The limits of torque production for a given weight of motor

[SolarRay]

We need to turn them into better structures.

 

[Miles]

Here's a graphic comparison

 

[Miles]

Just a thought: Is the magnet thickness optimal?

I was aiming for 1T airgap flux density. I used N45SH to give some thermal headroom. I think we're ok WRT the permeance coefficient. We might be able to save a bit of weight on the magnets. I haven't tweaked the width yet, for instance. We'll see.....

Any reasonable ways to increase winding fill factor?

Well, unless we also applied it to the Astro..... I doubt if that has a fill factor 0.6 so, we're already pushing it.....

 

[Punx0r]

Fair enough

The graphical comparison is interesting to see - the prototype design really does look like a wheel rim. It's hard to fight the tendency to think it wants supporting like one.

 

[Teh Stork]

One Norwegian startup tried to get rid of the gearbox and run windmills direct drive. Part of the invention was a stator and rotor supported like a bicycle rim, with spokes. One set to hold the rotor, and one to hold the stator. Regarding air-gap fluctuations due to shifts in wind and load -the stator is ironless and therefore more forgiving to variations in the air-gap. Motor specs: ~12 RPM operating speed, 10 MW, 25 meter stator diameter

They went out of money and currently an investment-firm holds the patents and IP. The future looks bleak tbh for this venture.

 

[Miles]

I tried the winding calculator here http://www.bavaria-direct.co.za/scheme/calculator/ , to get a graphic of the phase layout, but it wouldn't accept more than 2 digits for the slot number :lol:

So, I've added it to the model. You can see the three symmetries.

 

[Miles]

This would seem to be a good candidate for a "slinky" lamination stack. Otherwise, there'd be a 99% wastage factor

 

[Hillhater]

Respect for your work and efforts here Miles. and these results are inded impressive for such low weight.
Am i missing something here ?... I thought there was some basic relationship between air gap dia and torque...T~~ cD^2. .?
But we now have a motor with approx 10x the dia, similar weight of active materials, .so theoretically we ought to be expecting 10^2 ~ 100 times the Torque output..........but at the moment its only 16 times the Torque .?
Why this huge "orders of magnitude". difference ?

 

[parabellum]

But we now have a motor with approx 10x the dia, similar weight of active materials, .so theoretically we ought to be expecting 10^2 ~ 100 times the Torque output..........but at the moment its only 16 times the Torque .?
Why this huge "orders of magnitude". difference ?

Do not forget, it got slimmer.

 

[liveforphysics]

I tried the winding calculator here http://www.bavaria-direct.co.za/scheme/calculator/ , to get a graphic of the phase layout, but it wouldn't accept more than 2 digits for the slot number :lol:

So, I've added it to the model. You can see the three symmetries.

Drool!! That is so beautiful Miles!

 

[speedmd]

It is beautiful. I can see some shared support on the rotor side with some spoke attachments and outer brake rotor used as a radial support. Thin carbon shell would be trick. Brake Caliper would have some tremendous leverage at these diameters. Torque arm can share as brake support and stay stiffener.

 

[Farfle]

I have seen a high specific torque motor with magnets on both sides of the iron. Essentially replacing the yolk iron with magnets, similar to the way a two rotor single stator axial flux motor works, but with radially configured flux instead. i wonder what that would look like in FEMM.

 

[Miles]

I have seen a high specific torque motor with magnets on both sides of the iron. Essentially replacing the yolk iron with magnets, similar to the way a two rotor single stator axial flux motor works, but with radially configured flux instead.

There's the Sineton design. That's two stator one rotor:
http://www.sineton.com/web/index-10.html
http://www.sineton.com/web/index-4.html

 

[Miles]

But we now have a motor with approx 10x the dia, similar weight of active materials, .so theoretically we ought to be expecting 10^2 ~ 100 times the Torque output..........but at the moment its only 16 times the Torque .?
Why this huge "orders of magnitude". difference ?

Do not forget, it got slimmer.

Airgap area x airgap radius, the difference is 23 times. So, not so far off.....

 

[Miles]

I've put the data onto the comparison spreadsheet.

I added a new column ( H ) for airgap area * airgap radius

 

[parabellum]

I've put the data onto the comparison spreadsheet.

I added a new column ( H ) for airgap area * airgap radius

You marked it as cm, it does not mean centimetres right?
P.S. I mean H column.

 

[Miles]

You marked it as cm, it does not mean centimetres right?

Yes, that's the base unit. Sorry, that is confusing..Forgot to finish that off...
It's column E/20 * column G.
Division by 20 to get radius and to convert to centimetres.

 

[toolbag]

The double rotor or double stator layout that Farfle mentioned gets you a little bit more torque per amp, and in an ideal world you can hope to see marginally more efficiency with one of those (on the order of 0.2% improvement on a 97% simulated efficiency). The trouble with those is that they suck structurally.

The high pole count outrunner you have posted is not so crazy. One important thing to note with a motor like this is that you will want to try to keep your inductance as low as possible. Motors of this type often have a very high per-unit inductance, which will absolutely kill the power factor at the high electrical frequency that you will need. The end-turn to useful copper ratio also gets pretty bad with a short stack motor like this.

A useful constraint for this thought experiment is cooling. With good cooling you can push >20A/mm2, but if you assume that you're using air cooling then your acceptable losses go waaay down. In general it takes a lot of flux to get to a point where you're not making more torque with more current. With water cooling the same electromagnetics can do 5 times as much torque as a well air-cooled motor.

It's cool to see people upping the specific torque ante

 

[Miles]

Thanks for your overview toolbag.

The end-turn to useful copper ratio also gets pretty bad with a short stack motor like this.

The end turns are 40% of coil length, in this case.... We could certainly try keeping this configuration and reducing the diameter a bit to get a longer stack.

It would be fun to scale this motor up and build it into a monowheel

 

[Miles]

The double rotor or double stator layout that Farfle mentioned gets you a little bit more torque per amp, and in an ideal world you can hope to see marginally more efficiency with one of those (on the order of 0.2% improvement on a 97% simulated efficiency). The trouble with those is that they suck structurally

In the Sineton introduction, that I posted above, they claim 40Nm peak torque per kg of active mass for their dual stator design (@ 170mm diameter, liquid cooling) and 10Nm per kg continuous torque for their claw pole design (@ 150mm diameter, passive cooling).

At 10Nm/kg (20 Nm) we are getting 54 Watts of heat, according to the simulation.

 

[Miles]

https://www.indiegogo.com/projects/bimoz-the-world-s-smartest-e-bike-drive#/story

The Bimoz motor gives a maximum torque of 50Nm, for a 1.37 kg motor, according to the specification....

 

[John in CR]

https://www.indiegogo.com/projects/bimoz-the-world-s-smartest-e-bike-drive#/story

The Bimoz motor gives a maximum torque of 50Nm, for a 1.37 kg motor, according to the specification....

I'd take anything they say with a grain of salt, since they claim up to 150km of range with their 2kg package including battery. I should set up a fundraiser campaign to mock ridiculous EV claims by promoting a package with a 100g motor powered by a single 18650 cell that will go 1000km.

 

[Hillhater]

I thought it was an April Fools joke !

 

[Miles]

I just thought it would be interesting to include references to projects that are attempting to push the limits. Leaving aside the 50Nm max., they'll need to be making 26Nm continuously, to justify the 250 Watts in the specification........

I love the answer to this question.

"Question:
"Direct drive systems like the BionX do have a drag torque when turned of like a traditional dynamo has when it’s turned on. Your system claims not to do so, how is that possible?"

Answer:
We took the term „direct drive“ serious in the development of the bimoz. The power transmission from the engine to the pedals takes place through an electromagnetic field. Our patented solution neither has gears, timing belts or other mechanical parts. Therefore it is a real direct drive. The powerful 250 W highefficient magnetic drive works electromagnetically. The result is no friction at all when you turn it off."

 

[nutnspecial]

Hey Miles, I wasn't sure where to post this or make a thread. The answer might be obvious?
In the nature of the idea that volts are gearing, and even the distance from shaft to drive wheel as mentioned in the 'gearboxes' thread . . . I guess design is also gearing, and torque is a value of rpm x power . . .


What would be any technical drawbacks or benefits in making and running a low enough kV motor to equal 1 motor rpm equalling 4rpms of a 24" wheel, such as on my bike (example)?

The reason I ask is I like the 30kV version of 'big block' for mid drive, and would consider it difficult for volts vs amps ratio or a higher sprocket reduction ratio to use the same power @ same speed on a 70kv motor version.

I wonder what the motor would look like if my reduction of 11t to 50t was (as an example) actually reversed (55t on motor) for the same speed on that wheel (which is ~40mph on 80v and 30kV)?

If the motor would be possible, what voltage and amperage ratio would it use, and what implications would there be?

Thanks