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# Re: [femm] Circuits in FEMM

- To: femm@yahoogroups.com
- Subject: Re: [femm] Circuits in FEMM
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Tue, 04 Sep 2001 16:45:33 -0400
- Organization: Foster-Miller, Inc.
- References: <9n34jm+g9bk@eGroups.com>

frank.e.lenning@boeing.com wrote:

> How does the circuits feature in FEMM work for multiple conductors?
> Let's say I have two conductors of different materials that I define
> with Circuits to be carrying a total current of 1 amp together. How
> does FEMM solve this problem? Does it iteratively solve the problem
> by choosing different deltaV's for each conductor until it gets the
> right total current or is there some easier way that it uses to set
> the deltaV's? It appears to get the right answer, I'm just trying to
> understand how.

It's best to visualize this situation as being like two resistors driven in parallel. For two resistors driven in parallel, the voltage drop across each resistor is the same, so to get a given total current, you just have to pick the right voltage drop. In femm, the situation is analogous. For your model where you have two conductors labeled with the same circuit property, the voltage gradient is the same for each conductor. The program just has to pick the applied voltage gradient to give the right total current for the conductors. For magnetostatic problems, there is enough information to do this calculation up front, before doing anything else. For harmonic problems, eddy currents induced in the conductors have an effect on the total current, so the program ends up solving an extra equation for the voltage gradient that is required for each circuit to enforce the circuit's current constraint.

In any case, you can choose View|Circuit Props off of the main menu of the postprocessor to see what voltage gradient was required to enforce the circuit's current constraint, from which the program also infers impedance.

Dave Meeker

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