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Virtual Work vs. Maxwell stress tensor

Posted Dec 11, 2014, 4:02 a.m. EST Low-Frequency Electromagnetics, Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 5.0 3 Replies

Thomas Bender
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Hi @ all,

I'm (still) try to calculate a force on a magnetic ball in the field of an electromagnet.
For the permanently magnets I use an Ampère's Law with Magnetic Field: Constitutive relation: Remanent flux density Br=1.3 [T] in y, x and z=0 [T], because the south- and north-pole should be on the y-axis. Do I have to enter parameters like bHc and BH? My magnetic ball parameters can be viewed here N42: www.ndfebmagnets.de/DE_Pros_NdFeB.htm

The cylindrical coil has a ferritic and is crossed with a negative current, that is for the orientation of the magnetic field in relation to the permanent magnet. The permanent-magnetic ball is to be attracted to the coil.

The individual results of Virtual Work and Maxwell stress tensor differ strongly.
the procedure with Virtual Work: i do two simulation and move the ball 2.5mm on the x-axis and use the difference of the Total magnetic energy (mf.intWm [J] ) to calculate the force F [N]= d_mf.intWm [J] / 0.0025 [m], are the steps correct?

If I change the radius of Sphere 1, the air around my model, the force of Force Calculation 1 varies greatly, why?

It would be very nice of a pro might look at my model and can give me some advices or hints.

Thank you for your help, with best regards Thomas
3 Replies Last Post Dec 15, 2014, 5:03 a.m. EST
Nirmal Paudel COMSOL Employee

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Posted: 10 years ago Dec 12, 2014, 9:43 a.m. EST
Hi,

The method of virtual work can be employed by using the features for "deformed mesh" and "sensitivity analysis" in COMSOL Multiphysics. Please refer to the "Deformed Geometry and Moving Mesh" and "Sensitivity Analysis" sections in the COMSOL Multiphysics Reference Manual.

There is an example "axial_magnetic_bearing.mph" in ACDC Module in which the deformed mesh and sensitivity analysis is performed to evaluate the magnetic stiffness. You could use the same approach here. In addition, you might need to further refine the mesh in the domain where "Force Calculation" node is added.

Hope this helps.

Best Regards,
Nirmal Paudel
---------------------------
COMSOL Employee
USA
Hi, The method of virtual work can be employed by using the features for "deformed mesh" and "sensitivity analysis" in COMSOL Multiphysics. Please refer to the "Deformed Geometry and Moving Mesh" and "Sensitivity Analysis" sections in the COMSOL Multiphysics Reference Manual. There is an example "axial_magnetic_bearing.mph" in ACDC Module in which the deformed mesh and sensitivity analysis is performed to evaluate the magnetic stiffness. You could use the same approach here. In addition, you might need to further refine the mesh in the domain where "Force Calculation" node is added. Hope this helps. Best Regards, Nirmal Paudel --------------------------- COMSOL Employee USA
Thomas Bender
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Posted: 10 years ago Dec 15, 2014, 4:30 a.m. EST
Dear Nirmal,

thank you for your tip, I have adapted the model for my simulation. It would be great if you could check my simulation, the results looks better so far.


Thank you for your help, with best regards Thomas
Dear Nirmal, thank you for your tip, I have adapted the model for my simulation. It would be great if you could check my simulation, the results looks better so far. Thank you for your help, with best regards Thomas
Thomas Bender
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Posted: 10 years ago Dec 15, 2014, 5:03 a.m. EST
I have modified a model with a Multi Turn Coil as magnets. But i can't find any difference in the global evaluation: mf.intWm [J] to calculate the force with the method of the virtual work.
The force calculation from Comsol Maxwell stress tensor locks better but i cantt compare it with the virtual work ;(
Can someone show me a solution how to get the differences in the total magnetic energy?

Thank you for your help, with best regards Thomas
I have modified a model with a Multi Turn Coil as magnets. But i can't find any difference in the global evaluation: mf.intWm [J] to calculate the force with the method of the virtual work. The force calculation from Comsol Maxwell stress tensor locks better but i cantt compare it with the virtual work ;( Can someone show me a solution how to get the differences in the total magnetic energy? Thank you for your help, with best regards Thomas

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