Hub motors - Why such bad power to weight

Electric Motors and Controllers
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Toby82   100 mW

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Hub motors - Why such bad power to weight

Post by Toby82 » Feb 03 2020 3:07am

Having a fairly basic understanding of the workings of the brushless DC hub motor I'm curious as to why all currently available Hub motors have poor power to weight when compared to mid drive motors (Emrax, Zero, motenergy etc) ??

Is it simply the fact that development has not occurred in the Hub motor market or are there aspects of the hub motor (low rpm etc) that require designs which are subsequently heavy?

Punx0r   100 GW

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Re: Hub motors - Why such bad power to weight

Post by Punx0r » Feb 03 2020 3:55am

Because comparitively they are high-torque, low speed machines with a large diameter. Torque production requires active materials (iron, copper) in quantity and the diameter requires significant support structure for not only these active materials but also to carry the weight of the vehicle (being the wheel hub).

One way to improve things would be to greatly increase the pole count while reducing the size of individual teeth and coils. This would require thinner (more expensive) laminations though - morel ike those of a typical mid-drive motor - and would be difficult for the controller to drive as the switching frequency would be quite high.

The more practical way to boost the power (and often efficiency) of a typical hub motor is to spin it faster: Same torque x faster speed = moar powah. This is done but either putting it in a smaller wheel or using it as a mid-drive. You'll hit the limits of those thick lams at some point, though.

In short, DD hub motors are both sturdily built and typically spun too slowly to make the power they are capable of.

John in CR   100 GW

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Re: Hub motors - Why such bad power to weight

Post by John in CR » Feb 03 2020 10:12am

To make big power at low weight requires high rpm. There are small and light hubmotors, but they're enclosed inside a hollow hub with a planetary gear reduction of one or two stages. Because they are insulated from the outer shell by air, they have relatively poor heat transfer and generally go up to only 1000W or so of power, unlike direct drive hubmotors that spin only as fast as the tire. That makes DD hubmotors bigger and heavier.

Voltron   10 MW

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Re: Hub motors - Why such bad power to weight

Post by Voltron » Feb 03 2020 2:45pm

With the Motenergy type, another difference is they are typically inrunners, with the coils on the outer shell, where they cool better. Hub motor has to have the coils inside with the outside spinning, so the coils don't have as good a thermal path to the outside.

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