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Electric Currents Boundary Conditions Question

Posted 9 sept. 2012, 19:17 UTC−4 Low-Frequency Electromagnetics, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.2a 3 Replies

Robert Sanderson
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Hello,

I am a relatively new user of Comsol, and I am trying to learn to use the electric currents Physics module. I want to model two copper cylinders with a contact resistance, and 1V applied across the whole structure. I also want to see what the electric potential around this structure would look like in air.

If I do not have a box of air around the cylinders, Comsol solves the potential drop along the cylinders with no problem, but when the air box is introduced, a "Singular Matrix" error shows up**, which I understand is usually due to poorly formed boundary conditions.

How can I form boundary conditions to have Comsol complete this simulation? Also, in general, what do I need to be careful with in the electric currents module?

Thank you so much,
Robert



**The error is:
Failed to find a solution.
Singular matrix.

There are 21056 void equations (empty rows in matrix) for the variable mod1.V
at coordinates: (-3.5355,-3.53557,-5), (-3.17193,-3.86508,-5), (-2.77782,-4.15737,-5), (-4.15732,-2.7779,-5), (-3.86502,-3.17201,-5), ...
and similarly for the degrees of freedom (empty columns in matrix)
Returned solution is not converged.
3 Replies Last Post 10 sept. 2012, 17:30 UTC−4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 10 sept. 2012, 08:01 UTC−4
Hi

which physics are you using ?

if its MEF then you need to give an electrical conductivity on ALL domains including "air" that jhas "0" conductivity. try 1-10 S/m that should work (and read the doc ... ;)

--
Good luck
Ivar
Hi which physics are you using ? if its MEF then you need to give an electrical conductivity on ALL domains including "air" that jhas "0" conductivity. try 1-10 S/m that should work (and read the doc ... ;) -- Good luck Ivar
Robert Sanderson
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Posted: 1 decade ago 10 sept. 2012, 14:01 UTC−4
Ivar,

Thank you so much for your help! I am using the electric currents physics, and changing the air's conductivity from 0[S/m] to 1[S/m] fixed everything.

I am curious, though.... I have read the documentation, and I still don't quite understand why the model doesn't work with zero conductivity. Is it that the electric currents physics depends on the movement of charge so much that setting the conductivity to zero is too big a constraint on the model?

Thank you,
Robert
Ivar, Thank you so much for your help! I am using the electric currents physics, and changing the air's conductivity from 0[S/m] to 1[S/m] fixed everything. I am curious, though.... I have read the documentation, and I still don't quite understand why the model doesn't work with zero conductivity. Is it that the electric currents physics depends on the movement of charge so much that setting the conductivity to zero is too big a constraint on the model? Thank you, Robert
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 10 sept. 2012, 17:30 UTC−4
Hi

then check the equations you are solving for EC implies resisitivty, if you put it to "0" your solution is trivial or undefined, so this physics, the way how it is set up, implies some conductivity.

Note when the ratio of highst to lowest conductivity is comparable or smaller than sqrt(eps) or about some 1:1E-6 you might get numerical issues and your results (at least the low conductivity regions) might be only marginally correc., But so long the integrated current is small they should not disturb your main results significantly

--
Good luck
Ivar
Hi then check the equations you are solving for EC implies resisitivty, if you put it to "0" your solution is trivial or undefined, so this physics, the way how it is set up, implies some conductivity. Note when the ratio of highst to lowest conductivity is comparable or smaller than sqrt(eps) or about some 1:1E-6 you might get numerical issues and your results (at least the low conductivity regions) might be only marginally correc., But so long the integrated current is small they should not disturb your main results significantly -- Good luck Ivar

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