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# Re: [femm] Circuits in FEMM

- To: femm@yahoogroups.com
- Subject: Re: [femm] Circuits in FEMM
- From: "you se-ho" <sxy28@hotmail.com>
- Date: Thu, 06 Sep 2001 16:59:47
- Bcc:

`
To Dear Everyone:`
`How are you? Again some conceptual questions that makes me confused.`
`(1) I think the gradient term comes out from the displacement term in the Maxwell's equation. In class, when the conductors are good conductor, we are usually told that the term can be ignored. Is it true when frequency is high so skin effect is outstading?`
`(2) If I send the +1A one one conductor and get back -1A the other conductor, should the resistive loss (I guess this is from power loss, and the power is also 0.5*I^2*R where I=1A ) be same as the impedance (or its real part only) on each conductor?`
`(3) Is there any way to use the impedance in the circuit property for Spice circuit input (i.e frequency independent parameter R) ?`

    Any comments will be appreciated
    Sincerely,
    SE-HO YOU

    ================
    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`
``
``

    _________________________________________________________________
    Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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
`Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.`

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