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Outrunner Magnets Decomposing to Dust

No wonder the motor felt stiff!

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Gordon Whitehead 120/09/2018 19:14:50
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During the past week I've been working up the enthusiasm to design a nice 3S-powered Hawker Fury. Today I took the intended motor, an Actro C-6 1150kv outrunner, out of storage to check it out. I was a bit nonplussed to find that on fitting the prop this carefully stored motor felt really stiff when I turned it by hand. Rather than apply power, I took the motor apart and the photo show what the innards look like.

actro c-6 decomposing magnets.jpg

The motor is well over 10 years old and has sat in a warm drawer in my workshop for at least the past 10 years. I do remember having to re-araldite some of the magnets back in place when the motor was only a month or so old, but haven't seen anything like this before. Applying sellotape doesn't remove hardly any of the magnetic dust, btw.

Clearly I need a new motor. The Actro was only good for 30A so I'm planning to buy a replacement with rather more current handling capability.

Gordon

Former Member20/09/2018 19:32:49
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Former Member20/09/2018 19:57:52
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Chris Bott - Moderator20/09/2018 21:18:31
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How very strange. I've never seen anything like that. It looks like there's a shiny coating that's degraded?
Gordon Whitehead 120/09/2018 21:21:45
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344 forum posts
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The magnets are flaking and pitting Chris and David. The only hydrogen they'd have ever been near is that chemically bound in the water vapour in the damp flying conditions we often have to suffer in this country. I'm wondering if the magnets are made from powdered magnetic alloy bound with some kind of glue which is degrading with age. The epoxy fillets which you can see securing the magnets is araldite which I used many moons ago when re-installing detached magnets, and that has stayed whole. The motor wasn't particularly powerful but the current Aeronaut advert has it at a breathtaking 129 euro. I remember it did cost an arm and a leg when I bought it but nowhere near that. Perhaps the current limit was dictated by the magnets' composition.

The magnets in my equally old AXI 4130/16 of about the same age and stored in the same box are still whole, thank goodness, as are those in a couple of old Hacker A50s I regularly use.

Gordon

Edited By Gordon Whitehead 1 on 20/09/2018 21:22:26

Former Member20/09/2018 21:53:19
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Former Member20/09/2018 23:11:26
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onetenor21/09/2018 04:34:21
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Could you replace the magnets?

onetenor21/09/2018 04:34:21
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Could you replace the magnets?

Former Member21/09/2018 06:31:52

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Alan H21/09/2018 17:15:36
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I had exactly the same thing happen to a Waypoint 2 series motor a few years ago. I ran a few tests on it then put it away pending a suitable project. It was stored indoors in a warm dry location but when I got it out some time later it was stiff and the magnets looked just like your motor. In my case I just binned it. I also have a 3 series Waypoint motor from the same era and that is still fine.

PatMc21/09/2018 17:42:24
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I think the problem & it's cause may described in this Wiki article :

Corrosion problems

Sintered Nd2Fe14B tends to be vulnerable to corrosion, especially along grain boundaries of a sintered magnet. This type of corrosion can cause serious deterioration, including crumbling of a magnet into a powder of small magnetic particles, or spalling of a surface layer.

This vulnerability is addressed in many commercial products by adding a protective coating to prevent exposure to the atmosphere. Nickel plating or two-layered copper-nickel plating are the standard methods, although plating with other metals, or polymer and lacquer protective coatings are also in use.

Also looking at the stator, IMO it doesn't appear to be very good quality. The windings seem to be poorly wound & only single strand thick gauge wire.

Peter Christy21/09/2018 18:23:30
1781 forum posts

Its not a new problem! Many years ago (alright, VERY many years ago!), I replaced the motors in my Bonner Digimite servos. I kept the old ones as spares, just in case, and on opening the box a couple of years later, found the ferrite magnets on those motors turning to ferrite dust!

indecision

--

Pete

Gordon Whitehead 122/09/2018 17:54:51
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344 forum posts
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Thanks for your inputs guys. I feel a little bit better knowing it's not an isolated problem. I agree with PatMc regarding the poor look of the windings, and thanks for the wiki extract too. I've noticed that small sintered magnets can crumble when used for holding large hatches on i/c models, which I assume is caused by vibration making the opposing magnets jitter against each other.

Oh well. After seeing clubmates enjoying their Easy Streets with Ripmax Quantum II motors which are quite cheap, I'm thinking of getting my LMS to get the 25-size motor for me to to try.

Cheers

Gordon

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