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Old Nov 15, 2008, 03:58 AM
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Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Metal Motor Mount May Steal Your Mojo!

Today I tested a Keda A22-20L outrunner. Unfortunately the results were a bit disappointing. No-load current draw was quite high, and when propped up it also drew a bit more current and produced slightly lower rpm than expected . Did I get a dud, or was something else going on?

This motor has a very thin bell which is not very effective at containing the field from its magnets. In fact the external magnetic field is so powerful that the motor can easily suspend a 150g (5oz) pair of pliers by the pointy end! So I thought, What if the rotating bell was inducing eddy currents in my aluminium motor mount? Could that be causing the high no-load current and poor performance?

I re-mounted the motor in reverse and repeated the test. This produced much lower Io and better performance . I then tried holding a sheet of aluminium close to the bell, and Io jumped back up to its previous value. Finally, to prove that metal was the problem, I also tried a sheet of balsa - which had no effect.

Results:-
Io:- Reverse Mount = 1.08A, Front Mount = 1.58A (+0.5A, 46% higher).

HD8060:- Reverse Mount = 9731rpm @ 13.3A, Front Mount = 9647rpm @ 13.8A (84rpm lower, 0.5A higher).
According to Motocalc, the metal mount caused a 4% reduction in peak efficiency.

Several Aluminium stick mounts are available which have clearances similar to my thrust stand (eg. Hyperion Z22-MNTSTIK-AL, GWS EMM28A) although since they are shorter the effect may be less. If you are making your own metal mount for use with this motor (or others of similar construction, eg. KDA20-XX series, Turnigy 22XX series) try to maintain a large clearance between the motor bell and the mount!
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Old Nov 15, 2008, 12:18 PM
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landru's Avatar
Vancouver, Canada
Joined Mar 2003
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Very neat discovery!

Along similar lines, I wonder how much motor performance is diminished by eddy currents induced in the aluminum bearing tubes which typically run through outrunner stators. Those tubes can get quite hot. Is all of that heat just radiated from the stator?
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Old Nov 15, 2008, 04:22 PM
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United States, CA, San Diego
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Originally Posted by landru
Very neat discovery!

Along similar lines, I wonder how much motor performance is diminished by eddy currents induced in the aluminum bearing tubes which typically run through outrunner stators. Those tubes can get quite hot. Is all of that heat just radiated from the stator?

That can be a problem is with stator designs that are not done right. If the motor is designed well you should nearly all the flux contained with in the motor. With outrunners if you have steel objects sticking to the case there is clearly too thin of steel behind the magnets. If the core heats up from the eddy currents then again the stator does not have enough "back iron" between the teeth at the base. The problem is pretty common.

Steve
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Old Nov 15, 2008, 10:35 PM
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jackosmeister's Avatar
Auckland NZ
Joined Aug 2007
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Very interesting - I Currently have ali motor mounts in most of my planes...
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Old Nov 16, 2008, 01:15 AM
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Skycruiser's Avatar
Fairlie, New Zealand
Joined Nov 2006
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Quote:
Originally Posted by Bruce Abbott
I then tried holding a sheet of aluminium close to the bell, and Io jumped back up to its previous value. Finally, to prove that metal was the problem, I also tried a sheet of balsa - which had no effect.
That does it. Only balsawood motor mounts for me from now on.

Actually I mounted one of these motors last week, and noticed the same strong magnetic field. Strong enough that it can be a struggle stopping the screwdriver from sticking to it.
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Old Nov 16, 2008, 02:09 PM
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Vancouver, Canada
Joined Mar 2003
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Quote:
Originally Posted by sneu
... With outrunners if you have steel objects sticking to the case there is clearly too thin of steel behind the magnets. If the core heats up from the eddy currents then again the stator does not have enough "back iron" between the teeth at the base. The problem is pretty common.
Steve,

Yes, definitely a common problem from what I've seen.

How much back iron is enough? I suppose the pros use FEMM to optimize back iron thickness. The rest of us often use the 'paper clip test'. My experience has been that increasing the flux ring thickness beyond the 'paper clip' threshold can significantly extend the power range of a motor. The cost is a smidgen knocked off the peak efficiency.

It's no so easy to play with different stator designs!

Regards,
Andrew
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Old Nov 16, 2008, 02:45 PM
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United States, CA, San Diego
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Quote:
Originally Posted by landru
Steve,

Yes, definitely a common problem from what I've seen.

How much back iron is enough? I suppose the pros use FEMM to optimize back iron thickness.
It's no so easy to play with different stator designs!

Regards,
Andrew
FEMM is fun to play with --it is sort of a trial and error process to come up with a good design--but given that the tool for a stator die costs $$$ it is worth a few hours of cad work with FEEM making cool pictures of the flux lines. I have some plots saved at work that I will post if I can dig them up.

Steve
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Old Nov 16, 2008, 02:47 PM
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Sure, post them. Did you teach yourself FEMM?
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Old Mar 15, 2009, 04:12 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
near Nijmegen, Netherlands
Joined Feb 2001
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Quote:
Originally Posted by sneu
... I have some plots saved at work that I will post if I can dig them up. ...
Have you found them Steve? Would not mind adding to this "animations and simulations" compilation:
http://www.rcgroups.com/forums/showthread.php?t=216928

Vriendelijke groeten Ron
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Old Mar 15, 2009, 04:46 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
near Nijmegen, Netherlands
Joined Feb 2001
10,504 Posts
Quote:
Originally Posted by Bruce Abbott
... This motor has a very thin bell which is not very effective at containing the field from its magnets. In fact the external magnetic field is so powerful that the motor can easily suspend a 150g (5oz) pair of pliers by the pointy end! ...
All the information is available, and still some manage to produce cr*p This has nothing to do with cost-shaving, it's just plain stupid.

Vriendelijke groeten Ron
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Old Mar 15, 2009, 05:06 PM
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Vancouver, Canada
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It's very poor engineering, that's for sure.

I wouldn't be surprised, though, if some people haven't made lots of money by selling such efforts.
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Old Mar 15, 2009, 05:49 PM
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Martin_G's Avatar
Joined Feb 2008
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Hi,

I'm quite sure you can design an efficient motor with a lot of flux outside...

Just remember: eddie currents come from flux density variation. Saturated iron varies much less than not-saturated iron...

But for sure it should not be the prefered way.

servus,
Martin
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Old Mar 15, 2009, 06:58 PM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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Originally Posted by Bruce Abbott
... This motor has a very thin bell which is not very effective at containing the field from its magnets. In fact the external magnetic field is so powerful that the motor can easily suspend a 150g (5oz) pair of pliers by the pointy end! ...


Forget the paper-clip test - Littlescreamers have no trouble holding 150g, but I managed to beat that the other day - I was able to suspend a 305g hammer from a Dynam/KDA A22-20L look-alike (we can't quite figure out its origin)!!!
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