Dual rotor axial flux motor design

Electric Motors and Controllers

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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Re: Dual rotor axial flux motor design

Postby Miles » Thu Mar 01, 2012 10:12 am

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Re: Dual rotor axial flux motor design

Postby bearing » Thu Mar 01, 2012 10:50 am

Interesting motors. But I still don't get why the "kt" is constant in the Agni, and has a significant slope in other motors. The Agni and the motors above has a special layout of the windings, but how the windings are layed out shouldn't have anything to do with saturation of iron. Using a solid conductor or 100 parallel insulated wires only affects fill factor.
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Re: Dual rotor axial flux motor design

Postby Arlo1 » Thu Mar 01, 2012 11:10 am

fechter wrote:
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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Re: Dual rotor axial flux motor design

Postby Miles » Thu Mar 01, 2012 11:15 am

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Re: Dual rotor axial flux motor design

Postby rhitee05 » Thu Mar 01, 2012 8:44 pm

Miles wrote:Anyone an IEEE subscriber?
http://ieeexplore.ieee.org/Xplore/login ... ision=-203


Yes. That paper and it's references look like interesting reading.
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Re: Dual rotor axial flux motor design

Postby fechter » Thu Mar 01, 2012 10:42 pm

bearing wrote:Interesting motors. But I still don't get why the "kt" is constant in the Agni, and has a significant slope in other motors. The Agni and the motors above has a special layout of the windings, but how the windings are layed out shouldn't have anything to do with saturation of iron. Using a solid conductor or 100 parallel insulated wires only affects fill factor.


The flux in the iron is going to be a function of the amps x number of turns. If the number of turns is 1, it takes a lot of amps to saturate. The Agni also does not have 'teeth' on the iron that are more prone to saturation.
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Re: Dual rotor axial flux motor design

Postby bearing » Fri Mar 02, 2012 3:40 am

fechter wrote:The flux in the iron is going to be a function of the amps x number of turns. If the number of turns is 1, it takes a lot of amps to saturate. The Agni also does not have 'teeth' on the iron that are more prone to saturation.


Makes perfect sense. I guess what I'm wondering is: what's the drawback? why isn't everyone making motors which doesn't saturate?
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Re: Dual rotor axial flux motor design

Postby fechter » Fri Mar 02, 2012 10:06 am

bearing wrote:
fechter wrote:The flux in the iron is going to be a function of the amps x number of turns. If the number of turns is 1, it takes a lot of amps to saturate. The Agni also does not have 'teeth' on the iron that are more prone to saturation.


Makes perfect sense. I guess what I'm wondering is: what's the drawback? why isn't everyone making motors which doesn't saturate?


The drawback is you don't get much BEMF from one turn, so you need a lot of poles to make up for it. In smaller sizes this would become a challenge to get enough poles and still have a reasonable size for the iron pieces.
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Mar 07, 2012 1:05 pm

Latest update of the model:
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Re: Dual rotor axial flux motor design

Postby Miles » Wed Mar 07, 2012 1:15 pm

How about the following idea for connecting up the coils?

- Before assembly, crimp the ends of each coil strip around a gold-plated tube (bullet connector?).

- Use gold-plated copper wire to form the bridging links.

- Wire is a push fit into the ID of the tube (end could be tapered).
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Re: Dual rotor axial flux motor design

Postby fechter » Thu Mar 08, 2012 12:22 pm

I'd solder the connections after pressing them in. Using high melting point solder might be good there too (but it's even better if these parts don't get hot enough to worry about that).

On your rotors, it might be good if there was a small lip on the OD to help keep the magnets from flinging off at high speeds. I don't trust glue that much. Also, if the rotor hub was mild steel, it could help the back iron or replace the back iron.
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Re: Dual rotor axial flux motor design

Postby flathill » Thu Mar 08, 2012 7:22 pm

Miles wrote:How about the following idea for connecting up the coils?

- Before assembly, crimp the ends of each coil strip around a gold-plated tube (bullet connector?).

- Use gold-plated copper wire to form the bridging links.

- Wire is a push fit into the ID of the tube (end could be tapered).


How about some poki-poki style
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Also know as a joint lapped core
http://www.mitsubishielectric.com/compa ... 12_tr3.pdf

Not exactly applicable but it gets you thinking :)
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Re: Dual rotor axial flux motor design

Postby Miles » Fri Mar 09, 2012 7:31 pm

fechter wrote:On your rotors, it might be good if there was a small lip on the OD to help keep the magnets from flinging off at high speeds. I don't trust glue that much. Also, if the rotor hub was mild steel, it could help the back iron or replace the back iron.

Yes, I'm intending to put a lip on, possibly by milling shallow pockets for each magnet, which would also give the spacing.

Your second point has prompted me to try something that we talked about before. With this, all the internal height of the motor could be used for active material.

Increasing the height of the cores to match would lower Rm whilst also increasing heat dissipation but would mean higher iron losses. Thoughts?
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Re: Dual rotor axial flux motor design

Postby fechter » Fri Mar 09, 2012 8:13 pm

I'm missing something.

In the pic above, the gap needs to be shortened. I guess you're suggesting making the cores longer to fill the space. It would be better to move the rotors inward to shorten the gap. Adding iron behind the magnets can't hurt.
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Re: Dual rotor axial flux motor design

Postby Miles » Fri Mar 09, 2012 8:19 pm

Yes. It was/is work in progress:
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