Dual rotor axial flux motor design

Electric Motors and Controllers

Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 3:15 am

Next decisions:

How many slots? 18, as modelled or I could possibly reduce it to 15? without increasing the lead in/out angle across the laminations too greatly...

How many poles?
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Re: Dual rotor axial flux motor design

Postby Lebowski » Wed Feb 01, 2012 3:17 am

Miles wrote:Next decisions:

How many slots? 18, as modelled or I could possibly reduce it to 15?

How many poles?


These choises have no effect on efficiency... just make sure the motor is physically big and has lots of copper in it.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 3:26 am

Lebowski wrote:These choises have no effect on efficiency... just make sure the motor is physically big and has lots of copper in it.
But they will affect the amount of copper in the links between coils... If I used 15 slots and 14 or 16 poles, most of the coils could be linked to their adjacent partner, I think. As I'm intending to use rectangular copper strip and the connections are outside of the case this would be an advantage...

Need to find out the trade-offs WRT the winding factor?
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Re: Dual rotor axial flux motor design

Postby bearing » Wed Feb 01, 2012 4:22 am

I don't think a stator with 15 teeth is going to work well. With 14 or 16 poles, windings will, as you say, be put in groups of 1/3 of the diameter. Every time a phase pair is energized it will "bend" the rotor(s) and shaft. See the summary at the bottom of this page: http://powerditto.de/15N14P.html
It might work better with an axial design with small diameter, but I would personally avoid it.

From a mathematical perspective, it looks like you end up with a solution like this (windings in groups of 1/3 of diameter) every time the number of stator teeth is 3 * prime number, and at the same time use a pole count as close as possible to number of teeth. (3*5 and 14/16 in this case.) It will, however, give a very good winding factor. To solve this "imbalance", use twice as many teeth, which will put windings in two groups, on opposite sides of the centre. 18t (2*3*3) 18p/20p are twice as many teeth as the "imbalanced" 9t (3*3)p 8p/10p. Following this logic, 30t (2*3*5) with 28p/32p would be a good solution, or 42t (2*3*7) with 40p/44p. This can be confirmed with http://powerditto.de/bewicklungsrechner.html
The famous 12t (2*3*2) with 10p/14p also follows this logic/design of 2*3*prime number and then the closes possible number of poles.
Last edited by bearing on Wed Feb 01, 2012 4:57 am, edited 3 times in total.
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Re: Dual rotor axial flux motor design

Postby Lebowski » Wed Feb 01, 2012 4:26 am

Miles wrote:
Lebowski wrote:These choises have no effect on efficiency... just make sure the motor is physically big and has lots of copper in it.
But they will affect the amount of copper in the links between coils... If I used 15 slots and 14 or 16 poles, most of the coils could be linked to their adjacent partner, I think. As I'm intending to use rectangular copper strip and the connections are outside of the case this would be an advantage...

Need to find out the trade-offs WRT the winding factor?


What is a winding factor ?

I thought about using copper strips as well, an idea which I had with this respect that you may find usefull:
at farnell they sell (quite cheaply) heatshrink tube in all diameters and up to 5 meters in length (5 meter for 4 euro
for the thinner stuff). You can sheeth your copper strip with heatshrink tube before winding it to isolate it....

Another thing I thought about at the time was a tripple stator design with one phase per stator. You can
wind the strip in a cloverleaf design... disadvantage is the big piece of iron (a.k.a. axle) going through
the middle of your coils :(

Also usefull, think Lorentz force, not magnetic attraction / repulsion.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 4:32 am

bearing wrote:I don't think a stator with 15 teeth is going to work well. With 14 or 16 poles, windings will, as you say, be put in groups of 1/3 of the diameter. Every time a phase pair is energized it will "bend" the rotor(s) and shaft. See the summary at the bottom of this page: http://powerditto.de/15N14P.html
It might work better with an axial design with small diameter, but I would personally avoid it.
Thanks bearing. That's a very good point. I guess the "unbalanced" rotor/pole combination would be noisy, too...
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Re: Dual rotor axial flux motor design

Postby bearing » Wed Feb 01, 2012 4:46 am

Winding factor has got to do with the amplitude of EMF compared to the maximum possible EMF based on peak magnetic flux. If the number of teeth is the same as number of poles, the winding factor is 1, since all flux will generate EMF in windings. But you can't use a teeth/poles ratio of 1:1 with an iron core (probably with ironless core though) because of the super high cogging. If the number of teeth is different from the number of poles, the winding factor will go down. With 12t/2p, the winding factor is 0.25. With 12t/4p, WF is 0.5. With 12t/8p or 12t/16p, winding factor is 0.866 (=SQRT(3)/2). With 12t/14p or 12t/10p WF=0.93.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 4:46 am

Lebowski wrote:What is a winding factor ?

I'm hoping someone else will explain that.... :)

[Ed. Thanks bearing!]

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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 4:54 am

bearing wrote: To solve this "imbalance", use twice as many teeth, which will put windings in two groups, on opposite sides of the centre.
18t and 20p (winding factor 0.945) seems the best solution for me, I think. I'll see what a 12t layout looks like, though.
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Re: Dual rotor axial flux motor design

Postby Lebowski » Wed Feb 01, 2012 5:26 am

My axial flux motor made me think...

The magnetic force from the iron-less windings is very small. The torque comes from Lorentz-force which
is B*I*L. I can't rememer B at the moment but the torque matched the force I calculated with this Lorentz
force equation. For Lorentz force it's necessary that there is a current carrying conductor in a magnetic field.

But if you have a motor with an iron core all the magnetic field channels through the iron, there will be no
Lorentz force. It works base on magnetic attraction and repulsion.

I found it very interesting that apparently there are more than one way of generating electro magnetic force
and that different motors use different principles. My main conclusion however was that adding iron to my motor
would allow the flux to go through a smaller diameter area which would lead to a shorter bit of wire necessary
for 50 the turns I use -> less resistance -> more efficiency.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 6:12 am

Lebowski wrote:an idea which I had with this respect that you may find usefull:
at farnell they sell (quite cheaply) heatshrink tube in all diameters and up to 5 meters in length (5 meter for 4 euro
for the thinner stuff). You can sheeth your copper strip with heatshrink tube before winding it to isolate it....
Thanks. This is a good idea but ordinary heatshrink tube (even the thin stuff) might still be a bit thick walled to get a good fill factor? Maybe I can get some very thin walled heat shrinkable Kapton/Polyimide tubing in a suitable size? Otherwise, I was planning to use a layer of Kapton tape.

Ref: viewtopic.php?p=230735#p230735
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 8:05 am

This is what the flux path looks like with a 12t layout:
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12t-layout-flux-path.jpg
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Wed Feb 01, 2012 8:44 am

Your desired speed/torque/Kv also might be a factor in choosing the number of teeth/poles. All other things being equal (which they won't be...), the 18t motor will have 2/3 the speed of the 12t motor, but 1.5x the torque (not accounting for different winding volumes, etc). You could still make the Kv of either motor almost anything you wanted, but this is probably a good time to decide about what range you're shooting for.

Miles wrote:I was planning to use a layer of Kapton tape.


That's what I would have suggested, even using really thin heatshrink you'll end up with 2x the thickness between wraps. Kapton tape should be pretty easy, since you could just apply it to one side of your copper strip and then roll the whole thing up.

Lebowski wrote:I found it very interesting that apparently there are more than one way of generating electro magnetic force
and that different motors use different principles.


That's not accurate. There's more than one way to analyze and calculate the force, but Maxwell's laws haven't changed. If you've figured out how to do that, let me know and we can get stinking rich together! Calculating the torque using an iron core is trickier because it really requires software to calculate the flux, whereas it's constant in an ironless machine. Once you know the flux, it's a fairly simple matter to calculate the Kv (V = N*dPsi/dt) and from there the Kt.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 8:58 am

rhitee05 wrote:Your desired speed/torque/Kv also might be a factor in choosing the number of teeth/poles. All other things being equal (which they won't be...), the 18t motor will have 2/3 the speed of the 12t motor, but 1.5x the torque (not accounting for different winding volumes, etc). You could still make the Kv of either motor almost anything you wanted, but this is probably a good time to decide about what range you're shooting for.

I'm aiming for a Kv of around 150 rpm/V.

I guess I have to find out what copper strip thicknesses are readily available, too. Strip width needs to be about 8mm.

The thicker the strip I can use (fewer turns), the better the fill factor :D
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 9:03 am

rhitee05 wrote:
Miles wrote:I was planning to use a layer of Kapton tape.


That's what I would have suggested, even using really thin heatshrink you'll end up with 2x the thickness between wraps. Kapton tape should be pretty easy, since you could just apply it to one side of your copper strip and then roll the whole thing up.
If the core is epoxied, there's only the exposure between the two coils to worry about..... A Kapton "washer" separating them would sort that.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 9:43 am

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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 9:45 am

ISIS YASA design: download/file.php?id=66703

ISIS patent:
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 12:38 pm

You can get enamelled copper strip down to 1mm thick from Essex. See PDF
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Wed Feb 01, 2012 12:59 pm

8mm x 1mm is pretty hefty stuff - comparable to 8-ga round wire. Bending radius could be an issue - you'd probably need to round the edges of the lam stack to accommodate it. Definitely suitable for a high-current motor, though...

If you want to provide me with some of the dimensions you're looking at, I can toss together a quick FEMM model as a place to start considering some of the tradeoffs.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 1:04 pm

rhitee05 wrote:8mm x 1mm is pretty hefty stuff - comparable to 8-ga round wire. Bending radius could be an issue - you'd probably need to round the edges of the lam stack to accommodate it. Definitely suitable for a high-current motor, though...
The insulation thickness values seemed quite high, too. But with only 3 turns x 2 ....... :mrgreen:
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 1:07 pm

rhitee05 wrote:If you want to provide me with some of the dimensions you're looking at, I can toss together a quick FEMM model as a place to start considering some of the tradeoffs.
Thanks Eric. That would be very useful. I'll model up the stator modules properly for 12t and 18t layouts.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 01, 2012 5:31 pm

Reworked 18t stator:
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Re: Dual rotor axial flux motor design

Postby markobetti » Thu Feb 02, 2012 1:48 am

Miles wrote:Reworked 18t stator:


will it be possible ti add this way one more lineup of magnets on top of allready mounted mags; for small increase in flux . If you remember ; the lynch motors have this option on their high power models . Modul per modul based motor..
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Re: Dual rotor axial flux motor design

Postby Miles » Thu Feb 02, 2012 4:37 pm

Hi Marko,

I'm not up-to-date with what Lynch do so I'm not certain exactly what you mean.

This is a double rotor design and the second rotor is essential to complete the flux path.

If you mean stacking another layer of magnets on each rotor, I guess it's possible as long as it's allowed for in terms of space and rotor stiffness.

I've no plans to go into motor production.... :)
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Re: Dual rotor axial flux motor design

Postby Miles » Thu Feb 02, 2012 6:49 pm

This is what the present 18t stator looks like in relation to 20 rectangular magnets:
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