Dual rotor axial flux motor design

Electric Motors and Controllers

Re: Dual rotor axial flux motor design

bearing wrote:
Miles wrote:
Miles wrote:How about multiplying whatever no load amps value is available by Kt, dividing by mass and keeping it as a separate figure for "specific parasitic torque"?

Scorpion S-5545:........0.0813 Nm/Kg [1.31A @ 10V]
Astro 3210:..............0.0395 Nm/kg [0.7A]
Turnigy C80100:.........0.1025 Nm/kg [3.5A]

Good suggestion!
Since it's no load torque, I suggest calling it Specific Tnl.

Code: Select all
`Motor               Specific Km     Specific TnlScorpion S-5545     0.36 Nm/√(W)    0.081 Nm/kgAstro  3210         0.20 Nm/√(W)    0.040 Nm/kgTurnigy C80100      0.22 Nm/√(W)    0.10  Nm/kgThingap TG2311      0.14 Nm/√(W)    0.077 Nm/kg`

I found a that Thingap have good specs on their motors. Added TG2311 for comparison. It's a 0,6kg ironless core radial flux PMSM.
http://www.thingap.com/pdf/2011/tg2311ss.pdf

Ok, "Specific Tnl"

Did you forget to divide Tnl by 0.595 kg? I make Specific Tnl 0.1290 Nm/kg Certainly makes it look even less impressive Where are all the losses coming from? Eddy currents in the copper?

Miles
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Re: Dual rotor axial flux motor design

Miles wrote:How about multiplying whatever no load amps value is available by Kt, dividing by mass and keeping it as a separate figure for "specific parasitic torque"?

This is a good start, but this is a little bit of a slippery quantity since the RPM will factor significantly into those losses.

Option: Take this quantity and multiply by no-load speed (convert units to rad/s first); the resulting units will be loss per mass in W/kg. This is the same way that losses for electrical steel are expressed. May penalize high-RPM motors? Not sure.

In other news, my FEMM simulations are going less than well. Getting useful numbers for torque is proving difficult as I can't get the different methods to agree on a single number. I just started a new run of simulations to try out a different approach. Should have some results late this afternoon.
Eric

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rhitee05
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Re: Dual rotor axial flux motor design

rhitee05 wrote:
Miles wrote:How about multiplying whatever no load amps value is available by Kt, dividing by mass and keeping it as a separate figure for "specific parasitic torque"?

This is a good start, but this is a little bit of a slippery quantity since the RPM will factor significantly into those losses.

Option: Take this quantity and multiply by no-load speed (convert units to rad/s first); the resulting units will be loss per mass in W/kg. This is the same way that losses for electrical steel are expressed. May penalize high-RPM motors? Not sure.

A lot of the time we're already starting with a slippery quantity, though.... Most specifications don't even give a voltage to go with the Io figure

Good suggestion, though.

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Re: Dual rotor axial flux motor design

Design, Comparison and Experimental Evaluation of
Non-OverlapWinding Radial Flux Permanent Magnet Hub
Drives for Electric Vehicles
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Re: Dual rotor axial flux motor design

Those thin gap motors are damb cool!!!
Thanks Justin at http://www.ebikes.ca/
Thanks to Methy at http://www.methtek.com/
And Dave who has Nomex, fets and current sensors etc. STUFF
My YSR build. viewtopic.php?f=10&t=18183
RC lipo and most other types of Lithium batteries you MUST know your individual cell voltages charging and discharging.
Don't keep them under your bed or were you cant afford smoke or fire!
Never above 4.2v never below 2.7v EVER!!!

Arlo1
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Re: Dual rotor axial flux motor design

Miles, I had looked at that reference before. So I learned that my rotor/stator layout has negative periodicity (42,36 and 84,36 gives 12/6=2 based on page 38). What puzzles me is that in the chapter on optimization, the tables indicated that embedded magnets in the rotor are better than surface magnets. To start with, I was planning the rotor, with the almost flush magnets, to be Aluminum, not steel and use a back iron disk (mu-metal?) to shape the PM return flux lines. Obviously, semi closed slots are better than open slots, back to the maximum trapezoidal flux director face on the coil relative to the overall efficiency. I am not clear why particularily embedded magnets in a steel rotor are better overall. Of course, this is an outrunner, not an axial flux design, still some of the results do apply. To much reading can sometimes be confusing.
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Re: Dual rotor axial flux motor design

Miles wrote:Ok, "Specific Tnl"

Did you forget to divide Tnl by 0.595 kg? I make Specific Tnl 0.1290 Nm/kg Certainly makes it look even less impressive Where are all the losses coming from? Eddy currents in the copper?

Maybe Tparasitic was better, but Tnl is shorter, and says what figure it's based on. I take these figures with a big grain of salt since we don't know how they were measured. Changing the RPM could double the no load torque.

I also find it strange that the Thingap no load torque is so high. In pics I've seen it seems like the conductors are wound diagonally with quite a big angle. Maybe that angle makes the overlap a bit too big. Or maybe they have a hard time driving the motor properly. The motor probably doesn't have more inductance than two loops of wire of the same diameter as the motor.

You are right, I did forget to divide by the mass. Will correct the post.
Last edited by bearing on Wed Feb 22, 2012 3:50 pm, edited 2 times in total.
bearing
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Re: Dual rotor axial flux motor design

Arlo1 wrote:Those thin gap motors are damb cool!!!
But not that impressive, performance wise.....

Miles
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Re: Dual rotor axial flux motor design

rhitee05 wrote:In other news, my FEMM simulations are going less than well. Getting useful numbers for torque is proving difficult as I can't get the different methods to agree on a single number. I just started a new run of simulations to try out a different approach. Should have some results late this afternoon.

I think we've hit the same wall. Since Saturday I have been doing the math longhand, using Excel to store the formulas and evolving a design forward. I got to a point where the math looked promising and spent yesterday modeling it in FEMM only to find that it reports significant force & torque pulsing.

I still like FEMM for 2D analysis, and I think it could be still for these types of motor designs, but we’ll have to come up with different maths for AF.

Checking the website… Eric, I just noticed the author D. Meeker pushed out an update on October 2011 although kept the revision the same; there could be something valuable there. Link to Readme.

Downloaded the source; app is written in C… not my language of choice. I might take a stab at fixing one pesky problem: When running scripts, the secondary helper apps pop to the top which is annoying if you want to read mail or post or browse whilst it’s working in the foreground instead of being well-behaved and minimized in the background.

ADDENDUM: The latest drop has a rendering issue with a typical AF model and gets stuck in a loop. I've tried both the x64 and x86 versions and there is no difference, both having pokey-slow calculus, and incomplete rendering stuck in a loop. Reverted to the November 2010 release. Bummer
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Re: Dual rotor axial flux motor design

Okay, very promising results from my new FEMM modeling technique and this afternoon's simulation run. Even better, I don't have to turbocharge the mesh density to make this method work so the simulation runs 2x faster or better.

I'll take the time to explain what I'm doing more thoroughly a little later, after I've tested it a little more thoroughly, collected more data, and rung out any bugs. For now, suffice it to say that I'm not bothering with any actual force calculations within FEMM, everything is based off the flux and BEMF. I expect this to be a little less accurate, but should be valid within appropriate bounds. Then again, given how much trouble I was having getting results which passed the sniff test before...

One thing I still need to do is a little bit of reading on what the usual conventions are for defining Kv, Kt, and such in terms of the waveforms. I may have been a little sloppy about this when reporting previous results, and it's difficult to make an apples-to-apples comparison against a commercial product unless I'm using the same definition, but I can at least be precise about what I'm calculating here and how it's specified.

Initial Phase BEMF.png
Phase BEMF waveforms

These are the simulated phase BEMF waveforms (phase-to-neutral). Note that the shape of the A and C waveforms are slightly distorted - this is due to the approximate nature of the model (only half of the stator). All of my other calculations rely on the phase B waveform which should be correct. Peak phase-to-neutral voltage is 7.004 mV/RPM. I had previously been using the inverse of this number as Kv, which would be 142 RPM/V (this version of the model is 18 turns/tooth). However, I think using the line-to-line number is more correct which is below.

Initial PhB Flux.png
Phase B Flux and BEMF at various currents

In this initial simulation run, I ran the model for a handful of currents in the range 0 - 100 A. The above plot shows the flux linkage waveforms and the BEMF waveforms for phase B over this range of currents. Note that for a current of 100 A the flux linkage is nearly constant (cores heavily saturated), which results in a very small BEMF and as will be shown later, significantly reduced torque. For very small currents (5 A here), the flux waveform is shifted slightly but the shape is not significantly altered, which results in a BEMF waveform almost identical to the zero-current case. These results make sense.

Initial Line-Line BEMF.png
Line-Line BEMF at various currents

This is the plot that I think is more useful for talking about Kv. This plot shows the line-to-line BEMF waveforms, accounting for the commutation changes every 60 degrees. The vertical red dashed lines indicate the commutation changes (I have aligned the model and the commutation so this happens at the correct instants). The peak line-line voltage is 11.44 mV/RPM, which equates to a Kv of 87 RPM/V. I believe this is the correct number for comparison. This also happens to be pretty close to 7.004*sqrt(3) = 12.13, which is what we'd expect from theory. Based on this definition, I obviously need to back the turns count way down to get back to the 150 Kv range. Note also that the voltages drop off significantly for the higher currents.

Initial PhB Torque.png
Phase B Torque at various currents

Here's the plot we've all been waiting for, torque calculations for various current levels. The instantaneous torque curves are shown, and on the left side the shorter lines show the average torque value across the active portions (note that I'm only exciting the B phase here, over 240 degrees of rotation) and the resulting Kt values. If we convert the above Kv into the appropriate units (11.44 mV/RPM * 60/2/pi) we get a theoretical Kt of 0.109 N-m/A, which is pretty close to the value we get from the lowest current. Kt drops off as the current rises, which we should expect as saturation kicks in. I didn't run enough different currents to get a really good picture, but the peak torque here is 1.75 N-m at 40 A. Now that this model seems to be working, it's pretty obvious that some design tweaks will be in order to get to the specs Miles wants.

Initial Torque vs Current.png
Torque vs current curves

Finally, curves of torque and Kt versus current. There will obviously be a peak torque, probably somewhere above 40 A, then it drops off again. Kt is roughly level at low currents and then drops off at higher currents. I'm a little bit torn about the phenomena that this model shows where the torque peaks, and then decreases for really high currents. My intuition was expecting it to level off, but remain more or less flat into saturation. However, despite the fact that this is sort of a back-door approach to the calculations the theory should be valid. The power output of a motor should be a function of the product of current and BEMF (rate of change of flux), and as the second plot shows, the BEMF flattens out as the stator gets heavily saturated, so this does seem to imply that the results might be correct. In the end I don't think it really matters because 1) we should consider this a region of operation where the model becomes suspect, and 2) the motor would never be operated in this regime anyhow. Mostly for the second reason, I think the question of validity is mostly academic.

Tonight I'm going to leave my computer running the model again for a broader range of currents. Assuming that everything still looks good, it should then be time to update the model for Miles' latest design tweaks and then start twiddling parameters. I also need to go over the model and my calculations one more time. I'm confident that the basic technique is correct, but I need to make sure that I didn't slip a factor of 2 or something in there somewhere where it shouldn't be.
Eric

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rhitee05
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Re: Dual rotor axial flux motor design

Thank you very much, Eric. I really appreciate this.

These results look more promising.... I'll take a closer look in the morning.

This is my latest iteration of the core section (mid-section).
The "neck" is now 4.5mm wide and the heads have increased to 3mm. Overall height is now 26mm. Depth increased to 13mm.

What do you think?
Attachments
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Re: Dual rotor axial flux motor design

I think those changes are definitely in the right direction. I can either make adjustments to the existing model, or you can put up a new DXF. I can also include that hole, although I don't think it will make a significant difference. Is it still 1 mm dia? It looks like it's centered in the head in both x and y dimensions. I'll adjust the turns count to get back to something close to 150 Kv.

The other thing I want to play with is the magnet and air gap thickness, particularly w.r.t. the static flux density in the cores.
Eric

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rhitee05
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Re: Dual rotor axial flux motor design

Miles wrote:
Arlo1 wrote:Those thin gap motors are damb cool!!!
But not that impressive, performance wise.....

They have one they are claiming 60kw!!!
Thanks Justin at http://www.ebikes.ca/
Thanks to Methy at http://www.methtek.com/
And Dave who has Nomex, fets and current sensors etc. STUFF
My YSR build. viewtopic.php?f=10&t=18183
RC lipo and most other types of Lithium batteries you MUST know your individual cell voltages charging and discharging.
Don't keep them under your bed or were you cant afford smoke or fire!
Never above 4.2v never below 2.7v EVER!!!

Arlo1
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Re: Dual rotor axial flux motor design

rhitee05 wrote:I think those changes are definitely in the right direction. I can either make adjustments to the existing model, or you can put up a new DXF. I can also include that hole, although I don't think it will make a significant difference. Is it still 1 mm dia? It looks like it's centered in the head in both x and y dimensions. I'll adjust the turns count to get back to something close to 150 Kv.

The hole is now 1.3mm dia. and, yes, centered in x and y. If it's possible to make adjustments to the existing model, that would be great. I've got 3 weeks worth of work to do in 8 days. I might have to resort to switching off the computer, even After that, I'll have lots of time to play.

The changes give close to 1:1 iron to copper ratio.

Miles
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Re: Dual rotor axial flux motor design

Arlo1 wrote:They have one they are claiming 60kw!!!

Yes, but since they have lower Specific Km than a typical BLDC outrunner, the motor will weigh a lot compared to motors with better numbers. The TG14010 produces 18kW continuous, and it's weight is 37kg. Thats the weight of 20 C80100 motors, which I'm sure would produce a lot more than 18kW continuous in total.
bearing
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Re: Dual rotor axial flux motor design

Eric,

Is there any way to simulate grain-oriented steel in FEMM?

It seems to be what they are doing here..

http://inwe.hogent.be/elektriciteit/Des ... hines.html

Also, I've just noticed that they use it in the Lynch motors:
http://www.agnimotors.com/home/index.ph ... &Itemid=66

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Re: Dual rotor axial flux motor design

Miles wrote:Is there any way to simulate grain-oriented steel in FEMM?

I think probably not. It's possible to enter different x- and y- parameters for anisotropic linear materials, but there doesn't seem to be a way to enter anisotropic data for nonlinear materials. I'll consult the documentation to verify, but at a glance it doesn't appear possible.

Miles wrote:The hole is now 1.3mm dia. and, yes, centered in x and y. If it's possible to make adjustments to the existing model, that would be great. I've got 3 weeks worth of work to do in 8 days. I might have to resort to switching off the computer, even After that, I'll have lots of time to play.

Yeah, I can make the appropriate changes to the model. It's probably about even anyhow, between the time it would take me to modify this model or import and re-configure a new DXF. I'll update the model geometry, work out appropriate changes to magnets, airgap, and turn count, then start running some new simulations.

Initial Torque vs Current2.png
Torque vs current curves

Here's a more detailed curve for torque versus current using the current model. Peak torque is about 1.8 N-m at 33 A. I still need to verify that I'm not missing a factor of two anywhere.

The behavior is very interesting - there is a distinct optimum current which gives peak torque, then the torque drops off but appears to level out at very high currents. I find this phenomenon very interesting and am still trying to think about whether this is would match the physical behavior or if this is a modeling artifact. I'm actually leaning a little bit more toward the former now, actually. Here's why: as steel becomes saturated, the permeability begins to decrease. Eventually, at very very large magnetic field intensity, the permeability will approach that of free space (mu_r = 1). So, a very heavily saturated iron-core motor will behave more or less like a coreless motor (in terms of torque production). A coreless motor will product much less torque for a given geometry, so it does seem to make some sense that the torque would peak and then drop off to a lower value. Still, interesting behavior. If anyone's reading this with access to a dyno, I'd be really interested in trying some experiments along this line...
Eric

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rhitee05
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Re: Dual rotor axial flux motor design

Thanks Eric.

Interesting...

Maybe we'll need even more iron....?

Miles
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Re: Dual rotor axial flux motor design

The updated geometry is definitely an improvement. I left the magnet thickness and air gap alone for now and changed the windings to 2x4 for 8 turns per core. This resulted in a Kv of 155 RPM/V. I ran a very limited number of currents, but this configuration produced something like 2.2 N-m at 50 A. I'm going to leave the computer running overnight on a more extensive selection of currents for this geometry. Unfortunately, it takes about 40 minutes to run the model per current, so this takes time... Based on the results from the previous model, peak torque was achieved at 18*33 = 594 amp-turns. The new geometry increases core area by about 52%, so that implies that peak torque might occur around 900 amp-turns, or about 112 A for this 8-turn configuration. With a little hand-waving prediction I think the peak torque might be around 3 N-m here.

The next thing I want to try is decreasing the magnet thickness to reduce the static flux density in the cores. The current configuration has a peak flux density of about 1.6 T. I think I will try to make a large change, maybe down to 1 or 1.2 T, with an appropriate increase in the turn count to keep the Kv around 150. I have a hunch that this will permit more torque output before the cores saturate. The tradeoff will be higher losses since it will require more turns to get the same Kv. Another thing I would like to try is reducing the air gap, but then decreasing the magnet thickness to keep flux constant.
Eric

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rhitee05
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Re: Dual rotor axial flux motor design

The alternative would be to increase the core to 5.5mm, which would leave 2mm either side for 2x3t or 2x4t...

Interesting to see which would be better.....

Fewer turns would give better thermal conduction to the outer surface of the coils and the core.....

We could undercut the teeth a bit, too......?

Mid-section:
Attachments
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Re: Dual rotor axial flux motor design

Hm, is it "normal" that Kt decreases like that? Whats the cause of it?

A normal PMDC motor seems to have constant Kt.
bearing
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Re: Dual rotor axial flux motor design

Miles wrote:The alternative would be to increase the core to 5.5mm, which would leave 2mm either side for 2x3t or 2x4t...

That's definitely on the list of configurations to try as well. The end result will likely be a combination of both, more iron and lower static flux density.

bearing wrote:Hm, is it "normal" that Kt decreases like that? Whats the cause of it?

It's an effect of saturation in the iron. In this configuration, the static flux density in the cores is around 1.6 T, which is right around the knee of the B-H curve for that steel alloy (using the model for M-15 here). Above the knee, as saturation occurs, it takes more and more incremental H to get each extra increment of B. This translates into requiring more and more current to get each incremental bit of torque, i.e. the Kt starts to drop off.

This is the main reason I think we'll benefit from reducing the static flux density. Not only will that allow for more amp-turns before heavy saturation, but moving below the knee means that for at least moderate currents we'll be operating in the linear portion of the B-H curve where the Kt should stay more or less constant before it begins dropping off for higher currents.

We'll see how the data back this up, but the tradeoff seems to be between torque and efficiency. A high static B requires fewer turns to achieve a given Kv, but at the cost of lower ultimate torque potential. Lower static B should allow greater torque potential, but will require more turns and thus have higher losses.
Eric

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rhitee05
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Re: Dual rotor axial flux motor design

rhitee05 wrote:
Miles wrote:Is there any way to simulate grain-oriented steel in FEMM?

I think probably not. It's possible to enter different x- and y- parameters for anisotropic linear materials, but there doesn't seem to be a way to enter anisotropic data for nonlinear materials. I'll consult the documentation to verify, but at a glance it doesn't appear possible.
Just wondering whether you'd need to enter different x and y parameters? Could we not just assume uni-directional flux flow? Or is that too simplistic? In any case, one for the future - it doesn't work in favour of the immediate issue.

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Re: Dual rotor axial flux motor design

Miles wrote:Just wondering whether you'd need to enter different x and y parameters? Could we not just assume uni-directional flux flow? Or is that too simplistic? In any case, one for the future - it doesn't work in favour of the immediate issue.

Unfortunately, yes. The defining characteristic of the grain-oriented steel is the different x- and y-direction properties (anisotropy).

Promising results! I don't have plots to post, but my simulation run overnight showed that model produced a peak of about 3.9 N-m of torque in the 110-120 A range. This was with 5.5 mm wide cores, 2x4t coils (the Kv ended up lower than desired, something around 130 RPM/V), and I made the magnets thicker and airgap smaller to boost the static flux. Based on a few other results, it looks like my theory about lower flux was not correct and higher flux is better. I'm running another model now with the flux boosted even more and 2x3t coils, which should bring the Kv closer to 150. It looks like we're closing in on a configuration to get that 4 N-m, although we still need to figure out peak vs continuous. It's also probably a good idea to set a higher target for the simulations to better guarantee you'll hit that 4 N-m when built.

Is boosting the core width to 6 mm an option? Not sure how much room is left for copper. I think the best option might be to increase the depth and/or radius a little bit. Maybe like 10%? That would likely get us to the 4.5 N-n peak range...

In other good news, after a morning of tinkering I finally figured out how to run FEMM in parallel, which is going to reduce my run time by a factor of 4. Very helpful!
Eric

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rhitee05
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Re: Dual rotor axial flux motor design

Going to 6mm width might be squeezing the copper a bit fine.......

There should just be room to increase the height of the cores by a couple of mm to 28mm. That would allow the height of the coils to increase from 9mm to 10mm. Perhaps try that first? Then, on top of that, see how it looks taking the neck from 5.5mm to 6mm width? I can only increase the radius 1mm at most. I'd prefer to leave that as the adjustment of last resort...

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