Dual rotor axial flux motor design

Electric Motors and Controllers

Re: Dual rotor axial flux motor design

Postby rhitee05 » Tue Mar 13, 2012 1:48 pm

Miles wrote:Do you think the reduced cross section (approx 1/3rd for <1 mm), where the two coils are connected in the middle, will cause any problems?


I seriously doubt it. We won't be coming anywhere near the level of current where that would act like a fusible link, and it's such a short segment that the increased local heating should dissipate quite well into the rest of the coils. I doubt there will be any noticeable impact at all.

Miles wrote:Magnet shape modified to give a more equal flux density across the laminations.


This actually won't matter. I can't demonstrate this without full 3D modeling, but the flux won't be limited to just the narrow lower section of the core - it will spread across the entire volume of the core, so any worry about the narrow section of the core saturating first are mostly unnecessary.
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Mar 13, 2012 2:39 pm

rhitee05 wrote:
Miles wrote:Do you think the reduced cross section (approx 1/3rd for <1 mm), where the two coils are connected in the middle, will cause any problems?
I seriously doubt it. We won't be coming anywhere near the level of current where that would act like a fusible link, and it's such a short segment that the increased local heating should dissipate quite well into the rest of the coils. I doubt there will be any noticeable impact at all.

Great! I was just worried that there was some effect that I hadn't considered.

rhitee05 wrote:
Miles wrote:Magnet shape modified to give a more equal flux density across the laminations.
This actually won't matter. I can't demonstrate this without full 3D modeling, but the flux won't be limited to just the narrow lower section of the core - it will spread across the entire volume of the core, so any worry about the narrow section of the core saturating first are mostly unnecessary.

I guess I just assumed that it would stay separated because of the reluctance of the inter-laminar insulation. Or, do you mean that it spreads across only as each lamination approaches saturation?
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Mar 13, 2012 3:00 pm

One thing that occurred to me, looking at this pic.... Why don't we pack more copper in (or add more iron) until the coils meet (not electrically!) and then, if necessary, ventilate the inside, axially....? Better not to generate the heat in the first place.....

Still got to get the termination wires out somehow, though..... :mrgreen:



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Re: Dual rotor axial flux motor design

Postby Miles » Tue Mar 13, 2012 6:49 pm

Miles wrote:Just wondering if it would be feasible to laser cut the laminate stack....?
I got in touch with Polaris Laminations (recommended by Biff). They can't cut stacked laminations but don't seem to have a problem with cutting and assembling the 70 different lam sizes to make each module. :shock: They stock Arnon 7 NGO steel http://www.arnoldmagnetics.com/Non_Grai ... Steel.aspx No GO steels, though.
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Re: Dual rotor axial flux motor design

Postby flathill » Tue Mar 13, 2012 6:53 pm

The biggest challenge is likely going to be maintaining the axial gap (bearing system):

"The bearings were absolutely critical to enabling the modular design of the Yokeless and Segmented Armature [YASA] motor that we helped to design with Oxford," says Nick Carpenter, technical director at Delta Motorsport. "Ultra Low Energy Vehicles, such as the E-4 Coupe, rely on lightweight components to compensate for the inherent problem of the low energy density of lithium batteries when compared to gasoline.

When I first came up with the idea of the new bearing arrangement that would help to optimise the space available for the stator, while accurately controlling the air gap to the rotors, I almost dismissed it, due to the fact that standard bearings would have been too big and too heavy. However, following detailed discussions with Schaeffler UK, the company's engineers were able to offer a special design of hybrid double-row angular contact ball bearing, which has a split inner ring that was able to meet the demanding criteria of reduced weight and cross-section."

This was the breakthrough that Delta Motorsport needed, if the company was to fulfil its ambition of developing a highly efficient plug-in battery electric 'car for the people' that is both stylish in its design and financially viable, but also capable of overcoming the 'range anxiety' perceived by many purchasers of all-electric vehicles.

As Stewart Davies, senior applications engineer at Schaeffler UK, recalls: "It was a very interesting project to work on, with very demanding criteria to meet. The challenge was to keep the bearing as narrow as possible, in order to meet the dimensional constraints of the design envelope inside the YASA motor, whilst at the same time offering a commercially viable bearing solution within a very tight timescale."

Schaeffler UK was able to supply the first samples four months after it started work on the project. Davies continues: "The initial bearing design concept proved to be too expensive and threatened to jeopardise the project. Following intensive design calculations, we were able to offer a single hybrid bearing that met the key design criteria of high performance, reliability and ease of assembly, but that could also be competitively produced in volume."

One of the problems encountered by Schaeffler's design team was the high seal lip speed. With the bearing rotating at such high speeds, all calculations indicated that the seal would wear out and that the bearing would fail prematurely. By specifying a non-contact shield, this problem was solved. Another advantage of using a non-contact shield is the increased life expectancy of the grease. According to Davies, "a hybrid bearing has all the insulation qualities required for use in electric motors, including low friction, but the non-contact shield has doubled the life expectancy of the grease, which has further enhanced the performance of the bearing"

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I would like to see a YASA hub motor :wink:
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Mar 13, 2012 7:00 pm

flathill wrote:The biggest challenge is likely going to be maintaining the axial gap (bearing system):
I was about to turn my attention to that aspect...... Thankfully, I've got a reasonable amount of space to work within and the forces are quite modest in comparison with the YASA.... :)
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Re: Dual rotor axial flux motor design

Postby flathill » Tue Mar 13, 2012 7:19 pm

Miles wrote:
Miles wrote:Just wondering if it would be feasible to laser cut the laminate stack....?
I got in touch with Polaris Laminations (recommended by Biff). They can't cut stacked laminations but don't seem to have a problem with cutting and assembling the 70 different lam sizes to make each module. :shock: They stock Arnon 7 NGO steel http://www.arnoldmagnetics.com/Non_Grai ... Steel.aspx No GO steels, though.


Maybe EDM wire cut for the prototype...not sure with the geometry
http://www.hvwooding.co.uk/motor-laminations
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Mar 13, 2012 7:24 pm

flathill wrote:Maybe EDM wire cut for the prototype...not sure with the geometry
http://www.hvwooding.co.uk/motor-laminations

I've not had much to do with laser, water-jet or EDM wire cut....

I guess the bonding of the laminations is going to be the critical factor for post machining....

Woodings look to be an interesting contact. I'll get in touch with them. Thanks.
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Re: Dual rotor axial flux motor design

Postby flathill » Tue Mar 13, 2012 7:41 pm

flat foil winding.JPG
flat foil winding
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Mar 13, 2012 7:55 pm

The snag with using that method is that my strip is 0.9mm thick :mrgreen:
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Tue Mar 13, 2012 8:55 pm

The two-rotor design has good and bad aspects for the bearings. The good is that the magnetic forces will nominally cancel each other out as far as the bearings are concerned - the internal braces between rotors will take care of those loads and there will be net zero force on the entire assembly. The bad is that it's an unstable equilibrium, such that a slight shift to either side will unbalance the forces in such a way that the imbalance wants to get larger.

I'm not an ME and I certainly don't know much about bearing design. But, given the above, it seems like you'd want bearings that are a) precise, and b) capable of handling the unbalanced forces due to modest misalignment. So you would need bearings with at least some thrust capability, but probably not enough to withstand the entire magnetic attractive force.
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Re: Dual rotor axial flux motor design

Postby Kingfish » Wed Mar 14, 2012 1:29 pm

I can help with bearings…
For most needs, follow examples by the automotive industry; they use tapered bearings for axles. These can be externally greased ala trailer and hub axles, or sealed (maintenance-free). There are three companies that I have dealt with in the past, all fabricating reliable product.

The issue generally is with supply; pick a rep that is prompt with low shipping costs in your neck of the woods. My local SKF supplier took about a month to send me the replacement hub bearings, whereas one supplier delivered SKF seals from half-way across the country in three days. Personally, I enjoy calling up the reps and negotiating with the sales because they often have great insight on matching product with the application and saving a lot of time and expense in the process.

Know before Ordering
You will need to spec out your axial load, radial loads (static and dynamic), maximum shaft speed, maximum temp, ID (shaft diameter), OD, and allowable thickness (affecting cup design). It’s a little bit cumbersome with the wide selection, though I would lean towards both Automotive and Fuel-Efficient designs. Agreed that you will want the sealed bearings.

Regardless which manufacturer you select, I am certain that tapered bearings will suit your design well for precise positioning :)
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Mar 14, 2012 2:25 pm

Thanks guys.

I'm not sure I could get a tapered roller bearing small enough... I'm only using a 12mm shaft. :)

I guess the first thing is to estimate the maximum axial force that's likely to be experienced.
Some bearing pre-load should take care of axial movement.
Need to find a way to balance the two gaps accurately.
It may be that deep groove radials will be sufficient.

Otherwise, I was thinking to use a pair of these 40deg. angular contact bearings:
http://medias.ina.de/medias/hp.ec.br.pr ... A7?clrsb=1
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Mar 14, 2012 2:49 pm

This is the smallest tapered roller bearing listed:
http://medias.schaeffler.de/medias/hp.e ... I4?clrsb=1

I could go to 15mm hollow shaft........ :)
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Re: Dual rotor axial flux motor design

Postby bearing » Wed Mar 14, 2012 2:57 pm

I think the angular contacts are better for your application. They should have less drag.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Mar 14, 2012 3:01 pm

The other possibility is an opposed pair of spindle bearings. You can get 15 deg. or 25 deg. contact angle in the small sizes.
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Re: Dual rotor axial flux motor design

Postby Kingfish » Wed Mar 14, 2012 3:02 pm

Timken makes a single-taper bearing that goes down to 8mm, though I think you are wise to upsize the axle for more common product as it is bound to be less expensive :)

If weight is an issue, you could make use of hollow shaft stock. Just an idea...

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Re: Dual rotor axial flux motor design

Postby Miles » Wed Mar 14, 2012 3:15 pm

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Re: Dual rotor axial flux motor design

Postby Miles » Wed Mar 14, 2012 8:02 pm

Crikey! :D

Core cross-sectional area has gone up 1.375x

Volume of copper has gone up 1.45x

Number of turns down from 6t to 4t

2 phases length down to 3 metres.
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Re: Dual rotor axial flux motor design

Postby fechter » Wed Mar 14, 2012 9:24 pm

36W of wasted heat will barely get warm. And that's at full power. Who needs cooling?
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Re: Dual rotor axial flux motor design

Postby Miles » Thu Mar 15, 2012 3:55 am

fechter wrote:Who needs cooling?
Quite. :D

You have to add the parasitic losses to the 36W but if I use G-O steel and 0.178mm laminations, they should be pretty low.
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Re: Dual rotor axial flux motor design

Postby Miles » Thu Mar 15, 2012 4:41 am

If we don't need the through venting, we can pack more copper in and get:
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Re: Dual rotor axial flux motor design

Postby Miles » Thu Mar 15, 2012 4:52 am

Km would then be 1.21

Weight has crept up to 1.4kg because of the additional material, so specific Km is 0.87

Miles wrote:Scorpion S-5545 10t Delta: http://www.scorpionsystem.com/catalog/m ... 545-150KV/

Weight 1.026 Kg

Kv 150 rpm/V

Kt 0.0637 Nm/A

Rm 0.03 ohms

Km is 0.0637 / SqRt 0.03 = 0.37

[ 0.37 / 1.026 = 0.36 ]


Astro 3210 8t Wye: http://www.astroflight.com/pdfs/3210WEB.pdf

Weight 1.0 kg

Kv 169 rpm/V

Kt 0.0565 Nm/A

Rm 0.08 ohms

Km is 0.0565 / SqRt 0.08 = 0.20


Ok, the Scorpion is in the lead.... :)


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Re: Dual rotor axial flux motor design

Postby bearing » Thu Mar 15, 2012 5:34 am

Number of turns down from 6t to 4t

2 phases length down to 3 metres.


With the same kV?
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Re: Dual rotor axial flux motor design

Postby Miles » Thu Mar 15, 2012 5:35 am

Yes, that was the assumption. There's quite a lot more iron, now...... We'll see..... :mrgreen:
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