Dual rotor axial flux motor design

[rhitee05]

Another thing I need to do is review how Kv is defined in terms of the EMF waveforms. I think the simple definition is:

Kv = Vpp/2/RPM

Assuming trapezoidal BEMF. The definition is slightly different for a sinusoidal motor, although I'm pretty sure this design will end up with a more trapezoidal BEMF.

[Miles]

But, going over to the topic of Thud's thread, the calculator shows that 16p has 144 cogging steps while 20p has 180, so 20p will probably have somewhat lesser cogging force.

But the number of cogging steps per revolution won't directly affect the total parasitic torque (and therefore efficiency)? Right?

[Miles]

Assuming trapezoidal BEMF. The definition is slightly different for a sinusoidal motor, although I'm pretty sure this design will end up with a more trapezoidal BEMF.

At some stage, I thought it might be interesting to try round magnets, in order to get a more sinusoidal BEMF.

[bearing]

I believe it was stated earlier in this thread that this design wouldn't have any end turn losses. Is that true?

In a normal radial "outrunner" design, only the part of the windings that are parallel to the magnets will contribute to torque, correct? won't the same apply to this design?

[Miles]

I believe it was stated earlier in this thread that this design wouldn't have any end turn losses. Is that true?

I certainly didn't intend to imply that.

In a normal radial "outrunner" design, only the part of the windings that are parallel to the magnets will contribute to torque, correct? won't the same apply to this design?

Yes, in this case, only when the coils are running radially will they contribute to torque. If you have a wedge shape inner core (mine is uniform), you can take advantage of wedge shaped magnets but the proportion of the coils which is non-contributory will increase significantly. It would be interesting to do the comparison.

[Miles]

Deciding the ratio of iron to copper:

If the goal is to maximise the continuous torque output, the amount of iron only needs to be sufficient to avoid saturation up to that specific torque level. By reducing the amount of iron and so being able increase the amount of copper, you decrease both parasitic and direct losses and raise the maximum sustainable torque level by sacrificing the peak levels of torque.

So, working back from the maximum heat that the design can dissipate.........

Does this make sense, as a strategy?

[rhitee05]

But the number of cogging steps per revolution won't directly affect the total parasitic torque (and therefore efficiency)? Right?

Yes, the parasitic eddy current losses are separate from cogging. Cogging is an annoyance at low speeds, but is not a net loss to efficiency. The eddy losses will be proportional to the number of poles and the RPM, as bigmoose stated in that snippet you quoted previously.

At some stage, I thought it might be interesting to try round magnets, in order to get a more sinusoidal BEMF.

That should be the effect, but it would be difficult to simulate that - the less uniform the cross-section, the less accurate a 2D linearized model will be. A better approximation can be had by breaking it up into a number of slices, but that still won't capture all of the effects. Maybe something to try after we've iterated the basic design a few times and reached an optimum.

Deciding the ratio of iron to copper:

If the goal is to maximise the continuous torque output, the amount of iron only needs to be sufficient to avoid saturation up to that specific torque level. By reducing the amount of iron and so being able increase the amount of copper, you decrease both parasitic and direct losses and raise the maximum sustainable torque level by sacrificing the peak levels of torque.

This sounds like the correct approach. I should be able to run simulations to compare a few different cases. We'll have to see if this works, but what I think I should be able to do is generate a plot of torque vs. current for a given geometry configuration. What I would expect to see, if this works correctly, is a linear relationship (slope = Kt) at lower currents and then leveling off as the iron saturates. That should make it possible to determine the peak torque for that geometry configuration. Max continuous torque is a slightly different matter, since losses and heat dissipation come into play... but I suspect that when we reach that point we can probably make some assumptions that will let us at least make a reasonable estimate for the purpose of comparing different geometries. If we can estimate the continuous power dissipation capability, then we should be able to work backwards to max continuous torque.

[Miles]

Thanks Eric.

I think what I should do is fully parametrise my model, to make it easier to change.

[markobetti]

I've saved it as an A'CAD 14 DXF

Magnets to be N48

Air gap circa 0.7 mm

Allowing for insulation, copper fill ratio is 0.75 of marked area.

Possibly 2 x 6 turn coils (12t) ? This gives slightly under 0.5mm thick strip.

Magnet flux backing 2mm thick steel.

i would prefer if you d use N42SH they are easy available

max at 150C (302F)



we used this one in last colossus 12kw. Belive me , this is what you want...

http://www.kjmagnetics.com/specs.asp

n48 N48 13.8-14.2 KGs >11.0 KOe >12 KOe 45-48 MGOe
n42sh 13.0-13.3 KGs >11.4 KOe >20 KOe 40-42 MGOe

[Miles]

Ok. N42SH for Marko, N48H for me.

[markobetti]

hhehhe ,Miles , you rock... Isnt he the nicest man here ? Insted of saying ; Marko shut up...he is like this ))

[fechter]

You may have some extreme forces on the cores when assembling the motor, especially if the magnets 'stick' to them. I couldn't see exactly what holds them to the frame, but whatever it is, it needs to be very strong.

[Miles]

Yes, I wouldn't like to be making an axial flux motor any larger than this.....

The cores are restrained axially by a recess in the can. They are held into the recess by the mutual wedging action. The last core and separator in is the "keystone". They could also be bonded into the recess. I'll definitely need to make sure this is sufficient at an early stage, using an equivalent non-magnetic force

I need to devise a way to move both rotors equally with respect to the stator....

[rhitee05]

I'll add to my simulation list estimating the attractive force of one rotor to the stator with and without the other rotor present. That should give you some idea of how much the assembly challenge will be.

[Miles]

Thanks Eric.

I've just done the 1/6 & 5/6 linearised sections.

I discovered that the mid radius section I did yesterday was off centre by 0.6mm. Should I redo it?

[Miles]

Here's a comparison between the 1/6 & 5/6 linearised sections

[Miles]

Two more DXFs

[rhitee05]

I discovered that the mid radius section I did yesterday was off centre by 0.6mm. Should I redo it?

Probably should, although I bet it won't affect the results much. Still, it's easy enough for me to import the new model. I should have some early results soon.

What's the radius for that center section? By my math, I put it at about 36.4 mm.

[Miles]

What's the radius for that center section? By my math, I put it at about 36.4 mm.

When it's consistent with the other 2 sections I've just done, it will be 35.5mm.

I'll redo it.

[Miles]

Re-done 3-6 section

[rhitee05]

Teaser:

Prelim BEMF.png

Rough BEMF Simulation

(28.04 KiB) Downloaded 3 times

Here's an early simulation of the motor BEMF. There are a couple of issues with this simulation, so it's pretty rough. The shape on the Phase A and C waveforms aren't right, so I made a couple of changes to the model to reduce (hopefully eliminate) the edge effects and am re-running (takes some time). I also just checked my math and realized that the units here are off by a factor of 64x. Still, the shape of the phase B waveform looks promising. Correcting the units, I get a Kv of about 174 RPM/V for this geometry. Is that ballpark of what you're trying for? Caveat: this simulation uses N40 magnets, because FEMM doesn't have a built-in model for N42 or N48, so the Kv will end up being lower for either of those strengths.

[Miles]

Still, the shape of the phase B waveform looks promising. Correcting the units, I get a Kv of about 174 RPM/V for this geometry. Is that ballpark of what you're trying for? Caveat: this simulation uses N40 magnets, because FEMM doesn't have a built-in model for N42 or N48, so the Kv will end up being lower for either of those strengths.

Thanks Eric.

Yes, definitely sinusoidalish.

Crikey! I didn't expect to get that close! I was aiming for 150 Kv with N48 magnets

That's with 12t? Right?

[rhitee05]

Crikey! I didn't expect to get that close! I was aiming for 150 Kv with N48 magnets

That's with 12t? Right?

Yes, this is for two 6t coils on each core, with all 6 pairs for each phase in series. Also using 3 mm thick N40 magnets, with 3 mm thick back iron. I think I figured out how to model N42 or N48 magnets, so I can try those in the next run.

[Miles]

Resistance per phase will be something in the order of 25 - 30 milliohms? Rm say 55 milliohms?

An 8t Astro 3210 (weighing about the same) with a similar Kv has an Rm of 80 milliohms.

Looks like I have a fighting chance......

[panurge]

Hi
I'm Impressed by your motor model, Miles, very interesting, as by your last works, like the beautiful bottom bracket/freewheeling cranck model....

Unfortunately in motors I'm nearly Zero...... but just today I've been in a Comsol workshop where they showed some multiphysic simulation models of this motor....not useful for terrestrial vehicles, indeed , but how not to post a Reaction Sphere Electric Motor on the Sphere?

reaction sphere.jpg (32.68 KiB) Viewed 239 times

http://csnej106.csem.ch/detailed/pdf/e-proj-SPHERE.pdf

More here: http://www.comsol.com/papers/9385/

Fascinating thing......just image it applied to CNC.....

(sorry for the OT )

Jules