Miles' 90mm inrunner build thread

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Miles   100 GW

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Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 4:21am

Progress, so far:
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Re: Miles' 90mm inrunner build thread

Post by Jeremy Harris » Nov 07 2012 4:22am

I'll be watching this with serious interest, Miles!
Please ask questions on the forum, rather than by PM, as it helps others and you'll get a better range of answers.

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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 4:30am

Jeremy Harris wrote:I'll be watching this with serious interest, Miles!
Looking forward to your comments, Jeremy!

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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 4:56am

Progress on the model:
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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 5:08am

The idea is to have something that runs at 3000rpm and can generate up to 5Nm of torque, continuously... We'll see...

I was originally going to do this with 20 poles but I've dropped to 16 to keep the fundamental frequency below 400hz, which seems to be the maximum recommended for 0.35mm laminations. This will help keep the costs down.

Magnets are N30 SH, again for economic reasons.

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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 5:23am

Calculated weight, sans copper, is 820 grams.

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Re: Miles' 90mm inrunner build thread

Post by circuit » Nov 07 2012 6:59am

I'm wondering how you are going to keep the magnets from flying away. Would not it be better to simply embed the magnets under the surface of rotor's lamination? Should not be hard to do in this configurarion.
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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 7:08am

circuit wrote:I'm wondering how you are going to keep the magnets from flying away. Would not it be better to simply embed the magnets under the surface of rotor's lamination? Should not be hard to do in this configurarion.
I did experiment with that idea. It's terribly wasteful of magnet, though, a lot of flux is inevitably short circuited.
Ref: http://www.endless-sphere.com/forums/vi ... 30&t=41153

Anyway, 3000rpm isn't so fast. I was planning to wrap a single strand Kevlar thread around the magnets and impregnate it with epoxy.

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Re: Miles' 90mm inrunner build thread

Post by John in CR » Nov 07 2012 8:31am

Great to see you going for one Miles. How much do you think the laminations and whatever insulates them from each other will cost? If it helps you get a better price, I'd be interested in buying some too.

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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 8:49am

I haven't got any quotes for laminations yet. I'd guess a one off .35mm C5 lamination stator/rotor set would be somewhere between $150 and $200? To get things cheaper, you'd need to have them stamped in large quantity.

Let's try this out as a prototype, first. If it's any good, I'll think about putting a kit together for people to make their own.

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Re: Miles' 90mm inrunner build thread

Post by John in CR » Nov 07 2012 9:25am

Miles wrote:I haven't got any quotes for laminations yet. I'd guess a one off .35mm C5 lamination stator/rotor set would be somewhere between $150 and $200? To get things cheaper, you'd need to have them stamped in large quantity.

Let's try this out as a prototype, first. If it's any good, I'll think about putting a kit together for people to make their own.
Great, that sounds even better. I especially like that you chose a tooth count divisible by 6.

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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 9:29am

Magnets are going to be something like 1 euro each, sourced in europe. So, that's 32 euros per motor....

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Re: Miles' 90mm inrunner build thread

Post by liveforphysics » Nov 07 2012 12:00pm

Taught kevlar thread wrapped magnets impregnated with epoxy seem to stay on pretty well. Don't underestimate getting it taught while wrapping though, putting the rotor in a lathe set for a very slow speed seems to help. Wet the magnets first as well, so the thread lays in epoxy, then gets it brushed on the back side as well. Kevlar doesn't seem to just soak in the epoxy when you wet it from the outside nearly as well as fiberglass.

I agree though, low RPM's and a small magnet radius should make it pretty easy to keep these magnets attached.


Looks like generous room for copper with those narrow teeth. Definitely going for efficiency over burst torque, and I think that's a good move for your application.
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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 12:28pm

Thanks for your thoughts, Luke.

Ryan gave me some other ideas but I thought I'd try the Kevlar thread technique first. I have some experience of winding strings on musical instruments, so I'm not too daunted by this :) I've allowed an 0.8mm gap which will include the Kevlar wrap and hopefully allow sufficient clearance/tolerance room....

It will be interesting to compare the performance of this with the Astro motors. Although both are inrunners of a similar size and weight, they are very different in almost every other aspect.

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Re: Miles' 90mm inrunner build thread

Post by Jeremy Harris » Nov 07 2012 12:47pm

The forces on the magnet bond are small, not worth worrying about in my view. By all means add a Kevlar binding, but it almost certainly isn't needed at this sort of rpm and rotor radius.

If you're using magnets that weigh, say, 10g (probably pessimistic), have a rotor diameter of 90mm and spin it at 3000 rpm then the centripetal force acting on the magnet/rotor bond is only around 44N, or about 10 lbsf. The motor torque reaction will be primarily in shear, I would think, and probably adds little to the overall bond stress. The bond strength of a good epoxy on a well-prepared metal to metal joint will be around 6 to 8 N/mm² in shear and maybe 2 to 3 N/mm² tensile. My guess is that your magnets will have a bond area of somewhere between 200 and 300 mm², so just a good epoxy alone will be OK for around 8 times the load that the magnets will see.
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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 1:26pm

Thanks Jeremy. That's reassuring. The bonding surface on the rotor will be whatever the laser cut gives, which may not be ideal. The 'spacer' between the magnet pairs should help the integrity, though... (see pic below).
.
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Re: Miles' 90mm inrunner build thread

Post by Jeremy Harris » Nov 07 2012 1:33pm

That configuration is great for bonding, as adhesives are generally far better in shear than they are in tension. I suspect the bond areas on the small locating upstands will be more than enough to hold the magnets against the modest centripetal force, and the adhesive overall stress levels will be lowered by not having to take any of the shear reaction from torque.
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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 2:02pm

BTW, assuming a density of 7.5gm cm³, the magnets weigh as much as 3.35 grams each :)

Magnet dimensions are 30mm x 5mm x 3mm.

Rotor diameter is only 60mm (sorry, I should have made that clearer...) 90mm is the outside diameter of the case.

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Re: Miles' 90mm inrunner build thread

Post by Jeremy Harris » Nov 07 2012 2:14pm

In that case my assumptions were well and truly pessimistic!

I reckon the magnets would almost stay in place just from attraction to the back iron. A decent epoxy adhesive (my favourite would be Hysol - I've used it to bond alloy aircraft wings together) would be more than strong enough to hold the magnets in place at around two or three times the design rpm (bearing in mind that the tensile force increases in proportion to the square of rpm).
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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 2:27pm

As a comparison, this is a section through one of the Astro motors.

Stator OD is 76mm and the rotor diameter is 42mm
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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 2:36pm

The continuous torque rating of the Astro 32XX series is 1Nm per cm of stator length.

The rotor of my motor has 1.43x the surface area of the Astro, per unit length.

The airgap radius is also 1.43x greater, of course.

I'm hoping this will give me around twice the torque rating per unit length......

30mm x 2Nm = 6Nm. My target is 5Nm - I don't want to run as hot as the Astros......

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Re: Miles' 90mm inrunner build thread

Post by liveforphysics » Nov 07 2012 3:19pm

Jeremy Harris wrote:That configuration is great for bonding, as adhesives are generally far better in shear than they are in tension. I suspect the bond areas on the small locating upstands will be more than enough to hold the magnets against the modest centripetal force, and the adhesive overall stress levels will be lowered by not having to take any of the shear reaction from torque.

If the magnets are chrome/nickel plated, I find most any glue tends to bond better to the coating than the coating bonds to the magnet material. Just something ti remember when working on bonding designs.
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Re: Miles' 90mm inrunner build thread

Post by Miles » Nov 07 2012 3:29pm

Could I take the plating off of the face of the magnet to be bonded?

I don't relish the idea... Just wondering how reactive the NdFeB would be bonded directly to steel.

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Re: Miles' 90mm inrunner build thread

Post by liveforphysics » Nov 07 2012 6:23pm

A Joby motors magnets do have each magnet masked, and sand blasted off to remove the coating do the epoxy can bond.

You can alternatively order magnets with a black epoxy coating.

And O2 exposed uncoated magnet surface begins to oxidize very rapidly. On the order of hours to a day or so.
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Re: Miles' 90mm inrunner build thread

Post by Farfle » Nov 07 2012 6:35pm

Not sure if you have a source or not, but laser laminations out of Warrior AL, was very easy to work with, and were very interested in a small motor project. Plus the price quote from them was only $300 for my big outrunner's lams (.014" ~ 0.35mm)

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Last edited by Farfle on Nov 07 2012 6:39pm, edited 1 time in total.
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