So here we are again, running cogging torque simulations.
This time I automated the thing based on the percentage of stator circumference occupied by the stator coil cores. I gave the cores a “hammerhead” shape to make the design more realistic, and then specified an occupation from 50% to 90%.
Here are 2 pictures, showing the difference between a 50% occ. stator and a 90% occ. stator, respectively:
On the upper image, you can see the hammerheads are narrower than the stator core; on the lower image, you can see the hammerheads are wider than the stator core.
I ran the batch for a good day or so, and accumulated the data. The script made the motor rotate 90 degrees in 1 degree steps, calculating the torque at each step. The output data is very noisy (the thing is yucky, click to see in full size): raw_cogging_torque_vs_occupation.png .
So I made use of a little statistical power to get some insight. First I turned all values into absolute values (removed the minus signs), and then applied the Max(), Median(), and Average() functions. I’m no maths or stats wizard, so I just threw everything at it and drew a graph.
Now that’s more like it! 🙂
Once again we can see that the worst case maximum cogging torque will always be very small (3 Nm!), but I can’t be sure of the remaining interpretation of the data. There seems to be a “nice spot” at 70% occupation, but I’m not so sure… the data noise could come from the precision of calculation (FEMM mesh size vs. rotation step)… maybe if I pump up the volume and specify a super-fine mesh I get a close to zero torque?…
Anyway, I’m satisfied for the time being, regarding the cogging torque of this model. I think it is good enough to be tested again only in the prototype phase. Of course, I could run more cogging simulations ad nauseum, but I’ve got bigger fish to fry right now.
MyOwnHybrid 
Njay
January 16, 2008
Ok… 3Nm seems relatively small.
Is it just my impression or you do have a very wide rotor iron shell? Can that have a negative influence?
Vasco Névoa
January 16, 2008
It’s 3 Nm in the worst case. 0,5 Nm average in the best case. 🙂
Hmm… a negative influence on what, exactly?
Well, the thicker the rotor core, the more magnetic flux it can carry. That’s a good thing, and it was my main worry in this design (I was afraid it wouldn’t be able to fit all the stuff and also have a thick enough core). But it’s looking good (although the model isn’t finished yet).
The main disadvantages of a thick magnetic core are:
– increased total wheel weight;
– increased wheel inertia;
– higher manufacturing cost (more material).
That’s why modelling is so important, I’m trying to determine the thinnest core dimension that will effectively carry enough magnetic flux for a good amount of torque and a good efficiency level.
Njay
January 16, 2008
In your drawings, the rotor thickness is much bigger than the stator thickness. That’s what I’m talking about. Either you have a rotor too thick or a stator too thin (I don’t think so, comparing with the thickness of the magnets).