Reducing Rotor back-iron

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hias9   1 kW

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Reducing Rotor back-iron

Post by hias9 » Jan 10 2022 2:29am

Can back iron on a DD outrunner be reduced to save weight without a negative effect on efficiency as long as there is still no magnetic field measurable on the outside of the ring?

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Re: Reducing Rotor back-iron

Post by zzoing » Jan 14 2022 9:06pm

What do you mean? can you post a fast picture please?
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Re: Reducing Rotor back-iron

Post by calab » Jan 14 2022 10:44pm

The metal on the rotor shell, back of the magnets its been done with success.

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Re: Reducing Rotor back-iron

Post by amberwolf » Jan 15 2022 12:04am

hias9 wrote:
Jan 10 2022 2:29am
Can back iron on a DD outrunner be reduced to save weight without a negative effect on efficiency as long as there is still no magnetic field measurable on the outside of the ring?
If it doesn't affect the return path of the fields, it shouldnt' change the motor operation.

It's been done to various outrunners over the years, including hubmotors (IIRC some middrives using DD hubs, perhaps even the original stokemonkey, with their spoke flanges machined off along with as much backiron / shell as they could get away with).

Of the DD hubmotors I've worked with that have a separate steel ring for backiron with an aluminum shell and spoke flanges, the backiron ring is typically 5mm or less. (Stromer's UltraMotor, early 9C, etc).


But structurally:

If the DD outrunner is a hubmotor, and the backiron is also the rotor shell and the spoke flange: Removing metal will weaken this structure and make it more susceptible to deformation from spoke tension, and dependng on how the machining is done between and next to the flanges, could even weaken the flanges themselves and if spoke tension is sufficiently high, allow failure of the flange to remain attached to the rotor.

This has happened without such mods for other reasons to some DD hubs over the years, though I think those were aluminum like some of the old 9C "500w" hubs, which had a separate steel ring inside the alumimum outer shell and flanges; there are also all-steel versions of them. (it has also happened to regular bike hubs, also for other reasons).


Additionally, for motors where the backiron / shell is a certain thickness to prevent deformation of it from the pull of the fields on the magnets attached to it (probably not the case for DD hubs) then thinning that increases deformation potential, and could not only cause changes in the fields because magnets can then move closer to stator teeth, etc., but also allow magnets to scrape the stator, if the deformation is large enough and the air gap started out small enough. I don't imagine either of these is much of a problem, but it might be worth considering for thin-shelled motors (some RC outrunners have shells so thin they have significant flux leakage outside the shell, according to some posts over the years--I think they might be in Kepler (or other ) friction drive threads, regarding "garbage" picked up and stuck to the outside of the motor shell, causing grip problems where the shell is directly used to drive the tire).
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Re: Reducing Rotor back-iron

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Re: Reducing Rotor back-iron

Post by hias9 » Jan 15 2022 10:11pm

Why do you think the manufacturers went away from using backiron inside an aluminium shell?

Did it have some kind of form-closure or was it just an iron ring pressed into an aluminium shell?

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Re: Reducing Rotor back-iron

Post by calab » Jan 15 2022 11:18pm

The cast molds and master molds were already made and other cost factors its not like the industry is really catering to us diy ebike emotorcycle escooter hobbyists. Everything for them comes down to cost to save a penny there earns them more money, also the commodity market for steel vs aluminum, labor costs, training costs, equipment costs. The only ones catering to the industry are like the ones in the video.
hias9 wrote:
Jan 15 2022 10:11pm
Why do you think the manufacturers went away from using backiron inside an aluminium shell?

Did it have some kind of form-closure or was it just an iron ring pressed into an aluminium shell?

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Re: Reducing Rotor back-iron

Post by hias9 » Jan 15 2022 11:49pm

Did anybody here who worked with those motors that have backiron inside an aluminium shell experience rotor deformation by spoke tension?
Was it just an iron ring pressed into an aluminium shell or did it have some kind of form-closure (to avoid problems with thermal expansion when the motor gets hot).

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Re: Reducing Rotor back-iron

Post by amberwolf » Jan 16 2022 1:49pm

I don't know if there was any rotor deformation or not; I didn't experience rotor / stator rub in any instances where I disassembled and reassembled the motors, but I don't know if the airgap changed or motor behavior changed; I wasn't paying that much attention to them at that time (if I didn't have a problem, I didn't worry about them).

The aluminum shell/flanges appear to be an interference fit with the steel rotor backiron. Meaning, they probably heated the shell and cooled the backiron and pressed them together, so when they are the same temperature they are locked thoroughly together.

I'm not enough of a mechanical engineer type to know exactly what happens at varying temperatures, but if they did that, then as long as the aluminum shell isn't heated beyond the temperature at which it was pressed onto the backiron, it won't expand enough to no longer be a press-fit. (and if the backiron is expanding too, it would take even a higher temperature). I don't expect cold would be as much of an issue because the motor would be creating heat countering that.
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Re: Reducing Rotor back-iron

Post by hias9 » Mar 20 2022 9:18pm

After removing the excessive back iron on a lathe, the available solutions for cooling fins like Hubsinks would not fit any more.
What would be a good way to machine custom cooling fins? I guess EDM cutting them would be extremely expensive.

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Re: Reducing Rotor back-iron

Post by Chalo » Mar 20 2022 10:34pm

hias9 wrote:
Mar 20 2022 9:18pm
After removing the excessive back iron on a lathe, the available solutions for cooling fins like Hubsinks would not fit any more.
What would be a good way to machine custom cooling fins? I guess EDM cutting them would be extremely expensive.
You could use the same lathe to bore a piece of thick walled aluminum tubing to fit closely to the back iron, and then cut circumferential grooves on the outside to make fins.

Might want to cut the resulting tube into segments and hold them on with circlips or thermal epoxy, because a solid aluminum sleeve will expand faster than the back iron as it warms up, and could separate from the thermal compound exactly when that would be least welcome.

Best would be to use a big enough, efficient enough motor that overheating isn't an issue.
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Re: Reducing Rotor back-iron

Post by hias9 » Mar 20 2022 11:35pm

Thank you. How would you cut the circumferential grooves?

I am pretty sure that a slightly lighter motor with the added weight of cooling fins can take more continuous power than a motor which has that weight added in terms of a slightly wider stator+magnets, but no cooling fins.

Do you have experience with thermal epoxies? If I do the math it should be fine, but I am a bit afraid that they could become loose at very high rpms or if a stone hits one segment while riding offroad.

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Re: Reducing Rotor back-iron

Post by Chalo » Mar 21 2022 12:12am

hias9 wrote:
Mar 20 2022 11:35pm
Thank you. How would you cut the circumferential grooves?
I'd use a parting tool. Here's an example setup I found on the interwebs:
fins.jpg
fins.jpg (87.32 KiB) Viewed 165 times
http://modelenginenews.org/techniques/fins.html
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Re: Reducing Rotor back-iron

Post by amberwolf » Mar 21 2022 11:43am

hias9 wrote:
Mar 20 2022 11:35pm
but I am a bit afraid that they could become loose at very high rpms or if a stone hits one segment while riding offroad.
Add a binding wire circumferentially around the central groove to tension them all against the rotor. (this will also hold them in place while the epoxy cures; just don't remove it when done)
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