# How to calculate the Lorentz Force, Damping Force and Damping coefficient in 3D transient Analysis

Member Posts: 10
edited January 2021

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• Posts: 74Ansys Employee

The strategy one should use for modeling the effect of Lorentz forces (presumably due to motion induced eddy currents) on structural motion depends on the principle of operation of the device. I would need more details of the device and how it works before I could confidently advice you on an analysis procedure. In the absence of such details, perhaps I can describe things you might try if you were analyzing something akin to an eddy current brake.

At MAPDL V16.0, the now undocumented single code multifield solver was still documented and supported. All the commands pertaining to this feature began with the letters MF - please locate and review these commands in the Help. Turn on the multifield solver with the MFANALYSIS,ON command. The idea is to create two models in a single MAPDL database - one structural and the other electromagnetic (use the MFELEM command to declare which element types belong to each the two "fields"). The BFE,,FVIN and MFVOLUME commands can be used to specify transfer of volumetrically distributed Lorentz forces from the electromagnetic field elements to corresponding structural elements used to model the same body. They can also be used to transfer displacement from the structural model to the electromagnetic one, thus effecting "bidirectional coupling" between the two fields. Before solving, the MORPH command should be used to enable the magnetic elements representing the space surrounding device components to morph (accommodate relative motion between the components it encloses).

Some very basic examples are provided in the Help, such as this:

I always use carefully prepared input files for these types of analyses - working interactively in the GUI, I tend to lose track of the stage of setup I am currently working on and what I need to do next.

If an electrically conductive structure is oscillating adjacent to a field source (e.g., a permanent magnet or coil), the decay in the calculated amplitude of motion can be used to infer an effective damping ratio.

If the geometry of the device lends itself to modeling in 2D (planar or axisymmetric), it is usually easier to use coupled field PLANE13 with keyopt(1)=4 (UX, UY, TEMP, and AZ DOFs). The AZ DOF is the the z component of the magnetic vector potential, whose curl is the magnetic flux density (B = curl(A)). Make the stiffness of the "deforming air" enclosing the moving bodies very low, and turn on nonlinear geometry effects (NLGEOM,ON).

I hope this helps you get started. System coupling in Workbench between Maxwell and Mechanical may be easier for you.

Kind regards,

Bill

• Posts: 22Member

Thank you very much for your guidance. My problem statement is,

a cylindrical conducting Tube (ring shaped) is placed in a permanent magnet cylindrical tube. The magnet is held stationary and rotation is given to conducting tube about z axis (through real constant option). Magnets are axially magnetized (along z axis). SOLID97 element is selected for analysis.

I tired to carried out the 3D transient analysis. Modeling, meshing is done and boundary condition are defined. After this transient solution option is selected and in solcontrol option I have given some step loading data . I have given the values as

time at the end of load step = 0.2

automatic time stepping is kept on

Selecting "time increment" option

time step size = 0.002

minimum time step = 0.001

maximum step size = 0.005

In frequency option selecting " writing every sub steps" and OK

By solving using Current LS

solution is done dialog does not come up or any error dialog box appears. In output window it shows

ERROR

3D velocity effects can be currently solved by harmonic analysis

<ANTYPE,HARM> A-V formulation <KEYOPY<1>=1>. Apply a negligible frequency to emulate a static analysis.

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and apart from i wanted ask that how to generate a magnetic field from a permanent magnet which is equivalent to two step loading step in 3D transient analysis?

Once again I am very thankful for your comment.

Yours Sincerely