Rotor Material

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hias9   100 W

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Rotor Material

Post by hias9 » Dec 25 2020 4:09pm

The 3kw 45mm magnet height motors I use (similar to mxus 3k) have a rotor made of about 8mm thick steel which is almost 6lbs without the 46 magnets and cover plates. On the outside of the ring I use aluminum Hubsinks.
How would a rotor made of aluminium or copper affect the motor properties?

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amberwolf   100 GW

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Re: Rotor Material

Post by amberwolf » Dec 25 2020 5:16pm

Is the rotor also the backing iron for the magnets?

If not, then it shouldn't matter, motorwise.

If it is also part of the magnetic circuit, then you would need to test to determine the actual difference in behavior, but there would be some (or a lot), and may vary depending on loading conditions and current flow.

hias9   100 W

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Re: Rotor Material

Post by hias9 » Dec 25 2020 5:38pm

Yes it is back iron. Are there ways to calculate the effect on the motor Kv?
Because machining a rotor made of copper or aluminium just to test it would not be cheap.

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Chalo   100 GW

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Re: Rotor Material

Post by Chalo » Dec 25 2020 8:15pm

hias9 wrote:
Dec 25 2020 5:38pm
Yes it is back iron.
Then don't. What makes you think a motor designed to use back iron in its magnetic circuit would work satisfactorily without it?
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hias9   100 W

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Re: Rotor Material

Post by hias9 » Dec 25 2020 8:23pm

I am not sure by how much the back iron actually decreases Kv and if the amount of back iron can be reduced without a significant increase in Kv.
But I am also not optimistic that reducing back iron is actually a good idea.

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Chalo   100 GW

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Re: Rotor Material

Post by Chalo » Dec 25 2020 8:25pm

It's a little like asking how many legs you can cut off a chair. It depends on the specifics, of course, but it's not going to be good even in the best case.

Moving magnetic flux lines across conductive material will result in eddy current losses even if those materials are not ferromagnetic. That's how most stationary trainers apply resistance.
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hias9   100 W

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Re: Rotor Material

Post by hias9 » Dec 25 2020 8:48pm

I came across a study and one result was that torque was halved by removing the back iron completely. I agree, reducing back iron is most likely a bad idea.
Looks like there is only a bit of weight reducing potential on the cover plates.

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Chalo   100 GW

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Re: Rotor Material

Post by Chalo » Dec 25 2020 9:16pm

hias9 wrote:
Dec 25 2020 8:48pm
Looks like there is only a bit of weight reducing potential on the cover plates.
Cover plates are a good place to save weight. Not only are they not part of the magnetic circuit, but you can replace them with lighter aluminum or magnesium parts that have better specific heat and thermal conductivity, and which have surface area increasing features that also improve their structural rigidity.
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Re: Rotor Material

Post by hias9 » Dec 25 2020 9:22pm

They are already made of cast aluminium so there is not that much room for weight reducing. I also thought about cooling fins on the sideplates, but machining them will not be cheap I think.

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Chalo   100 GW

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Re: Rotor Material

Post by Chalo » Dec 25 2020 9:35pm

hias9 wrote:
Dec 25 2020 9:22pm
They are already made of cast aluminium so there is not that much room for weight reducing. I also thought about cooling fins on the sideplates, but machining them will not be cheap I think.
That's true. If you have them remanufactured, use magnesium to reduce weight. Add fins or corrugations inside and outside. But seems like your motor might already be optimized in terms of power vs weight vs cost as supplied from the manufacturer. Go figure.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.

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