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Comment by kumarvvr | Hacker News Reader

This is a common tech, with many variations of such induction used.

What is important is that the wireless power transfer is lossy. It's not very efficient.

Designs that do not require permanent magnets are good, but I believe that using good quality carbon brushes reduces the complexity of power transfer and is reliable enough for use case of cars.

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Carbon brushes also have isolation issues. If your motor is to be air cooled, you will be blowing outdoor air through it, complete with fog/condensation. In brushed motors, that fog will cause a leakage current from the motor brushes to ground. The leakage current may be tiny, but in a system where you want 10's of megaohms between the high voltage system and the chassis for human safety, it becomes a problem.

Or you could try to hermetically seal your brushes in to stop moisture getting in, but now you have the difficulty of a spinning moisture seal and build up of carbon dust which is probably going to start to get warm/hot.

> What is important is that the wireless power transfer is lossy. It's not very efficient.

And that is why their reported efficiency of 95% is significant.

We have DC-Motors with brushes on work (50kW-500kW). Brushes really need a bit care all the time. And if something happened, you have to rework the whole thing on a turning machine.

Permanent magnets are handy for spinning up a power grid from zero, are they not? Many generators need power to produce power, so they’re useless if the grid is down, until they get fed some power by something else.

Starting up power grids from zero is called black start[1].

Synchronous generators can have external exciter systems, or be self+excited[2].

[1] https://en.wikipedia.org/wiki/Black_start

[2] https://en.wikipedia.org/wiki/Excitation_(magnetic)

Many induction generators are 'self starting' - even a tiny bit of the earth's magnetic field is enough to get them generating something, which in turn makes them generate more, etc.

Would it really be the earth's magnetic field? Residual magnetization of the steel laminations seems more likely.

They speak about 95+% efficiency, I'd hope they included this wireless connection in the calculation.

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