Dual rotor axial flux motor design

[Miles]

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...

[bearing]

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.

[Miles]

What is a winding factor ?

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

[Ed. Thanks bearing!]

Ref:

[Miles]

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.

[Lebowski]

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.

[Miles]

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

[Miles]

This is what the flux path looks like with a 12t layout:

[rhitee05]

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 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.

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.

[Miles]

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

[Miles]

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.

[Miles]

Schiller design: http://schiller-energy.com/pdf/schiller-energy.pdf

Schiller patent:

[Miles]

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

ISIS patent:

[Miles]

You can get enamelled copper strip down to 1mm thick from Essex. See PDF

[rhitee05]

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.

[Miles]

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 .......

[Miles]

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.

[Miles]

Reworked 18t stator:

[markobetti]

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..

[Miles]

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....

[Miles]

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

[Miles]

18t 16p:

[Thud]

When will you comit to making chips Miles?
This thread has my full attention..so far I like what i see.
my motor (s) langushes on a shelf...& I am almost out of Kudo's

[Miles]

Hi Todd,

I think I'll test out different construction techniques before I commit to a final design.

What do you think about the possibility of machining laminated blocks for the core modules?

How detrimental is minor bridging between the laminates at the edges, I wonder...?

[Thud]

Good question....
I would think the laminated block would be nearly impossible to machine conventionaly unless you actually held it under pressure in a vice or jig for the machining opperations....

My 1st though would be a grinding opperation (activly cooled) & some slow feed rates....depending on the lamination insulation material, I would think a quick clean up on a flat lapping table would be your best chance at elliminating any bridging between lams.

Just thinking outloud.

[Miles]

If one just machined the edges of the end sections it would be quite easy to lap them afterwards. A fully machined "I" section (from square blanks) would be more problematic, perhaps..

The laminations would be properly bonded (maybe pinned, too?) before machining.