Cogging check of the new design

Posted on January 16, 2008

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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:

stator_50_pct_occupation.png

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.

stator_90_pct_occupation1.png

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.

stats_cogging_torque_vs_occupation.png

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.

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Posted in: Motorfemmulator