Retaining sleeve for rotor magnets

[SPIGUY1]

What could be the effects of adding a thin 303 to 316 stainless steel sleeve to retain the rotor magnets in a BLDC motor? What would be the best material to use for this sleeve to avoid affecting motor Kt?

 

[electricpete]

My limited two cents: those are austinetic stainless steels which should not be ferromagnetic (might be a little paramagnetic but who cares relatively permeability for all practical purposes is 1). So I wouldn't think it matters. But I have certainly don't have any knowledge of design of the types of machines you're talking about.

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(2B)+(2B)' ?

 

[electricpete]

Now I was just thinking in addition to magnetic properties... conducting properties might be important if possibility for induced current... don't ask me if that is relevant.

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(2B)+(2B)' ?

 

[stardelta]

Does the retaining sleeve have to be metalic? Why not use a "Resiglass" type self-amalgamating banding tape such as that used to retain armature and rotor windings? Its non-conductive, has zero magnetic influence and therefore shouldnt effect Kt.

 

[Skogsgurra]

Carbon fibre is what we have used in PM motors running up to 120 000 RPM at around 100 kW. A thin carbon fibre/epoxy resin sleeve works very well.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[SPIGUY1]

The reason we are using a retaining sleeve is due to the fact that the rotor magnets are secured to the steel hub of the rotor by locitie. The locitie bond is only as good as the application process is. The motor is only running in the 600 rpm range but the locitie bond failed after thermal cycle. We have tried aluminum for the sleeve but had issues with stack rubbing after crimping the sleeve.
If we use a 316 stainless steel that is annealed will the magetic properties of the steel be reduced enough to not impact the motor Kt?

 

[electricpete]

Google 316 stainless steel magnetic and you will get several links that all say it's not magnetic. Permeability very close to 1.0

http://www.azom.com/details.asp?ArticleID=1140

http://en.wikipedia.org/wiki/Stainless_steel

http://www.ssina.com/faq/index.html

One small Caveat the 1st link indicates some austenetic steels may become magnetic thru cold-working, but apparently applies to 304 and not 316

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(2B)+(2B)' ?

 

[sreid]

Myn I humbly suggest that you are using the wrong type of Loctite. Call Loctite and get an expert recomendation.

 

[Skogsgurra]

PM coming loose is usually caused by heat caused by eddy currents in the rotor or in the magnets themselves. The heat softens the Loctite or CA or whatever 'glue' you are using.
It is a common problem when you use variable frequency to feed PM motors. You may need to filter the inverter output voltage or use a sine inverter.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jpts]

Ideal sleeve would have relative permeability close to 1.0 (non-magnetic, and low conductivity. In my opinion Aluminium might not be a good idea as it has high electric conductivity. If you have high slot-frequency, permeance harmonics due to slot openings will induce high eddy currents on it.

I guess the best compromise is to use stainless steel, as their conductivity is usually rather low, and they are usually non-magnetic.

I have seen also some manufacturers using some kind of hard plastic tube around rotor (with some small servomotors). From magnetic properties point of view, that is a good choice, but if there are some losses in the rotor, it will also insulate the rotor thermally. In worst case, magnets would heat-up and eventually demagnetize. Sleeves that have some conductivity, also act as a "magnetic mirror" for air gap harmonics, prventing them from penetrating the rotor. Non-conducting sleeve lets all the air gap harmonics to pass the rotor and cause extra losses there.