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Comment by ridaj | Hacker News Reader

It reads like an ad to me. It's hard to figure out what the catch is, if any (and in this world of tradeoffs, there usually is a catch).

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The catch is that radial motors are balanced magnetically and these are not. All that magnetic force is added to axial pull between the stator and rotor. That means they need a thrust bearing in addition to the guide bearings. This causes increased heat and mechanical losses and probably limits efficiency to somewhat less than a standard electric motor. Hi-E standard motors cap out around 98.8%, these will probably cap a point or two lower.

I don’t have a firm grasp of the article, but isn’t the force balanced in this case since the two rotors, linked together, sandwich the stator? If I understood correctly, this would mean the thrust bearing would only feel the imbalance between the two sides.

See: https://m.youtube.com/watch?feature=youtu.be&v=BTuCshX5bc0&t...

Agreed, this seems to be an incorrect conclusion. Maybe ew6082 just looked at the asymmetrical figure without reading the text.

Fair enough. They said that a double rotor version has been developed, but all their prototypes and marketing material are single rotor. We can probably assume that single rotor is all that would fit on an in-wheel design but there are other ways to skin the cat.

They would still need to include a light thrust bearing on the double rotor design since you can never assume perfectly balanced air gap and one side or the other will dominate, but the losses would be much lower with that setup.

In general, yes, though the article mentions one in-hub motor with a single rotor, for reduced size or weight. I guess an in-hub motor, which also acts as the wheel's axle, has to deal with much larger axial forces, from cornering, than those generated by the motor, so the latter are moot.

Also, during operation, is not each magnet in the motor being simultaneously pulled by the pole it is moving towards, and pushed by the pole it is moving away from? If so then, at any given point in the rotation, the axial force would be the net of the two effects.

They're claiming higher efficiency because of less iron loss. Does it balance out?

https://www.researchgate.net/publication/262642389_Axial_and...

"ABSTRACT: The application of permanent magnet machines as direct-drive generators for wind turbines is considered. The Axial-Flux Permanent-Magnet (AFPM) machine with slots is compared to the Radial-Flux Permanent-Magnet (RFPM) machine, for this application. Using a computer-based design model, the two machine types are optimized with respect to the lowest Cost/Torque. Design optimization for maximum Torque/Volume is also investigated. The calculations show that the RFPM machine has lower Cost/Torque than the AFPM machine with slots, and lower torque/volume than the AFPM machine with slots.

...

Conclusion... ...Based on the results presented herein, the RFPM machine still appears as a better choice for a direct-drive generator, than the AFPM machine with slots"

For vehicles (cars, planes, etc.) if they can mass-produce at 97-98% efficiency and the power-to-weight ratio they are claiming to achieve (15 kW/kg) then it's still a huge improvement.

You will find on enquiry, as I have, that under continuous load the power density is a LOT lower.

Oh, it's definitely an ad. But it's the good kind of ad, the one that informs you about the existence and advantages of a product that you hadn't heard about. Contrast this throwback style of ad with the modern style of ad, which emotionally engineers you into subconsciously wanting a product that you don't actually need. I would love it if more ads looked like Magnax's.

Yes, you could call it an ad, but it reads as much more down-to-earth and straightforward than what most people think of when they think of advertising. They're simply describing what they built, why it was difficult, and why they think it's important, with a minimum of marketing fluff.

One thing I remember is some friends I met with a warehouse full of crazy projects they had worked on. They had a few axial flux motors they were really excited about that they had custom made. They seemed to think it was a superior technology, though I can’t recall the specifics.

Heat dissipation.

Bingo... Of course there's plenty of apps where that doesn't matter... we'll see them implemented in "bursty" automobiles a long time before we see them implemented in "continuous" railroad traction motors, for example.

Its an interesting time to be alive. I'm not THAT old and when I was a kid a generic NEMA 50 HP industrial electric motor took a crane or at least a large engine hoist to lift, more than 500 pounds for sure, but now there are high end automotive motors that I can easily do bicep curls with (like 40 pounds)

VFDs are kinda like that too. What used to be a large rack is now a little tiny DIN unit that handles 10x the power.

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