Toyota develops first neodymium-reduced, heat-resistant magnet for motors

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

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

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.

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.

That's not true.

> You need a rotor field to generate power.

Residual magnetic field is enough to start the process, absent that a quick DC zap will do.

> In a permanent magnet machine this field comes from the magnet, but in an induction machine this comes from the motion of the stator field (Lenz' Law).

Yes. Fortunately rotors retain a bit of magnetism after you run them to a stop, which is enough to get things going again later on.

> To maybe put it another way, there is no way to charge a dead battery with an induction machine.

Yes there is, unless the induction machine is fresh from the factory and has never run at all.

I think there are other more significant trade-offs between induction and PM motors. Induction motors/generators are significantly heavier, and have significantly lower torque (i.e. can extract less lower at low speeds when used as generator). Their main attractiveness comes from lower cost, reliability, and decent efficiency at high speeds.