Dual rotor axial flux motor design

Electric Motors and Controllers

Re: Dual rotor axial flux motor design

Postby Miles » Mon Feb 27, 2012 12:26 pm

rhitee05 wrote:We can treat this as a tradeoff. For example, if we reduce the cores to 26 mm, that gains us 1mm on each side for additional magnet and/or iron. The tradeoff there should be a little extra loss (lower copper volume) for extra torque. We could run a few cases to get an idea of where the optimum is.
Ok. That makes sense.
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Re: Dual rotor axial flux motor design

Postby Miles » Mon Feb 27, 2012 12:59 pm

Eric,

Flux is 3D in the back-iron... Could we not increase the effective back-iron volume by reducing its inner radius?
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Mon Feb 27, 2012 1:39 pm

Miles wrote:Flux is 3D in the back-iron... Could we not increase the effective back-iron volume by reducing its inner radius?


Yes, that should work at least to some degree. I'm not sure if it would be a straight volume improvement (i.e. is 12 mm wide x 4 mm thick equivalent to 16 mm x 3 mm), but probably at least a fairly large percentage.
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Re: Dual rotor axial flux motor design

Postby Miles » Mon Feb 27, 2012 1:44 pm

I'll play around with the idea.
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Mon Feb 27, 2012 5:34 pm

Miles wrote:I have an overall length of 56mm, which is set by the length of my gearbox.

The 28mm core height seems right - I'm happy with that.

That leaves probably a maximum of 6mm for magnet plus back-iron height (43.5 mm over back-irons). I'll see if I can gain some more, though....


Trying to make sure I properly understand the length constraint. You said 43.5 mm total length, i.e. from the back edge of back iron on one rotor to back edge of back iron on the 2nd rotor? Using 28 mm long cores + 2*0.75 mm airgaps, that's 29.5 mm, which leaves 7 mm at each end for magnets + iron. Am I correct here? Going to 26 mm long cores would make it 8 mm each, and so forth. 24/26/28 seems like an appropriate range to explore here.
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Re: Dual rotor axial flux motor design

Postby Miles » Mon Feb 27, 2012 6:28 pm

rhitee05 wrote:Trying to make sure I properly understand the length constraint. You said 43.5 mm total length, i.e. from the back edge of back iron on one rotor to back edge of back iron on the 2nd rotor? Using 28 mm long cores + 2*0.75 mm airgaps, that's 29.5 mm, which leaves 7 mm at each end for magnets + iron. Am I correct here? Going to 26 mm long cores would make it 8 mm each, and so forth. 24/26/28 seems like an appropriate range to explore here.
:oops: Looks like I meant 41.5mm back edge to back edge of the back-irons. Sorry about that.

The only way I could get more would be to fix the back-iron to the top of the rotor (cantilevered out). The limit then would be 45mm.
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Re: Dual rotor axial flux motor design

Postby fechter » Tue Feb 28, 2012 12:42 am

Miles wrote:The only way I could get more would be to fix the back-iron to the top of the rotor (cantilevered out). The limit then would be 45mm.


Or use a Halbach array :wink:
I'd stick with back iron though. A Halbach array would require some hard to find magnets.

Increasing the volume by decreasing the inner radius will only help a little, as the iron between the magnets will tend to saturate first, though satruating the back iron isn't all that bad as long as there isn't any aluminum right behind it.

I'm not sure what happens to the back iron at high currents. I suspect the flux will increase, so you need to avoid saturation at the maximum current. If the back iron saturates and the motor case is close to it, you could get some nasty eddy currents (drag).
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Feb 28, 2012 3:35 am

The periphery of the back-iron is close to the case, so I definitely don't want it saturating
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Tue Feb 28, 2012 8:52 am

fechter wrote:Increasing the volume by decreasing the inner radius will only help a little, as the iron between the magnets will tend to saturate first, though satruating the back iron isn't all that bad as long as there isn't any aluminum right behind it.


That was my initial thinking as well, but after considering it some more I don't think this is as much of an issue.

At lower frequencies (where inductance is not an issue), electric currents will tend to spread over all the available copper, even if it's not the most direct path. If you have a PCB with ground plane, the return currents will tend to spread over the entire plane (more concentrated on the direct path between source and load, but widely distributed). This, incidentally, is where ground loops come from. The currents tend to be spread according to the resistances of the various paths.

Since we can make a good analogy to magnetic circuits, the same principle will hold true. As the iron directly behind the magnets begins to saturate, the flux will tend to spread out more and make use of all available iron even if the path length is a little longer. It might not be quite the same effectiveness as if you put the same additional volume of iron directly behind, but I think it would still be effective enough to be worthwhile. It's impossible to simulate this 3D flux pattern in FEMM, though.
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Re: Dual rotor axial flux motor design

Postby fechter » Tue Feb 28, 2012 9:31 am

MFEA is only so good. I don't really trust it fully but is a good tool for comparing alternate configurations.

I have a gaussmeter that I trust much more. If you take a pair of magnets and sandwich them between two pieces of iron to create a closed flux path, you can measure the flux on the back side. Maybe in this case, two pairs of magnets stacked and stuck together like having the two rotors stuck to each other without the stator might be close to operating conditions.

Based on what I've seen in other motors, I'd guess you want at least 4-5mm of back iron.

Even without a fancy gaussmeter, you can just take a paper clip and see how much attraction there is on the back side. If the iron is saturated, the clip is strongly attracted. On my Zappy motor, I know the back iron is saturated and it's close to 4mm. I think the magnets are about 5mm but I have no idea what grade they are.

Even if the back iron is saturated, the eddy currents will be greatly diminshed by leaving a good sized air gap between the iron and any stationary housing metal. Using stainless steel or titanium instead of aluminum makes a huge difference too.
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Feb 28, 2012 9:41 am

fechter wrote:Using stainless steel or titanium instead of aluminum makes a huge difference too.
Good point. I don't have to use aluminium. The case could even be CFC, as most of the heat will be lost through convection (hopefully).
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Tue Feb 28, 2012 10:56 am

Overnight results - 6mm wide x 28mm long cores, 2x3t coils, stainless (air) pins, 0.75mm airgap, N48 magnets of varying thickness.

Model5a Torque vs Current.png
1.5 mm thick magnets
(20.92 KiB) Downloaded 3 times

Model5b Torque vs Current.png
3 mm thick magnets
(20.04 KiB) Downloaded 3 times

Model5c Torque vs Current.png
4 mm thick magnets
(20.04 KiB) Downloaded 3 times

Model5d Torque vs Current.png
5 mm thick magnets
(20.05 KiB) Downloaded 3 times


There doesn't seem to be very much gain past the 3 mm thick magnets. Some increase in the peak torque, but the Kv/Kt are very similar and the current required to reach 4 N-m only drops from 75 A to about 70 A. Based on this, it looks like 2-3 mm thick is probably the sweet spot given the tradeoff vs. back iron thickness. I just started another set to look at the 3 mm thick magnets vs several back iron thicknesses. Later I want to drop the core width back to 5 mm and try some permutations. I made a few more tweaks to my code to make it a little more efficient, hopefully I'll be down in the 4-5 hour range to process a single model case for a range of currents.
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Feb 28, 2012 11:31 am

Thanks Eric. We're homing in :D

Regarding the copper losses, I've been playing with the calculator on this site:
http://circuitcalculator.com/wordpress/ ... alculator/
I like the way it sets a cookie to remember your settings/values.

Length of 2 phases, for 6mm wide cores with 2 x 3 turns is approx. 4 metres.
Wire section: 0.5mm x 10mm (for 28mm core height).

See also:
http://circuitcalculator.com/wordpress/ ... calculator
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Re: Dual rotor axial flux motor design

Postby Miles » Tue Feb 28, 2012 1:07 pm

Here's how things look in DriveCalc:
[With 2A @ 48V for no load current... :mrgreen: ]
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Re: Dual rotor axial flux motor design

Postby kenkad » Wed Feb 29, 2012 9:47 am

Regarding the recent torque constant vs phase current plots (model5d), I keep wondering 'if the torque constant is higher at lower phase currents', why are motors not designed so they have a multiple of 3-phase groups so that each 3-phase coil set is operating in the max range of 15-20 amps? Certainly, it seems that six 3-phase groups at 15 amps (90 amp equivalent) is much greater torque that one 3-phase group operating at 90 amps (seems like about 6 Nm vs 4.9 Nm on the last graph). Would this not also mean less concern about coil saturation, etc. Can someone provide an explaination? I cannot imagine more electronic losses with six 3-phase group of drivers. Just trying to understand what the simulation is showing.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Feb 29, 2012 9:56 am

kenkad wrote:I keep wondering 'if the torque constant is higher at lower phase currents', why are motors not designed so they have a multiple of 3-phase groups so that each 3-phase coil set is operating in the max range of 15-20 amps?
I'm not sure it will make any difference, Ken. The fact that you're dividing the work between 3 controllers won't make any fundamental difference to the motor. The only advantage that I can see in this is for the controllers.
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Wed Feb 29, 2012 10:17 am

kenkad wrote:Regarding the recent torque constant vs phase current plots (model5d), I keep wondering 'if the torque constant is higher at lower phase currents', why are motors not designed so they have a multiple of 3-phase groups so that each 3-phase coil set is operating in the max range of 15-20 amps? Certainly, it seems that six 3-phase groups at 15 amps (90 amp equivalent) is much greater torque that one 3-phase group operating at 90 amps (seems like about 6 Nm vs 4.9 Nm on the last graph). Would this not also mean less concern about coil saturation, etc. Can someone provide an explaination? I cannot imagine more electronic losses with six 3-phase group of drivers. Just trying to understand what the simulation is showing.


It's a function of saturation in the stator iron. As the current gets higher, it begins to require larger and larger incremental changes in current to get a certain incremental change in the B field, due to the non-linear magnetic properties, which in turn means larger incremental changes in current to get a certain incremental change in torque. Thus, the torque constant tends to drop off.

If you want to avoid that effect, as you say, just operate the motor at a lower current. But that means you'll have a much larger and heavier motor for a given power output. Picture an X5 limited to 500 W output. It doesn't matter how you have the coils configured, the more torque you want out of the motor the further into saturation you'll need to operate it.
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Re: Dual rotor axial flux motor design

Postby bearing » Wed Feb 29, 2012 11:59 am

How is it possible for the Agni motor to have a completely linear torque/current relationship in it's full published current range?
http://agnimotors.com/95_Series_Performance_Graphs.pdf

Look at figures 2a/2b to get a picture of the copper/iron ratio. Is it enough iron to not even reach the start of saturation @ 400A?
http://www.google.com/patents/US20020163258
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Wed Feb 29, 2012 2:06 pm

I'm not familiar enough with the Agni motors to speculate. It's possible they do have enough iron to avoid saturating.
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Re: Dual rotor axial flux motor design

Postby fechter » Wed Feb 29, 2012 2:59 pm

bearing wrote:How is it possible for the Agni motor to have a completely linear torque/current relationship in it's full published current range?
http://agnimotors.com/95_Series_Performance_Graphs.pdf

Look at figures 2a/2b to get a picture of the copper/iron ratio. Is it enough iron to not even reach the start of saturation @ 400A?
http://www.google.com/patents/US20020163258


It's a brushed design, and each 'coil' has like one turn.
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Re: Dual rotor axial flux motor design

Postby rhitee05 » Wed Feb 29, 2012 9:32 pm

I'm headed out of town for a few days, so posting will be sporadic or nonexistent until Monday. However, before leaving I set up my computer to run a series of model permutations across what seems to be the most promising range of parameters. I'm checking 4/5/6 mm core widths, 2, 2.5, and 3 mm magnet thicknesses, and 24/26/28 mm core lengths (varying back iron thickness to maintain 41.5 mm overall length). That's a total of 27 permutations, which should give us a fair idea of the tradeoff space when I get back and crunch the data. :-)
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Re: Dual rotor axial flux motor design

Postby bearing » Thu Mar 01, 2012 4:14 am

fechter wrote:It's a brushed design, and each 'coil' has like one turn.

Is it possible to make brushless version with the same properties?
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Re: Dual rotor axial flux motor design

Postby markobetti » Thu Mar 01, 2012 5:43 am

efficiency would fall
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Re: Dual rotor axial flux motor design

Postby bearing » Thu Mar 01, 2012 6:46 am

Why?
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Re: Dual rotor axial flux motor design

Postby fechter » Thu Mar 01, 2012 9:47 am

bearing wrote:
fechter wrote:It's a brushed design, and each 'coil' has like one turn.

Is it possible to make brushless version with the same properties?


Yes, it is possible. The eCycle solid slot motor is an example. Each 'coil' is a single turn of solid copper bar.
The Remy HVH is another example. The datasheet is here:
http://www.remyinc.com/docs/Remy_HVH_250_Sep09.pdf



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