Discussion Forum

Magnetic Fields, No Current

Hi everyone,

So I've been trying for a while now to bypass the problem of creating an external magnetic field in *other* AC/DC modules; but now I see that in the MFNC there are two options: External Magnetic Flux Density, and the same without external. The External seems to be a pointless node - it does not make any change at all in the model, does not affect anything - and you cannot provide it with any information;

so here's the first question - what does the External Magnetic Flux Density node do?

the other node, without external, seems to work if you have a stationary model, but if you have a time dependent, any field that is not 0 makes it not converge. attached a model; can anyone tell me what is wrong?

I'm working with 4.2a.

thanks,
~Shoval

5 Replies Last Post 24 Mar 2017 11:56 GMT-04:00
Posted: 5 years ago
Hi

why do you find it so difficult ?
In MNFC you can define the magnetic (scalar) potential Vm on two opposed boundaries (i.e. of a square) and you have your external magnetic field as a flux going from one boundary to the other.
Or you define one or more Magnetic boundary fluxes, but then you must somewhere define a magnetic potential to set a gauge values on Vm (or to define the integration constant)

In MF its the same, apart that we deal with the magnetic Vector potential A=(Ax,Ay,Az) and by default, in 2D only Az is considered (out of plane component) but that you can change that

--
Good luck
Ivar

Posted: 5 years ago
I really do not know why but this too does not seem to work. I attached the smallest and simplest model with it for you to see. It should be less than 5 minutes to check..

thank you for you time!!

~shoval.

Posted: 5 years ago
The External Magnetic Flux Density condition only works when you have an external (background) field, that is, when you select "Solve for reduced field" in the Magnetic Fields, No Currents node. In this situation, you solve for the reduced field redH and the total field H is the sum of the reduced field and the background field:

H = redH + H0

An External Magnetic Flux Density feature imposes the condition that the total field must be equal to the background field, i.e. redH = 0. In other words, it forces the perturbation to the background field to be zero on that boundary. Such condition does not require any user input, so its Settings window is empty.

As for your time-dependent model, the reason it fails to converge is that you did not provide any constraint for the absolute value of the potential, since you only imposed flux (Neumann) boundary conditions. It's mathematically equivalent to asking "what's the voltage of a wire carrying 1 A?": there is no unique answer, you can only compute the voltage drop but not the absolute value of the voltage in the wire, unless you specify the potential at least in one point.
In your MFNC model, you need at least a point condition somewhere that specifies the absolute value of the potential.

--
Andrea Ferrario
Electromagnetics Group
COMSOL AB

Posted: 5 years ago
Thank you very much!
it now works very nicely and I understand about the External too.

shoval

Posted: 6 months ago
Hello Ivar

you say "In MNFC you can define the magnetic (scalar) potential Vm on two opposed boundaries (i.e. of a square) and you have your external magnetic field as a flux going from one boundary to the other. "

I don't think this is correct : you can not specify a background magnetic field using specific values of the magnetic potential Vm on 2 sides of your domain, EXCEPT if there is no ferromagnetic material inside your domain.

The reason is the following :

Let L be the length of the domain along the X axis, and H0 the background field parallel to axis X.
If no ferromagnetic material is present, indeed, H=constant=H0= -grad Vm leads to a simple solution :

dVm/dx = -H0
dV/dy=0
dV/dZ=0

solution is

Vm(x=0)=L*H0
Vm(x=L)=0

It works !

But as long as you have a ferromagnetic material (µr>1), the magnetic potential Vm is no longer linear inside the domain, and having set your potential on the 2 opposite faces of your domain leads to an incorrect value of the field : grad Vm is no longer constant inside the domain, and there is no reason for if to be equal to H0 on the 2 boundaries.

My conclusion, if I am not wrong, if that there are only 2 solutions to impose a background field :

1) either solve in "full field" and impose the magnetic flux B on the 2 opposite faces using "magnetic flux density" node
2) either solve in reduced field and specify the background field

Do you agree ?

Best regards

Antoine