And magnets instead of electromagnets allow you to have brushless motors. You could go with a synchronous motor but I guess the efficiency is not very good
The induction torque speed curve is also very performant even when compared to PM machines so I don’t understand your criticism. Source: motor control engineer.
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
According to the article, the torque limitations are only for a poorly controlled drive current.
> 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
[1] https://insideevs.com/video-primer-tesla-model-s-electric-mo...
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...
https://electrek.co/2018/02/27/tesla-model-3-motor-designer-...
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.
To maybe put it another way, there is no way to charge a dead battery with an induction machine.
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.
As for the rotors and stators themselves, they are good old iron/steel laminates and will hold some magnetism just fine (they're laminates to cut down on the eddy currents). If the rotors themselves would be made out of copper the motor would not function at all, it needs a ferromagnetic part there to focus the field.
Source: have done a number of induction motor to windmill conversions. Also: have you ever tried any of this or are you coming at this from a theoretical angle?
Here is some good info for you:
Fortunately not everybody will take 'it can't be done' as the final answer :) It's not what can't be done in theory that matters but what can be done in practice.
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.