MULE1.2 Axial flux test motor/bicycle specific

[johnrobholmes]

It's funny, last year around this time everyone was hyped on the reduction designs for high rpm RC motors, now it's all about building your own motor. But we still don't have a good option for applying the power to the rear wheel. Gary's adapter comes closest to what we need, IF you have a three speed hub.

The ideas and ingenuity are top notch around here. In the end I think both will have their place. Larger diameter motors would be great for single stage reduction use. Small motors would be excellent combined with a multi-speed tranny.



A good material to prototype the housing would be delrin. Not much to add on the design that hasn't been said otherwise.

 

[fechter]

Nice table!

Don't totally give up on the aluminum parts yet.
Wood sucks.

On the rotor, you don't need to worry about it, since it travels with the magnets. It would be good to avoid it between the magnets like you have done with the epoxy.

On the stator, if you have windings/cores on either side of the rotor with adequate back iron, then all the flux should be carried in the iron. You can have aluminum outside of the iron. The iron core (if used) should be laminated to avoid eddy currents. I'm not sure if the back iron needs to be laminated or not. Most similar motors I've seen don't, but I suspect that's because they are cheap. I've seen some RC motors with 'moving back iron'. This eliminates any eddy currents, but adds a gap, so I think it's a trade-off. Moving back iron would probably be good for coreless motors.

In your design, the back iron would be a thick ring that sits behind the coils. Sorry, I suck at cad drawing.

Personally, I would avoid the horseshoe configuration because it requires more iron and the iron is lossy.

I bet if you took your existing prototype and tried to spin the rotor, you'd feel some serious drag.

 

[Jonathan in Hiram]

Personally, I would avoid the horseshoe configuration because it requires more iron and the iron is lossy.

I see it as a tradeoff, the motor with iron will have a lower Kv, lower rpm and higher torque at the cost of some efficiency. I suspect you may gain in mechanical efficiency by way of simplified gearing what you lose in having the iron in the motor.

Will the iron losses be less with lower commutation rate, rpm?

Cedric Lynch manages to get high efficiency in a brushed motor with iron, despite a considerable search I have yet to see any picture or diagram of what a Lynch motor looks like inside.

 

[Miles]

Cedric Lynch manages to get high efficiency in a brushed motor with iron, despite a considerable search I have yet to see any picture or diagram of what a Lynch motor looks like inside.

Have you seen these?
http://v3.espacenet.com/searchResults?locale=en_EP&IN=cedric+lynch&ST=advanced&compact=false&DB=EPODOC&submitted=true

 

[rhitee05]

Thud,

Regarding the two horseshoe designs you posted, I think the magnetic performance would be the same assuming that the number of turns was the same in both. If the amount of iron, area of the pole, number of turns, etc. are the same the flux will see the same magnetic circuit. Doesn't really matter if the coils are split up or where they're located (at least not much).

Jonathan,

Something just occurred to me regarding the suggestion that these could be made using transformer laminations. Great idea, but the laminations will be going in the wrong direction. To minimize eddy losses, you want the conductive path in the radial direction to be short. Assuming you did the easy thing, the laminations would be running in the radial direction and that wouldn't be helpful. Axial-flux motors with a stator core generally use some sort of spiral-wrapped lamination structure (AFAIK).

 

[Jonathan in Hiram]

Eric,

It would be a little more difficult and take a little longer to glue up shorter pieces of lamination into a bar, if I were going to do that I'd just overlap them a bit like the corners are done on the power transformers I've taken apart before. Sort of like a finger joint is used to join short pieces of stock in woodworking.

The laminations in a regular motor run parallel to the magnetic flux, do they not?

Or is it just that the winding core is short on most motors?

Of course the cores could be made of ferrite or magnetite bonded with resin of some sort.

I even thought of using toroidal cores with a section cut out to fit over the stator.

 

[Jonathan in Hiram]

Miles,

I've looked at the patents but they are remarkably obtuse, even for patents. I'd really like to see a picture or 3D drawing of one of those motors, then I could get it into my head.

I can generally get some idea of what a machine looks like from patent drawings if I puzzle over them for a while, not so with Mr Lynch's wonder motor.

Looking at these drawings I do believe the Lynch motor has a mechanical means of weakening the field from the permanent magnets.

http://www.freepatentsonline.com/4823039.pdf

 

[TylerDurden]

Looking at these drawings I do believe the Lynch motor has a mechanical means of weakening the field from the permanent magnets. http://www.freepatentsonline.com/4823039.pdf

Text seems to support that notion...

 

[Jonathan in Hiram]

Text seems to support that notion...

Well I'll be dipped in .. Ahem... I made the mistake of starting at the beginning and my head was starting to hurt and my vision get blurry long before I got that far in the text.

If Cedric wrote that patent his writing is every bit as affected as his mannerisms.

 

[rhitee05]

It would be a little more difficult and take a little longer to glue up shorter pieces of lamination into a bar, if I were going to do that I'd just overlap them a bit like the corners are done on the power transformers I've taken apart before. Sort of like a finger joint is used to join short pieces of stock in woodworking.

The laminations in a regular motor run parallel to the magnetic flux, do they not?

Or is it just that the winding core is short on most motors?

I'll try to be clearer in my explanation, it's a little tricky without a picture handy.

In a "normal" motor (i.e. radial flux), the stator core is made of a bunch of thin disks, insulated and stacked together. As the magnets spin around the stator, they see the narrow side of the laminations. The dimension that matters for eddy currents is one that is perpendicular to both the motion of the magnets and the magnetic field. On a more technical level, the EMF that causes eddy currents is defined by the cross product of velocity and magnetic flux, i.e. v x B. The right-hand rule applies here. If we're looking into a motor from the side, if the rotor is moving right-to-left and we assume the flux is flowing away from us, the EMF direction would be downward, parallel to the shaft. The stator laminations need to be thin and insulated in this dimension.

For an axial-flux motor, the eddy current-causing EMF ends up being in the radial direction (toward or away from the shaft). So, to minimize this, a laminated rotor has to be thin in the radial direction. The stators I've read about so far mostly seem to use spiral-would strips, like a clock spring.

If you're proposing what I think your proposing, I think the laminations would be in the wrong direction. What I think you mean is to take something like an E-core transformer, cut out the center bar of the 'E', then you'd have a 'U' shape like that in your drawings. Ideally you'd want the laminations perpendicular to the legs of the 'U', which would involve a lot of small pieces and be pretty annoying to build.

Of course the cores could be made of ferrite or magnetite bonded with resin of some sort.

I even thought of using toroidal cores with a section cut out to fit over the stator.

I think this would work out much better. Ferrite is non-conductive, so it can be solid and not worry about eddy currents. There might be some mechanical issues, but electrically/magnetically it should work well.

 

[enoob]

I do believe the Lynch motor has a mechanical means of weakening the field from the permanent magnets.

http://www.freepatentsonline.com/4823039.pdf

im easily confused but i saw something about a shaft rotating the magnets

BUT then it looks like theres small electromagnets that are used to boost the permenant ones

 

[Thud]

Thanks for all the input every one.
Here is the shop manual for Cedric's current motor:
http://www.agnimotors.com/workshop_manual_v1.pdf
Not many photos but there is no mention of moving parts & their adjustment. I interpret the lauguage as the normal flux patterns observed as the affected parts are rotated through the PM fields. (but I can't even spell inductance, & its 1am so take that for what its worth)

As for making a core lamination, my thought is to stack shim stock & press it into a shape on a male/female form. The extream angle could be addresd with the offset stack (finger joint referance) mentioned above. I am not to crazy about the horeshoe araingment as it may be a bit cramped for winding space,hmmnn Maybe no worse though than a 12 tooth out runner stator.(its late & I am free asociating ideas) but I have confidance I could do it.
My only concern is effort/return.

Twin rotor topography looks much more reasonable now, I have made stator molds in the past so thats no big deal.& I am fairly certain I can maintain .008" air gap on a cold motor. That may need adjustment to allow for material movemet throught the thermal range.
Maybe my design time would be better spent developing a modular stator for quick change out of winding setups?

I'll ask the group...has Mule 1 served its purpous & its time to follow a more conventional design scaled to our app.?

 

[liveforphysics]

Twin rotor topography looks much more reasonable now.


Twin (or more) rotor design rocks. And remember, once you've got a twin rotor, for only one more rotor, you can add an additional stator. We of course want to keep things as light as possible, but keep in mind though the rotor count is 2n stators for a single stator setup, it's only n+1 stators for as many layers as you want to do afterwards.


Maybe my design time would be better spent developing a modular stator for quick change out of winding setups?

With so many coils, the ability to pair them in series/parallel all ready gives us some freedoms with easy equilivant coil wind options for testing. I think what we need is a clutch-basket-like multi-plate rotor setup. Something 8" or so in diameter.

I've been wrapping my head around what the optimal design for an axial flux setup would be like for a couple of weeks now, and it still changes in my head as to what is optimal. Round coils are not optimal. Coils that start from near zero radius and work outward are not optimal. Copper-fill percentage is still just as important as other motors, so as much of the area on the rotor disk needs to be used by coil material as possible. Shape of the coil, choices on core or no-core, overlap, no overlap, and cooling all need to be thought through.

 

[enoob]

dont kill the mule yet, seems to me you may get some thing interesting if you used liveforphysics clutchbasket idea to hold and increase your stator count based on what youve already done with the rotor ?

the clutch basket system would let you swap out stators rather easy.

 

[Miles]

Twin rotor topography looks much more reasonable now,.......

Yes, you can see why it dominates for the highest efficiency/lightweight designs...

It's a pity that linking the rotors peripherally, precludes driving the axle, though

 

[Jonathan in Hiram]

I only found two pictures in the Lynch motor manual that gave much of a clue about how it is configured, here they are for your viewing pleasure.




Clearly the Lynch motor does not have a lot of poles and it's also clear that the conductors are arrayed in a radial fashion.

 

[Miles]

Briggs & Stratton licensed the technology from Lynch for the Etek motor.

http://www.veva.bc.ca/articles/etek.htm

 

[Miles]

There's also a mention in here: http://www.geminielectricmotor.com/Innovate%20and%20Prosper%20-%20Gemini%20Electric%20Motor%204.doc

 

[Jeremy Harris]

The neat thing about the Lynch design is the way that it doesn't need any windings, as such. Each rotor element is a single turn, in effect, made from copper sheet, with a shim of iron inserted between each layer of copper to concentrate the flux and reduce the air gap. The copper "winding" sheets are folded, so that the fol is towards the centre and the soldered join to the adjacent "winding" is on the outer edge.

The result is a motor with a large number of effective rotor "slots" (65 for the small Lynch design) and 8 magnet poles.

I've often wondered whether it would be possible to reconfigure the Lynch motor design to use a multiple of three for the windings, reverse the copper sheet so that the fold is on the outside, convert the rotor to a stator and find a way to wire up all the inner edges of the copper sheet windings so that they could be connected up to a conventional three phase controller. I think it could be done, using an annular ring design rotor like the CSIRO one. If each copper sheet had a tab, coming out at a radially stepped position for each of the groups of three, then it should be possible to fit three annular copper buss bars to connect the three phases together, plus a common buss ring to link the star point.

I like the idea of making a winding by simply folding strips of copper sheet, as it has the potential to make for an easy to build motor. The copper losses should be pretty low, too. Getting hold of insulated iron shim to make the flux concentrating cores should be fairly easy as well, just strip some old transformers, perhaps. I don't know how the Lynch design copes with the taper from the winding centre to the outer edge, perhaps stampings of copper and iron sheet are tapered in thickness?

Jeremy

 

[Jonathan in Hiram]

Miles,

I don't think that's the same thing as a regular Etek motor..

I did find a single picture of one disassembled..

http://www.b-e-t-a.com/etek/

The money shot..



As nearly as I can determine there are eight magnets on that steel plate, logically I can infer there is another plate on the other side of the rotor with eight more magnets.

It really seems to me that radial conductors are the way to go for an axial flux motor but I'm certainly no expert.

And I found this description of the Etek which is kind of interesting.

http://www.eng.mu.edu/crovettj/ses/lib/motors/brigs/

"What makes the Etek motor technology unique is the use of copper bus bars rather than steel and copper wire as the basic building block of the armature. These copper bus bars are stamped, bent, coated and assembled into a thin rotary disk. End clips connect the tips of the bus bars to shorten the air gap between the magnets. Steel is inserted between the bus bars to shorten the air gap between the magnets. One of the most unique characteristics of the motor is that simply machining the edges of the copper bars produces the commutator. Since commutator is built in, there is no need for a separate assembly.The motor uses neodymium magnet technology. The resulting motor construction is what conventional technologists would describe as a wave wound axial air gap brushed DC motor.

 

[liveforphysics]

Twin rotor topography looks much more reasonable now,.......

Yes, you can see why it dominates for the highest efficiency/lightweight designs...

It's a pity that linking the rotors peripherally, precludes driving the axle, though

Might preclude it for you , but my clutch basket concept solved this by having the ID of the rotor bearings slide over the shaft of the stator, which would have ground flats on it's end and a hole through it to give end shaft support in mounting. The rotor and output shaft are slid over the stator shaft, and a sprocket/gear/pulley whatever can mount on the rotor shaft.

 

[Miles]

Twin rotor topography looks much more reasonable now,.......

Yes, you can see why it dominates for the highest efficiency/lightweight designs...

It's a pity that linking the rotors peripherally, precludes driving the axle, though

Might preclude it for you , but my clutch basket concept solved this by having the ID of the rotor bearings slide over the shaft of the stator, which would have ground flats on it's end and a hole through it to give end shaft support in mounting. The rotor and output shaft are slid over the stator shaft, and a sprocket/gear/pulley whatever can mount on the rotor shaft.

That's what I implied. You're forced to mount the motor by the axle...

 

[liveforphysics]

It's a pity that linking the rotors peripherally, precludes driving the axle, though

That's what I meant. You're forced to mount the motor by the axle...

Only on 1 side my friend The back can be a big solid chunk of aluminum that is fixed to the stator. It's really no different than mounting a standard outrunner motor.

 

[Miles]

Sure, you can cantilever it from one side. I still don't like like it.... I don't like the way outrunners are mounted, either

 

[liveforphysics]

Sure, you can cantilever it from one side. I still don't like like it....

Gotta come up with something better then

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