Animation for High Torque ETM motor

[wturber]

We completed this overview animation for ETM earlier this year. The animation doesn't really get into the theory of the motor very deeply at all, but wonder if this kind of design could be adapted to make a DD bicycle hub motor that could have the size and weight of a geared hub but the simplicity and durability of a DD?

[youtube]w8xJKWsEKP0[/youtube]

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etmpower.com

 

[amberwolf]

If you mean a design with many more poles, there are some like that. The BionX D series is more than usual, and there are a few others. Some of the very small scooter hubmotors do this. Mostly, though, it's geared hubs that use greater numbers of poles, from the pics I"ve seen and the ones I've opened up. I don't recall any that have as many poles as this one, though.

However, unless I missed something (always possible) the motor in the video appears to be designed to operate with gearboxes, and/or at higher rotation rates, to get the higher power levels. That's incompatible with DD hubmotor use, unless it's in really tiny wheels, like the hand-sized scooter wheels, so you can have a high rotation rate but a reasonable ground speed.

I don't know enough about motors to know if a high pole count and a DD motor are completely incompatible, but I suspect the diameter of the motor must be very large, close to the wheel size, to be practical.

 

[Grantmac]

They must spec continuous output very conservatively because their 500w motor is ~9kg or something like that.

 

[wturber]

However, unless I missed something (always possible) the motor in the video appears to be designed to operate with gearboxes, and/or at higher rotation rates, to get the higher power levels. That's incompatible with DD hubmotor use, unless it's in really tiny wheels, like the hand-sized scooter wheels, so you can have a high rotation rate but a reasonable ground speed.

No. The specific point was to show the conventional motor and then show the unconventional ETM that doesn't use the gearbox. Also, the winding coils are quite small and unconventionally configured. You'd have to go to the website to start to get some insight on that. We didn't go there in the animation.

 

[amberwolf]

Ah...I watched it with sound muted, so maybe the audio explained what was being shown.

 

[lizardmech]

Adding loads of poles just increases torque density but it restricts the maximum RPM the motor can run at. Losses in the stator become too extreme if you try to run this type of motor at higher speeds. You can squeeze a little more with thin laminations and special materials but there's only maybe 20% or 30% to be gained, if you double the number of poles you're going to face a much lower practical maximum RPM and power density will typically be lower as the torque gained doesn't offset the reduced RPM.

Ebike hub motors more or less already work this way, they're optimized for torque and have a low power to weight ratio, a 6 pole inrunner the same weight might have 20 or 30kw but then you would need to some how gear it down from 50,000 RPM which is not easy or practical for a bike.

 

[neptronix]

Adding loads of poles just increases torque density but it restricts the maximum RPM the motor can run at. Losses in the stator become too extreme if you try to run this type of motor at higher speeds. You can squeeze a little more with thin laminations and special materials but there's only maybe 20% or 30% to be gained, if you double the number of poles you're going to face a much lower practical maximum RPM and power density will typically be lower as the torque gained doesn't offset the reduced RPM.

Ebike hub motors more or less already work this way, they're optimized for torque and have a low power to weight ratio, a 6 pole inrunner the same weight might have 20 or 30kw but then you would need to some how gear it down from 50,000 RPM which is not easy or practical for a bike.

Correctamundo :thumb:
For applications like these ones, high RPM doesn't matter too much. But eliminating a few sets of gears makes for a great boost in efficiency despite the higher rotating losses.

Same goes for hubs. The magic pies and the new 9C with a couple extra poles have higher rotating losses, but also produce a crapton of torque from a stall. Very ideal for a 29er ( or larger ) wheel.

 

[wturber]

Correctamundo :thumb:
For applications like these ones, high RPM doesn't matter too much. But eliminating a few sets of gears makes for a great boost in efficiency despite the higher rotating losses.

Same goes for hubs. The magic pies and the new 9C with a couple extra poles have higher rotating losses, but also produce a crapton of torque from a stall. Very ideal for a 29er ( or larger ) wheel.

Right. But the question I have is whether the scheme being used by ETM that uses a lot less copper than conventional hubs could be applied to bike hub motor designs.

 

[SwampDonkey]

Looks like a very high pole count motor, similar to industrial servos. Servos typically have torque far superior to other motors.