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Problems verifying model with Maxwell Tensor and Virtual Work

Posted 27.09.2010, 17:06 GMT-4 Version 4.1, Version 5.2a 4 Replies

Joaquim G. Detoni
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Hi every one,
I'm trying to design a damper using a brushless PM motor.

I'm using version 4 coupling the 2D AC/DC magnetic fields (mf.) with the moving mesh (ale). I have non linear iron and strong PM material obtaining really high saturation.

I have an 8 pole machine so I'm simulating only 1/8 of the model with antisymmetry boundary conditions and sector anti-symmetry for the pair condition.

My problem so far is that I can't get to obtain the same results for the cogging torque by calculating it using the Maxwell stress tensor and by calculating it with the virtual work method.
For the virtual work I set the problem by integrating the energy in the whole model and calculating the derivative with respect to the angle of the angular displacement from the (ale). The expression is this:

Tvw=-d(mf.intWm,mod1.omega)*Length*NumberOfSectors

Unfortunatelly, the Maxwell tensor gives me a response that is pretty much the same in shape but like 45 times higher using this equation

Tmax=mf.Tz_Cogg*Length*NumberOfSectors

I'm really confused with the way COMSOL calculates the total magnetic enery in the non linear case, but I suppose it is correct.

I attached a model with a coarse mesh just to have a smaller file, but I checked the results and the conclusions are the same.

Does any one have any hint or idea?

Attachments:

4 Replies Last Post 08.12.2016, 05:03 GMT-5
COMSOL Moderator

Hello Joaquim G. Detoni

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Oliver Vogel
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Posted: 1 decade ago 05.07.2011, 09:51 GMT-4
Hi Joaqui,

at first: nice example of an EC-Motor! I searched and already asked comsol support for such an example.

Back to Topic:
I am investigating the magnetic energy of linear actuators e.g. tubular solenoids or magnetic clamps.
I also experienced strange results and therefore contacted comsol support.

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.
Hi Joaqui, at first: nice example of an EC-Motor! I searched and already asked comsol support for such an example. Back to Topic: I am investigating the magnetic energy of linear actuators e.g. tubular solenoids or magnetic clamps. I also experienced strange results and therefore contacted comsol support. 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.
Ning Zhang
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Posted: 8 years ago 07.12.2016, 18:02 GMT-5
Hi guys,

Any updates on this issue ? I hit the same problem now and I just cannot get the virtual work force right for non-linear magnetic materials
Hi guys, Any updates on this issue ? I hit the same problem now and I just cannot get the virtual work force right for non-linear magnetic materials
Oliver Vogel
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Posted: 8 years ago 08.12.2016, 04:52 GMT-5
Hi,

as u can see, it has been a while, when I was working on this topic.
I did a paper for a comsol conference (Theory of proportional magnet and magentic force calculation) in which I also presentet some virtual work results.
So surely I finally got it running with the approach I postet above.
Maybe I can help u with an old model,
but I need some time to check my old files and then get back to you.

Best regards
Oliver

Hi, as u can see, it has been a while, when I was working on this topic. I did a paper for a comsol conference (Theory of proportional magnet and magentic force calculation) in which I also presentet some virtual work results. So surely I finally got it running with the approach I postet above. Maybe I can help u with an old model, but I need some time to check my old files and then get back to you. Best regards Oliver
Ning Zhang
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Posted: 8 years ago 08.12.2016, 05:03 GMT-5
Hi Oliver,

Thank you very much.

I think I have just got it to work a moment ago (I think).

I am not entirely sure yet until I run a full test tomorrow. I will let you know the result.

Cheers

Ning
Hi Oliver, Thank you very much. I think I have just got it to work a moment ago (I think). I am not entirely sure yet until I run a full test tomorrow. I will let you know the result. Cheers Ning

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