Hello,

I would like to know what the mass of Neodymium Iron Boron (NdFeB) is which is contained within the Volt if they are using permanent magnet motors. Does anyone know? I am a researcher from the UK and we work on these magnets. There has been a lot of publicity around the rare earth metal supply (including Nd) as China currently produces 95% of this material and it has introduced quotas (last year the quota was significantly cut and Nd prices rose from $40- > $120/kg). I had heard that this may be having an impact on the technology used in electric and hybrid vehicles and was hoping to find some information. Also does anyone know when the volt is coming to the UK. I would like a buy one. I hear great things about this car.

My understanding is the main traction motor/generator(s) in the Volt use AC motors. While there may be various DC motors controlling some functions of the Volt, I don't believe the main drive motors use rare earth elements.

As to availability in the UK? YMMV (or is it now YKMV?)

Yep, I examined the cuttaway electric motors in the Volt display at the L.A. Auto show, and they have no permanent magnets, use 3-phase AC motors with a special (Voltec) inverter to convert the 360 volt DC from the battery to AC for the motors, and reverse the generator output to DC to charge the battery. At least, that's the way I understand it.

Jerry, #536

Thanks for the information. Very interesting. The Prius has around 2kg of rare earth magnets in the motor and generator as I understand it. I believe you get better efficiency compared to an induction motor so I wonder if the price of Neodymium has driven this decision?

According to a SAE publication, the 111 kW traction motor is AC induction (no permanent magnets), and the 55 kW generator is a permanent magnet type.

GSP

Oh, Ok, I didn't actually look at the cutout of the generator motor, just assumed it would be the same. Hmm. Just guessing again but maybe the magnet style motor is more efficient for generating, and the induction (non magnet) motor is better for propulsion. I saw one article that said there were different advantages with each type, that might explain the difference.

Jerry, #536

Tesla claims that induction motors have a wider efficiency "sweet spot" than permanent magnet motors. They claim that this makes up for a slightly lower maximum efficiency. Since the Volt's main traction motor has to operate over a wider range of speeds and loads, induction may be a better choice for that one.

GSP, #1217 (built, but not yet delivered)

Tesla claims that induction motors have a wider efficiency "sweet spot" than permanent magnet motors. They claim that this makes up for a slightly lower maximum efficiency. Since the Volt's main traction motor has to operate over a wider range of speeds and loads, induction may be a better choice for that one.

GSP, #1217 (built, but not yet delivered)

Here is a link to the Tesla article you mentioned. I found it very informative: http://www.teslamotors.com/blog/induction-versus-dc-brushless-motors

Tesla claims that induction motors have a wider efficiency "sweet spot" than permanent magnet motors. They claim that this makes up for a slightly lower maximum efficiency. Since the Volt's main traction motor has to operate over a wider range of speeds and loads, induction may be a better choice for that one.

GSP, #1217 (built, but not yet delivered)

Not to mention 3Phase induction motors are better suited to contiuous use and can sustain higher rpm ranges because of no physical contacts.

Here is a link to the Tesla article you mentioned. I found it very informative: http://www.teslamotors.com/blog/induction-versus-dc-brushless-motors

Great article. I am wondering why they don't use 3 phase synchronous, DC excited motors for the traction motor. These are the standard for hi-power industrial applications. You would then get the best efficiency and control because you could very the DC excitation and with it the magnetic field strength based on load, and all without any permanent magnets. This motor type requires brushes to apply the DC excitation current to the rotor. Perhaps the issue of brush maintenance makes this solution impractical?

Great article. I am wondering why they don't use 3 phase synchronous, DC excited motors for the traction motor. These are the standard for hi-power industrial applications. You would then get the best efficiency and control because you could very the DC excitation and with it the magnetic field strength based on load, and all without any permanent magnets. This motor type requires brushes to apply the DC excitation current to the rotor. Perhaps the issue of brush maintenance makes this solution impractical?

You answered your own question. The brushes would be an undesireable maintainence item.

With a 3 phase AC induction motor the inverter can vary the AC voltage to optimize the magnetic field stregth (and hysteresis losses) for varying load conditions. It varies the ratio of drive frequency to rotor frequency to control accelerating / decelerating torque per driver input. In addition, the lack of permanent magnets saves cost and avoids dependency on China for rare earth materials. Finally, the 3 phase AC induction motor uses less expensive angular velocity feedback sensors rather than more expensive position sensors used in DC brushless motors.

Well, I am confused. I have heard that our Volt uses a three-phase AC induction motor (which needs no magnets or rare earths) and SharkVolt even saw a cutaway of the motor which confirms this.

Yet right here in this SAE paper from April 2011 it says something completely different:

http://papers.sae.org/2011-01-0355

Quote: "...Both the machines of the Voltec electric drive system are permanent magnet AC synchronous machines with the magnets buried inside the rotor."

A synchronous motor with permanent magnets is definitely not the same as a three-phase induction motor (which would need no magnets.)

So, which is it?

This makes a difference because permanent-magnet motors require rare earths, and induction motors do not.

Anyone have any way to settle this question? I'm currently being embarrassed by this paper in the comments over at SeekingAlpha.com

As the paper you linked to says, the Volt's Motor A and Motor B are both AC synchronous motors rather than AC induction motors.

So yes, the rotors do have interior permanent magnets. It isn't stated whether the magnets are of the rare earth metal type, but I would strongly expect that they are.

Over the years of the Volt's development GM was coy with us about the motor type, but this paper is definitive on the subject.