Electromagnetic force calculation

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Hi there,

I am modelling the force produced by a permanent magnet on a ferromagnetic plate. I am using the inbuilt force calculation within the mnfc interface, with a low permeability gap between the magnet and the plate.

My issue is that as I increase the thickness of the plate, (keeping the other dimensions constant), the calculated force drops. This does not follow the expected trend, since in reality a thicker plate will lead to a higher force up to a point where it would plateua.

I have attached the file below, any help or advice as to why this may be the case would be greatly appreciated.

Thanks in advancce, Owen



1 Reply Last Post Apr 23, 2019, 10:15 AM EDT
Magnus Olsson COMSOL Employee

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Posted: 4 weeks ago Apr 23, 2019, 10:15 AM EDT

Hi Owen,

I have uploaded a modified version of your model that addresses a few things that could affect the results:

1) I added a control sphere withe spherical infinite elements and use the Magnetic Insulation on the outer boundaries in order to better mimic an open boundary. Using a fixed/zero magnetic scalar potential on the outer boundaries offers a zero reluctance closing path for the magnetic flux density which is probably not what you want? It would be like having the device in a mu-metal box. To get a unique solution, I use a zero magnetic scalar potential point condition. 2) As the Thin Low Permeability Gap represents a discontinuity and as no discontinuous boundary features includes the limiting edges of the boundary selection, one can sometimes get unexpected results. The remedy is to extend the feature a bit beyond the contact surface only.

That said, the real force will not be very sensitive to the plate thickness. The force will essentially depend on the magnetic flux density in the contact gap and the limiting factor for that is the total reluctance of the magnetic circuit. That reluctance is dominated by the closure of the magnetic flux density through the air - the reluctance contribution from the iron plate is negligible unless you start making it extremely thin. Thus, the variation in force that you see is more likely to be indirectly caused by the plate thickness affecting the mesh density. If you add a mesh size of 0.5mm for the Thin Low Permeability Gap and compare the force for plate thicknesses of 1mm, 2mm, 3mm etc, you will probably not see much variation in the force (as then the plate thickness does not change the local mesh density in the gap).

Then, as you have established that the force computation is mesh sensitive, it is appropriate to start making the mesh size for the Thin Low Permeability Gap smaller and investigate the convergence.

Best regards,

Magnus

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Magnus
Hi Owen, I have uploaded a modified version of your model that addresses a few things that could affect the results: 1) I added a control sphere withe spherical infinite elements and use the Magnetic Insulation on the outer boundaries in order to better mimic an open boundary. Using a fixed/zero magnetic scalar potential on the outer boundaries offers a zero reluctance closing path for the magnetic flux density which is probably not what you want? It would be like having the device in a mu-metal box. To get a unique solution, I use a zero magnetic scalar potential point condition. 2) As the Thin Low Permeability Gap represents a discontinuity and as no discontinuous boundary features includes the limiting edges of the boundary selection, one can sometimes get unexpected results. The remedy is to extend the feature a bit beyond the contact surface only. That said, the real force will not be very sensitive to the plate thickness. The force will essentially depend on the magnetic flux density in the contact gap and the limiting factor for that is the total reluctance of the magnetic circuit. That reluctance is dominated by the closure of the magnetic flux density through the air - the reluctance contribution from the iron plate is negligible unless you start making it extremely thin. Thus, the variation in force that you see is more likely to be indirectly caused by the plate thickness affecting the mesh density. If you add a mesh size of 0.5mm for the Thin Low Permeability Gap and compare the force for plate thicknesses of 1mm, 2mm, 3mm etc, you will probably not see much variation in the force (as then the plate thickness does not change the local mesh density in the gap). Then, as you have established that the force computation is mesh sensitive, it is appropriate to start making the mesh size for the Thin Low Permeability Gap smaller and investigate the convergence. Best regards, Magnus

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