel-cage.

Its disadvantage relative to the induction motor is that it requires some
cleverness in its controller and/or sensors in order to run at all. I actually
wrote some control software for one as a side project - it was a somewhat
frustrating experience (though a good portion of the frustration was due to
malfunctioning hardware).

<http://en.wikipedia.org/wiki/Switched_reluctance_motor>

~~~
pjkundert
Also interesting is their capability to produce their full Torque output at 0
RPM. For applications requiring high torque for "station keeping", this is
crucial, and is pretty unique to SRM (Switched Reluctance Machine) motors.

I've been quite interested in repurposing the little Dyson "digital motor",
but I suspect that a complete drive system redesign would be necessary to
enable full variable RPM control (ie. 0-max, in both directions).

Question for turbofail: did your project happen to use the SRM motor from the
Neptune front-load washers, or some other motor?

~~~
turbofail
It was a custom-made SRM made by someone else. They were intending to use it
in an electric vehicle.

------
brianbreslin
Interesting point to note here (since this article seems to float from point
to point) is the fact that tesla's battery to wheels efficiency ratio is 88%,
which the article states is 4x the average. Seems to me like Toyota's
investment in tesla was a better deal than we initially assumed.

~~~
eru
Nothing to write home about. Electric engines are efficient.

If you look at the efficiency from, say, coal burned in the power station to
electricity in the batteries to driving, electric cars fare similar to
internal combustion engines. (Burning your fuel in the big power station is
more efficient than a car's internal combustion engine, but going through the
extra steps of transmitting the energy to your home and then into the
batteries eats some of that.)

~~~
rickmode
Consider transportation of oil and gas (petrol / auto gasoline) along with gas
refining versus electricity generation and transmission.

In any case full electric cars allow centralized power generation, in turn
allowing more opportunities for clean power.

~~~
protomyth
Liquid fuel does have one serious upside that electric doesn't: storage.

~~~
coryrc
And recharge time: I calculated once that refilling your gas car occurs at a
rate of approximately 5 Megawatts!

But, for 95% of trips, an overnight charge at 120V is sufficient.s

~~~
athom
What we really need for electric cars is battery swapout. Just pull into the
nearest station, trade out the drained battery for a fresh one, and you're
off! Meanwhile, the station sets the drained battery on the charging array,
and in a few hours, it's ready for the next customer!

In theory. I can see at least a couple of challenges in making such a system
feasible. It's not like you can just shove the batteries into a massive
underground tank, for example.

~~~
VladRussian
swapout is DOA. 30kw/h = 1000A x 120V x 15min. With coffee and wi-fi 15 min is
nothing for most of the people. Replacing pumps with electricity chargers, the
gas stations will easily morph into Starbucks and Starbucks/Safeways/etc...
will morph into charging stations.

~~~
chancho
15 minutes? Are you kidding? It takes that long to get your oil changed and
most people can't even be bothered to do that every 3 months / 3000 miles.

~~~
VladRussian
you are mixing things that can be delayed till next Monday and things that
can't

------
arepb
One of the biggest benefits -- which thankfully gets a line or so halfway down
-- is that induction motors require far less cooling accessories. This is big
for cost and weight savings.

~~~
adovenmuehle
A good point. With low weight and maximum torque from 0 to 6000 rpm, it will
be interesting to see what performance cars will look like in 20 years.

------
juiceandjuice
I think that a 350 mile range and 10 minute charge is ultimately the holy
grail for electric cars. Most cars built try to shoot for 400 on highway gas
mileage.

~~~
ars
10 minute charge is not possible with the current grid. You would need 1,500A
service - just for the car (most homes have 200).

The cables would be as thick as your arm.

You can do battery swaps instead, but it requires an infrastructure build out
first.

~~~
juiceandjuice
I never said the home would be the primary charge point, you wouldn't charge
like this at home, but at a charge station.

I'm implying that these are the specs necessary for an electric car to truly
be taken seriously as a replacement for a gas vehicle.

Furthermore, charging at a higher voltage, say even 720 volts (about twice the
current roadster's voltage) gets you down to 500 amps to charge a Tesla in 10
minutes, which is manageable for cables. Charge stations could have a
reserve/buffered system they can charge from that way they wouldn't be hitting
the grid all at once if 5-6 cars show up. You could do a similar thing at your
house (basically a large UPS)

The battery swap thing is a pipe dream, IMO.

------
1053r
The article incorrectly states that the Nissan Leaf needs a gearbox. In fact,
it has a single reduction gear and an AC induction motor, just like the Tesla
Roadster. (Well, ok, not as powerful...) As a Leaf owner, I can tell you it
makes for a very interesting torque curve. From 0-30 it is VERY fast, from
30-60 it is pretty average, and from 60-90 it is dog slow acceleration.

~~~
eru
Which units are you using?

~~~
sbierwagen
Leagues per hour, of course. Just like how I measure fuel consumption in rods
per hogshead.

~~~
kragen
Handy conversion factors: one league per hour is 4.82 kph, 1.34m/s, or if
you're stuck in a backward country that uses obsolete units like hogsheads,
3mph.

One rod per hogshead is 4.7 million liters per 100km, 0.021 m/ℓ, or 49μmiles
per gallon.

------
amitraman1
It's interesting how inventions from the early part of the 1900s are becoming
more useful today. I remember reading an article which stated that engineers
were re-discovering mathematical models from the early 1900s for use today.

The big concern I have is that we'll end up with more expensive hybrid cars
and China will have the cost-effective stuff.

~~~
eru
Yes, re-using old inventions is important.

As another example: In the last 20 years there has been an explosion in the
power of mathematical optimization, because numerics got fast and stable
enough that algorithmic techniques that had been put into the bottom drawer
decades ago have been resurrected. Those techniques were initially numerically
unstable (or needed too much computation to keep them stable).

------
d2
I drive a 2001 Ford F350. I get dirty looks from Prius drivers every day.
Little do they know (and the econ article mentions this) that their little
buzzbombs contain 2.2 pounds of neodymium which is both radioactive and
massively damages the environment in its production and disposal.

I bought my truck used. I bought it with 150k miles on it and I plan to put on
another 400k miles before it goes to the big diesel garage in the sky. These
f'ing Prius yuppies buy a car every 2 years thinking that saving a few gallons
of gas is going to save the planet. Try saving the environmental cost of
producing that horrible pile of radioactive and chemical garbage by buying a
10 year old used car and driving it for the next decade.

Edit: And buy American darnit!

~~~
kragen
How radioactive is neodymium? It looks like most of its isotopes are stable
and the rest have half-lives significantly exceeding the current age of the
universe, just like ²³⁸U.

~~~
d2
PBS video on rare earth mining in China, specifically mentioning the Prius
engine and battery e.g. the battery contains 22 pounds (10 kg's) of lanthanum.

<http://www.youtube.com/watch?v=JL4fluj004o>

~~~
kragen
I think you responded to the wrong comment? I wasn't talking about lanthanum,
although it's slightly radioactive too.

------
VomisaCaasi
This happened yesterday:

American mining company Molycorp has bought up a 90.023 share in Estonia's
rare earth metals producer AS Silmet, taking control of one of very few
sources of the much-sought materials outside China.

[http://news.err.ee/economy/c12a9355-2b41-46d3-b00e-875806144...](http://news.err.ee/economy/c12a9355-2b41-46d3-b00e-875806144126)

Relevant, I hope.

------
ck2
Rare-earth elements are not actually that rare.

A few Canadian companies hold the rights to many of the resources in the USA.

For example there are some nice lithium sources in the USA but the Canadian
companies will not mine them until there is a certain profitability level.

~~~
PotatoEngineer
I suspect that even if the resources were owned by US companies, they still
wouldn't mine them, for exactly the same reason.

~~~
ck2
Yes but it's an actual "homeland security" issue that we don't own the
resources in our own country that are critical for our future
infrastructure/transportation needs (vs. the theater issues at the airport).

~~~
elai
Being owned by canadians, an economy that is intimately tied and dependent to
the fate of the US economy, speak the same language and are fairly friendly
neighbours overall isn't that bad. And if the US really wanted to, they could
pass some BS law to retake those land claims.

------
nickpinkston
"it is puzzling to see why anyone should worry about potential shortages of
neodymium and other rare-earths for alternative power and transport."

Well, I'm sure the military doesn't want to redesign everything that already
uses them - we're broke enough as it is...

