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Hello Gerhard Edmound Stebner

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Hello Gerhard,

I had and still have trouble calcualting magnetic energy, too.
The problem with comsol is, that magnetic energy is calculated with a linear approach by default!!!
I do not know, which version of comsol you are using right now, but with version 4 and later you can check this fact by activating the equation view and search in the corresponding Ampere's Law node for variable mf.Wm. As told before it normally is calculated by mf.Wm = 0.5*(mf.Bx*mf.Hx+mf.By*mf.Hy+mf.Bz*mf.Hz), which is only correct for linear magnetic materials. For any non linear magnetic Material it is necessary to define a function (interpolation table) for magnetic energy density. You can calcualte the the magnetic energy density out of the BH - or HB - curve of the corresponding material by integrating H dB. Then you need to integrate the energy density over all domains of the model to get the total amount of magnetic energy. So far seems to work fine, but proving forces by virtual work approach I get forces wich are less than half of comsol results!?! Testing forces by virtual work with just linear materials works perfekt.
I got the feeling, that there might be a bug in the software. I will contact comsol support again help and advice....

Best regards
Oliver Vogel


I attached a file, so you can explore it yourself.

Hello Oliver,

unfortunately I am not at work... so I cant test your model. But incredible... you are right it is a linear approach. I will test my torque calculations on synchronous machines with the method described. Thanks a lot. I will share my new results as soon as possible. One question... did you multiply the result of the integral with the length?! Your equation would be right for 1 m of depth...

Hello again,

I guess your question assumes a 2D-model...
My test model is actually a axisymmetric 2D model, so there is no depth!

I also tried to simulate a small synchronous without success. I did not get the clue how to model the interface betwee stator and rotor correctly. I would be very very pleased, if you could provide a working model.

I mentioned that there still might be bug in energy caclualtion in comsol, so I am definetly interestet for your results.
I am also preparing a big request for comsol support about that. Kepp you informed when it is answered.

Best regards
Oliver

Hello again,

I was on vacation so i didnt answer. Sorry for that. I'm not allowed to post my models. But if you give me your email-adress I will send a modified geometry of my recent project. This will help you to understand how synchronous machines can be simulated.

Best regards,

Gerhard

Hi,

The same problem for me:

I am performing some simulations of a synchronous motor. Now I am dealing with cogging torque:

I'm using nonlinear material (Iron)

I checked the mesh to be symmetrical in the air gap, where I perform the stress tensor integration (I built a middle line in the air gap so as the mesh to be symmetrical: three stripes of element inner and outer it)

Then I integrate the magnetic energy in the rotor domains but what I get is a very irregular line (what I expect is a theta-derivative not higher than 0.01 Nm .. )

Could anybody help me?