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How do I determine the electromagnetic torque of a rotating machine in a good way?

Posted Sep 30, 2019, 3:38 a.m. EDT Electromagnetics, Low-Frequency Electromagnetics, Structural & Acoustics 1 Reply

Torkel Svenson Hagström
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Hello!

So I'm trying to calculate how the torque of an electric motor changes depending on the injected current. I have tried two methods of calculating this torque, one involves applying Force Calculation on the domains that make up the rotor and the other is to use a line integral in the middle of the airgap of the following form: intop1(LrBradBphi/mu0const) where r is the distance to the curve in question, L is the length of the machine and Brad and Bphi are the cylindrical vector components of the B-field.

This is similar to exercises I've done on this subject. The problem here is that neither of these methods give a consistent solution when I try for different mesh sizes. When plotting the B-field I don't get any visible differences at all whether I use Extremely Coarse or Extremely Fine, but the torque becomes completely different with no identifiable pattern on how it changes, and the two methods differ significantly from each other as well. Does anyone have any advice on how to calculate the torque in a way that gives a more consistent solution for different mesh sizes? It would be very helpful!

Due to secrecy I unfortunately cannot upload the actual model that I'm using, but I'll upload a model based on one of the built in applications in COMSOL where I use the same methods in case I've done something wrong in that regard.

Thank you in advance!

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1 Reply Last Post Sep 30, 2019, 4:38 a.m. EDT
Edgar J. Kaiser Certified Consultant
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Posted: 5 years ago Sep 30, 2019, 4:38 a.m. EDT

Hi,

for your tests you chose a rotor position close to the torque zero crossing. This is quite susceptible to large relative numerical errors. Try your model with some rotor deflection, e.g. 10° and you will see that the results of both different methods are quite consistent and also mesh dependence is acceptable. You still may go for a finer mesh in the airgap to improve results.

I am attaching a modified example with a small parametric sweep over the initial angle of the rotor.

Cheers Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Hi, for your tests you chose a rotor position close to the torque zero crossing. This is quite susceptible to large relative numerical errors. Try your model with some rotor deflection, e.g. 10° and you will see that the results of both different methods are quite consistent and also mesh dependence is acceptable. You still may go for a finer mesh in the airgap to improve results. I am attaching a modified example with a small parametric sweep over the initial angle of the rotor. Cheers Edgar

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