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Apr 29, 2019, 08:26 AM
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Axial flux motor stator configuration and materials


Hello I hope you guys can help me out a bit with this. I have a understanding of bldc motors and now would like to design and make my own motor.
Primarily for hi torque medium speed propellers, this particular one I would hope to be around 1kw. Quickly decided on axial flux layout because of the hi torque, relative ease of construction and high energy density along with being able to operate efficiently with a coreless stator meaning theoretically it can be made lighter.

So my questions are regarding the axial fluxmotor rotor layout and construction material. in my very sketchy sketches drawing 1 and 2 both show a layout of a high efficiency generator using round magnets, I understand the inner size of the coil should match the magnet size to allow maximum utilisation of magnet and coil at full voltage.

But as I want to create a motor and a high-density one at that, how would drawing 3 fair? Using the same size magnets it would create a much smaller lower inertia stator, of course slightly smaller coils so slightly less overall wattage and you would effectively lose some magnet coverage but have coverage for the entire phase.

Hopefully you can make out from the drawings the design I want to go for is a stator disc that is flush with the magnets front and back, so will secure them on that plane and a 'washer' type steel yoke backing. However as this is not a generator but a motor, so the rotor will be directly inside a changing magnetic field that is generated by the stator coils, I was going to use aluminium for rotor body but I feel I would be effectively putting an aluminium pan over an induction cooker hob, wasting power and creating excess heat directly in the stator. I feel the best option for creating a higher speed lightweight low-loss rotor would be fibreglasssheet that I can machine or I can easily make a fiberglass plug and mould(3d printer)?

Apologies for this being long-winded but in short:
1. For a high-energy density axial flux motors roror can the magnets be closer as in drawing 3?
2. If the magnets are close what is the affect on the magnetic flux Lines and if a double rotor was used would it stretch through the stator or will each magnet just return to its neighbour or a mix of both and dose it even matter?
3. For the main stator body will it be advantageous to use a non-conducting material so there are no Eddy current losses whatsoever?
4. Will the yoke ring have induced Eddy current aswell? or will the returning magnetic flux from the magnets protect it from this? As the magnets are so close should I ditch the yoke and use slightly larger magnets instead?


I know it is a long post but if anyone has some input and can help me along with this that would be fantastic!
Thank tom

Key
1, stator
2, yoke ring
3, magnet
4, coil
Last edited by tomtom001; Apr 29, 2019 at 11:22 AM.
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Apr 29, 2019, 12:17 PM
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Hi,
short to read , http://www.mojaladja.com/upload/elmo...%20machine.pdf .
later moore.

Happy Amps Christian
Apr 29, 2019, 05:37 PM
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Fantastic thank you Christian, clear and precise, confirmed a lot of issues I had and even provided awnsers.
Eddy current losses are indeed a problem, electrically insulating the magnets from the yoke is a simple but brilliant idea along with laminating magnets, I will employ both, along with a non conducting rotor.
Also slight magnet separation to encourage flux bridging in a dual rotor but to what degree I am not sure yet.
Last edited by tomtom001; Apr 29, 2019 at 05:42 PM.
May 11, 2019, 09:27 AM
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Hi Tom,

nice parts. Years ago i have designed at axial flux motor for direct small helicopter drive.
The part are made of litoprint plastic 3d print. Every side has Magnets, 20mm dia diske shape.
For the coils i used this side by side coil type i show in the second pic from an other diske drive motor. This is from flexdiske drive , early days of pc.

Happy Amps Christian
May 12, 2019, 04:29 AM
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Thanks Christian they look well designed, ergonomic and printed really clean, this isn't all that far from what I'm doing myself.
I've started a bit of a build blog on the motor design forum as well, I've been struggling to directly print a 3D mould for the rotor in abs.. so I will go the old route and print a plug rotor instead.

I still have one major question and that is indeed about the coils, I have used Liz and multi strand wire in a zvs switching power supply on a ferrite core, improved power, reduced arking and ran much cooler
I understand that Liz wire or multi strand has massive benefits in the reduction of eddy currents, however being a coreless motor is there any need for Lis or multi strand as there is no iron core to heat in the stator, perhaps multi strand would help Eddy Currents in the magnets and yoak even further.
May 12, 2019, 07:50 AM
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Hi Tom,
litzwire is in a slotless motor for my thought a must have. The magnetic flux is direct applied to the copper going true it, inducing good and mad current. I have searched for some papers they will show how . Dr. Richard Post from Lawrenz Livermore Institute designed his flywheel battery with Halbach magnet array and litzwire inside the rotating field reaching ,ok in vacuum, over 99% efficancy. https://patents.google.com/patent/US5705902A/en
,i have this paper also as a pdf i can send you a e-mail .

Have also a closer look at the NASA electric drive train , a axial, flux motor ,
https://ntrs.nasa.gov/archive/nasa/c...0170003042.pdf .

An other nice motor is this , http://projects.iqsoft.co.in/dual-ha...esentation.pdf

https://emediapress.com/2015/02/02/h...-7-horsepower/ .

I have build some flywheel energy storage that use slotless coils , made of litzwire , but with ferrite conducting parts to close the magnet circuite. And years ago a slotless ironless inrunner for high rev modelboat use with a two pole magnet and a star wind 5 turn litzwire , siede by side coils motor that has a kv of 7860 so on 2S battery it rev up to 55000 rpm.
on the pic i placed the litzwire we used in the flywheel storage with kevlar cover to hold high stress force as for high current discarge use.

Happy Amps Christian
May 13, 2019, 03:58 PM
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Thanks agane, yes I can understand that the coils themselves will bear the full brunt of the rotor fields, employing multi strand wire at the very least for this motor.

Nice motor, you have strong craft and knowledge. Have you seen the Dyson Digital motor? I believe they are 2 Pole reluctance quite interesting things.

That hall back arrangment on the generator would create a strong flux for sure, Kinetic energy storage has come along a long way also, I have seen some devices that use a vacume and also permanent magnets above to reduce the weight of the massive flywheel to almost nothing so there is little load on the bearings.

I like the NASA design ideas, I have a thing for duct systems lol. It also gives some detail on how much greater the energy density of a hall back array of magnets can be, however it is far harder to realise, as custom magnets are out of scope for most people.

And as for the uav motor, that is exactly what I am trying to achieve, it even uses composite rotors and is around 5kw, I was a little shocked to see it, difference being the hallback array. I did particularly liked integrated centrifugal cooling fins.

Wealth of knowledge and plenty to think about, thank you agane.
May 14, 2019, 04:17 AM
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Hi Tom
Fascinating project
So your goal is to build the lightest, high-torque motor you can? Plus of course, learn as much as you can along the way?

Hallo Christian
As always your contributions are perfectly on subject.

Looking at the links and with the help of Mr Google, it seems axial flux motors enjoyed a period of popularity before fading somewhat into the sunset. Why do you think that is? I guess in the case of LaunchPoint, it could be that they were successful in winning a government contract which, understandably, came with NDA's?

The award for most beautiful axial flux motor should I think go to Ben Katz. If you have not already seen it, have a look here: http://build-its-inprogress.blogspot...20Flux%20Motor

Name: BenKatzAxialFluxMotor1.JPG
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I wish you all the best for your project Tom, I'll be following with keen interest. I'm always on the lookout for a better motor for improving my little project - electric hang glider (the motor of which is currently 10kW @ ~3kg)
May 18, 2019, 10:06 AM
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Sorry for the delayed reply, slave to the modern wage after all...
Yes very much so, descovery is almost as good as the completion!
That is a awesome project, may i ask if that is a custom motor? On my travel through the internet I have seen people successfully using the turnigy rotomax 150 as a paramotor, there is in fact a gentleman on YouTube that rewinds his it to a even lower KV allowing use of a larger slower prop for increased low speed thrust necessary.
I would be inclined to use a ducted system with a smaller faster prop as it would be far more compact.
Do you have by any chance some performance figures? Perhaps watts at continuous power output and the static thrust generated? As it would give me a good bench mark
May 18, 2019, 10:15 AM
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Apologies I definitely should have looked through your started threads first!
I've actually looked for your threads and found your figures, congratulations that really is an amazing Project.
Last edited by tomtom001; May 18, 2019 at 05:38 PM.
May 18, 2019, 05:50 PM
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Now not that im detracting in anyway, but your figures and other people's I have found show these large Motors at WOT to only have an efficiency of 4 to 5 grams of thrust per Watt, I guess it is down to the prop RPM needed to generate the required thrust.
Tme off tommrow I should get the rotor plugs and then the moulds fully finished
May 20, 2019, 03:54 AM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Quote:
Originally Posted by tomtom001
Now not that im detracting in anyway, but your figures and other people's I have found show these large Motors at WOT to only have an efficiency of 4 to 5 grams of thrust per Watt, I guess it is down to the prop RPM needed to generate the required thrust.
Tme off tommrow I should get the rotor plugs and then the moulds fully finished
No detraction taken at all Tom! The numbers are what they are and I think its even a stretch to say as much as 5 gms/Watt.

The MP12090 I used was as it came from the factory. As 1boho commented in my thread, it's screaming for more copper A larger and slower prop would help a bit as well. To state the old rule of thumb - moving a greater volume of air slower, is more efficient than moving a smaller volume faster. Or another way to look at it, the more noise a system makes, the less efficient it is. Ducted fans are a little different in that they capture the air normally lost around the prop-tips, but this advantage is more than likely lost due to the minuscule diameter of the fan blades and number of blades competing for the circumference of air.

Sorry got a bit off topic There is a live axial flux motor build thread over on endless sphere ( https://endless-sphere.com/forums/vi...flux&start=275 ), fantastic build but looks like it will be heavy. The rotor-discs use 3mm backing-iron, which must weigh a lot. There was talk of using a Halbach magnet array to reduce the needed backing-iron, but it was decided to be too difficult. Also, each of the coils in APL's design is wrapped around iron laminates...so no light-weight all-up I imagine. APL found he got x4 the 'pull' with steel cores, vs a ferrite composite core he made. I imagine open-air coils would have an even more dispersed mag field By interpolation does this mean for an open-air coil it should be made as small and dense as possible? One thought I had was two stator disks (both with sets of coils) and one centrally mounted rotor with magnets on each side opposing (cancelling the mag field of) each other. That would be a very electrically-complicated motor!

Enough mumblings of a crazy dude how did your rotor plugs turn out?
May 20, 2019, 09:05 AM
Registered User
Hi,
to compare different motors i look allways about the force per squere cm of the rotorsurface .
Out- and inrunner are easy to do the math . Axial motors we have to calculate the ring surface where the magnets are placed. The other thing to rise the trust is to get as much surface inside the motor volume that is possible. So there are different ways to fold it or to use it from both side. One very torque dens motor design is a axial flux transversal motor design . The way of this motor is to have a thin rotor disce made of cfk to be very stiff and only placed magnets inside no other iron material. The stator in this motor is made from powder iron and there are on each side of the rotor disce one to get a big surface. The peak trust of this motor is not as high as by other designs but the very big surface catch them all and give the highest torque per motor volume. Here are two patents of this design , https://worldwide.espacenet.com/publ...C&locale=de_EP
and , https://at.espacenet.com/publication...C&locale=de_at .
About folding the trust surface here is the read, http://sternen-motoren.de/PCT_eng.htm , to see some drawings scroll down some of the track.

Happy Amps Christian
May 20, 2019, 02:52 PM
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That is more than ok! I've done an amount of research and prototyping with ducts, the interesting thing is with a high air flow and a smaller diameter prop that where the greatest increases in efficiency can be found, some gains from tip vortex and also the duct acting as a exhaust vain, but it is the low pressure created on the front surface that produces massive gains, however diminishing at increased air velocities.

The plugs took so much longer and I was expecting lol, but they are done thank you and I have tomorrow off so I will be finally making some moulds at last!

I believe double stators and a single rotor in the centre are fairly practiced so no mumbles there.

For what I have come to understand for these ironless stators to be effective the flux Fields affecting them differ from a normal motor.
The iron will concentrate the magnetic field through the coil meaning only the relatively close opposing stator and magnet surface need to have concentrated flux at that point.
The air core stator I believe relies on a highly concentrated magnetic flux being passed directly through the coil as it has no iron to 'carry' it.
So yes I believe coil thickness plays a large role on how effective it can be, I guess as increased power levels are needed the stator would become too thick so multiple stator and rotors are used or a single thick stator with an iron type core. Of course I stand to be corrected

Thank you Christian more food for thought! I started looking at electric helicopter efficiency vs multi-rotor platform there is a renowned investor backing a company that is currently testing electrically converted helicopters, using twin YASA motors, the basic idea is end of turbine life helicopters are then overhauled with the conversion.
Martine Rothblatt on converting an electric helicopter - TMC Connect 2016 (4 min 11 sec)
May 21, 2019, 04:07 PM
Registered User
Hi,
for Rotoraircraft this is a must to read, https://repository.tudelft.nl/island...m/OBJ/download

Happy Amps Christian


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