AC Induction motor. Why?

AC Induction motor. Why?

Should someone smarter than me explaine why Tesla uses an AC induction motor as opposed to a DC motor. The AC motor needs the inverter and there has to be some loss in efficiency doing that. Why not use a DC motor since the batteries are DC? My friends are always surprised that the motor is AC and I don't know the reason why that is.

carlk | June 26, 2013

Rare earth magnets are expensive and dangerous to handle. Rare earth resources are scarce and refinement is a very dirty process to the environment. The last one is probably a show stopper for green minded people. Rare earth magnet DC motor has some advantages in small horsepower applications that also need precise control (e.g. stepping motor). At MS power level the two types should be pretty close in performance and efficiency.

LDC | June 26, 2013

This blog entry might help explain the answer:

PaceyWhitter | June 26, 2013

It is also important to note that a DC brushless motor (the current standard DC motor) also requires an inverter to function properly.

carlk | June 26, 2013

Let me add something more on rare earth magnet. Almost all rare earth used in the world today are produced in China. China have known to try to use the supply as bargaining chip in trade disputes with other countries. Can you imagine what would happen when EV production increased to millions a year and all the basic materials are controlled by a single and not very friendly country which happens to have her own ambition on EV? It will be oil and OPEC all over again. Tesla has made a very wise decision to go AC motor imo.

george210 | June 26, 2013

thank you carlk !

your explanation re: rare earth and china use of it as bargaining chips is the best answer yet!!

Thank you Elon for making such a wonderful vehicle!

Any ICE car is "toast"

GeekEV | June 26, 2013

"Rare earth resources are scarce..."

@carlk - Not so. That's a misnomer that was once thought to be true, but now we know better:
Despite their name, rare earth elements are relatively plentiful in the Earth's crust, with cerium being the 25th most abundant element at 68 parts per million (similar to copper). However, because of their geochemical properties, rare earth elements are typically dispersed and not often found concentrated as rare earth minerals in economically exploitable ore deposits. It was the very scarcity of these minerals that led to the term "rare earth".

Xerogas | June 26, 2013

An employee at a Tesla showroom also told me that the AC motor is 3% more efficient as a generator when doing regenerative braking.

DonS | June 26, 2013

Once upon a time, variable speed motors all ran on DC because a DC motor can vary the speed by varying the voltage. Voltage was changed by putting resistors in series, essentially wasting energy as heat. Although modern DC controllers are more efficient, they are still no match for AC systems. There are still a lot of elevators in service using DC motors.

Variable speed AC motors became practical with the commercial introduction of high power IGBTs (insulated gate bipolar transistors) is the early 1980s. IGBTs allow speed control by varying the frequency. Since IGBTs are basically very efficient switches that convert DC to AC, they waste very little of energy flowing to the motor.

carlk | June 26, 2013

@GeekEV I was talking about refined rare earth metal not rare earth elements. Refining for the commercially useable rare earth is a costly and environmentally damaging process. That's the reason it is very expensive and still 95% of world's production are from China.

Read this if you're interested. It's really a foresight on Tesla's part that they chose not to use this material if this is one of the reasons for their decision.

olanmills | June 26, 2013

Wow thanks for the informative posts everyone. That Tesla blog post was great too.

dortor | June 26, 2013

as with all/most/majority of engineering decisions there is no one right answer - there are tradeoffs to be balanced and conditions to be optimized.

Successful engineering/design is the art/science of blending all the possible decisions to achieve a "whole" result that balances numerous factors (performance, efficiency, cost, reproducibility, manufacturing, supply chain...on and on and on)…

anyone who generally state's "why didn't you do thus and so - it's clearly better" is typically identifying themselves as lacking insight and generally fail to appreciate the reality of the situation…

Tesla so far has shown itself to be adept at balancing multiple factors to achieve a desired result…

this more than anything will define their continued success - their choice with regards to the AC induction motor is a bold decision - and one that I agree with - but it could've only been made by someone with a deep understanding of all the trade offs…the blog post is very very insightful and given that it was written in 2007 even more impressive given how far in the future the decision was going to resonate.

the next time anyone offers your a simple solution for a complex problem - they are probably grossly incorrect and massively uninformed - which is why the simple answer is so appealing.

Bubba2000 | June 26, 2013

The AC induction motor that Tesla uses and produces 416 HP, got to be the smallest and lightest motor for its output... only motor smaller than that would be a gas turbine. Back in my day as an engineer, inverters used to be huge. The brushless motor rotor has the magnet and the whole thing is heavy and bulky.

The Chinese tried to squeeze the Japanese companies on the Rare Earths and they would do the same to us. In fact, Elon said that the Model S have very few components from China.

Tesla actually designed the Model S motor, that is what I understand. I thought it was AC Motor? Anyway, it takes a lot of brains to understand EMFs. Gave me a headache. Impressive that they could fit all that in the size of a watermelon.

langzaiguy | June 26, 2013

Anybody know where the motors are manufactured?

KWTESLA | June 26, 2013

Ok JDpilot
AC 3 phase induction motor with a frequency drive ! This is genius . First the the tree phase motor allows very fast starting with near zero lag time . Also AC 3 phase motors have lots of torque this why your S takes off like a slot car on steroids! . A more traditional single phase electric motor might use a capacitor to help start the rotation of the motor giving the loud induction noise as it struggles to over come the starting load. This is the humming and rev- up you are accustomed to with poor quality tools and appliances. The really good ones use a chip controller which can match the power needed.
Next the the frequency drive allows its controller to monitor motor forces and tune the power to just the level needed to give you that flawless speed control that holds 65 MPH on the level or over hills. Have you noticed how smooth the car is to drive!

KWTESLA | June 26, 2013

The Motor is made upstairs in the Fremont plant!

pilotSteve | June 26, 2013

Anyone driving one has to be in awe of how smooth yet capable of controlled EXPLOSION of power the motor/inverter/firmware/battery system implemented by Tesla is.

Awesome pretty fine magic!

wcalvin | June 26, 2013

I was told at a Tech Talk yesterday in Seattle that it was a matter of incremental torque at higher speeds: AC motors (about 90% of all motors) have only slight reductions while DC motors have a fast falloff. If it were a DC motor and you pulled out to pass someone at 55 mph, you'd complain that it was wimpy.

wtdvorak | June 26, 2013

Its an AC motor because its a Tesla. If it used a DC motor it would be called an Edison...

Lush1 | June 26, 2013

Varying the speed of AC motors by changing the frequency has been around for a long time. I work with audio and I remember tape machines in the 60s being powered by a 300 or 500 watt audio amplifier with a variable oscillator driving the input. You could make the tape deck to run at almost any speed by changing the frequency of the tone going into the amp. For normal use, the machines ran off the line frequency of 60Hz, which is pretty dependable in most places. Blew my mind the first time I saw an Ampex tape recorder running off a big Crown amp. I never thought about the way Tesla implemented a continuously variable AC motor. Thanks for the info.


P.S. I also had slot cars in the 60s with DC motors and a variable resistor for the controller. That thing got pretty hot in my hands. Apropos of nothing. Just recalling my old toys as I contemplate my new one.

GeekEV | June 27, 2013

@wtdvorak +1 LOL

Brian H | June 27, 2013

But never an "Edsel". >;p

EVTripPlanner | June 27, 2013

In designing wind turbines, these magnet-free, variable frequency drives have also become the standard:

* no "cogging torque" - this would be annoying at low speed (when magnets align with winding poles)

* much better control (especially important at low speed) - very smooth, reduced mechanical stresses/wear

* higher efficiency (both in gen and regen)

* completely software-controlled (through 500V IGBTs)...this allows refining of controls over-the-air moreso than if using some type of electro-mechanical commutation. Remember that early update that smoothed out the acceleration profile? Do some math...write some code...send bits over 3G...make car better. Magnets are much more difficult to transmit via cellular.

* no brushes, of course (and related arcing, EMI, wear, dust, etc)

* high power density (but you've felt that!)

NASA explains:
Wikipedia even a bit more in depth:

Overall, I'd have to say it was another good choice.

sean | October 28, 2013

@wtdvorak is correct

my two cents is that too many engineers here are overthinking the obvious.

Nikola Tesla father of alternating current was manipulated by Thomas Edison father of direct current. Read books.

Elon Musk created a company to thumb the nose at the thinking and selling of Detroit and Europe's best as we all move into the next phase of motor cars and their engines, just like Nikola did before.

A DC motor of any kind; EVER, would remove the very soul of the intention and naming of Tesla Motors...

Car t man | October 28, 2013

AC motors have lower peak efficiency (the best at about 92%) vs permanent magnet DC motors (up to 97%) but higher overall efficiency across the entire operating range, which is in uses such as vehicles.

AC motor was invented by Nikola Tesla. The first proper controller/inverter was made in 1985 for industrial use and then in 1989 for cars (same guy).

He chose to develop AC drive trains because they don't use rare earths, are more durable (don't demagnetize at high temps), don't need maintenance, are more efficient in vehicular use, could be very smoothly controlled with his controllers, have a wider usable operating range, etc.. They are more refined
and you will find that serious full EVs were all designed on AC drives.

GM's EV1, Mini E, unnamed FIAT (today Chevy Spark), Tesla, Fisker, Mercedes A (not produced), Mercedes B, first EV Fords (got the drive knowledge by buying Norwegian Think program), etc. All of these programs basically use versions of drive trains developed by that one individual. All those programs use his knowledge today, directly or indirectly. The kicker is that he originates in the same village as Nikola Tesla. Must be something in the water there...

Nations use trade wars and blocks regularly so AC drives, much like zinc air batteries (he also made first zinc air batteries) were chosen because of using common and abundant materials.

OPEC members use oil, US technology limitations and sabotage, China rare earths (actually the late comer to the limiting games), so global supply chains aren't reliable, so removing narrow throats was part of the original
global electrification plans (the ones that were then buried by big auto) in 1990ies. So Tesla is reviving a plan already buried and is using the superior drive train technology.

When your friends ask you, you can tell them that expensive magnets demagnetize when overheating and are rendered useless, that magnets are still, in this day and age a nightmare to fit properly, that AC drive trains are more capable and efficient in actual real life use and that DC motors are inferior and older tech, quite usable in forklifts and cheap conversions. Tesla hasn't mastered AC motors quite fully yet, which causes some of the noise issues in some cars, but it will.

Today, batteries are the narrow throat because those inverters made the kind of leap in drive trains, as an affordable 500mile battery would be today. It is drive trains get far less attention than batteries today. The drive train issue was solved sufficiently but that gentleman is currently
working on the next leap in AC drives.

Curmudgeon | October 28, 2013

A 3 phase AC motor is much easier to reverse. The controller
only has to reverse two the phases.

pschrube | December 11, 2015

The Porsche Mission E will use permanent magnet synchronous motors. As I understand it, permanent magnet motors are slightly more efficient than induction motors, but more expensive to produce and maintain, susceptible to heat, and bad for the environment. How does the efficiency compare at high speed? Will the Mission E have a multi-gear transmission?

Does the 8 minute Nürburgring Nordschleife specification require a supercapacitor?

Will the substitution of cameras for rear-view mirrors fly in the US?