[raverbashing]

I stand corrected about not being able to use induction motors as generators, still

As per wikipedia

> An induction generator produces electrical power when its rotor is turned faster than the synchronous speed... An induction generator usually draws its excitation power from an electrical grid

Which is not what they want in a car

Also they do want good torque at zero speeds, while it can be done with induction motors it's not ideal

[smaddox]

The Tesla roadster uses PM free induction motors: https://www.tesla.com/blog/induction-versus-dc-brushless-mot...

According to the article, the torque limitations are only for a poorly controlled drive current.

[raverbashing]

Good article. And I quote:

> Today, all the hybrids are powered by DC brushless drives, with no exceptions. The only notable uses of induction drives have been the General Motors EV-1; the AC Propulsion vehicles, including the tzero; and the Tesla Roadster.

> Induction machines are more difficult to control. The control laws are more complex and difficult to understand. Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless. This means added development costs, but likely little or no recurring costs.

A hybrid has different characteristics and requirements from the motor than a purely electrical vehicle as well

[jacquesm]

> An induction generator produces electrical power when its rotor is turned faster than the synchronous speed...

Yes, that's called overdriving the motor.

>> An induction generator usually draws its excitation power from an electrical grid

> Which is not what they want in a car

You can generate very nice sine waves using modern electronics, tri-phase is also not a problem.

> Also they do want good torque at zero speeds, while it can be done with induction motors it's not ideal

Triphase motors have fantastic torque at zero speeds, see also every (diesel-)electric locomotive ever produced.

[stevendhansen]

Yes, induction machines do draw power in order to establish a magnetic field where as PM machines get it for "free" by virtue of the magnets. Also the torque production at zero speed is almost identical in induction machines vs. PM machines, so this isn't an issue.

For EV applications one of the cool things about induction machines is that they can actually achieve higher efficiency at high speeds than a PM machine. This is because at high speeds the spinning magnets in a PM machine induce losses (so called eddy-current and hysteresis losses) which are difficult to suppress. In an induction machine you are controlling the field directly so it can be weakened at higher speeds with a control loop to directly optimize the losses vs. torque production. This means that there is a trade-off speed at which an induction machine becomes more efficient than a comparable PM machine.

To mitigate this issue in PMs some vehicle manufacturers have tried to do cool tricks like partially demagnetizing the magnets at high speeds to reduce losses, but this is very difficult to do without damaging the magnets permanently.

Motor size is approximately proportional to torque, not power, so you can imagine a small, low torque induction machine that is fast and will have better efficiencies than a corresponding PM machine. In stop and go traffic or low speed travel the PM machine typically has better efficiencies.

Personally I hope in the long run we end up with induction machines in our electric cars. 1) No safety issues with spinning magnets at high speed during an inverter failure, 2) no reliance on rare-earth materials, 3) the control of induction machines is very cool in its complexity and gives an additional degree of freedom on field flux that is only partially controllable in PM machines, and 4) induction machines have longer lifetimes as there is practically nothing to wear out aside from the bearings.

[vvanders]

The Model S also uses an induction motor(s)[1] including power recovery via regenerative braking.

[1] https://insideevs.com/video-primer-tesla-model-s-electric-mo...

[sunstone]

Yes but apparently the Model 3 uses permanent magnet motors. I would be interested to see the detailed engineering trade offs between these two choices.

https://www.reddit.com/r/teslamotors/comments/6strnb/model_3...

EDIT: Here's a bit of thought on why they might have switched technologies in the motor:

https://www.edmunds.com/car-news/auto-industry/2017-tesla-mo...

[sunstone]

And for those joining in late here it is straight from the horse's mouth, the head of Tesla motor engineering on the trade offs:

https://electrek.co/2018/02/27/tesla-model-3-motor-designer-...

[jabl]

AFAIK practically all locomotives use induction motors, and those need massive torque at zero rpm. So it can certainly be done.