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# Re: [femm] Comparing inductance predictions: femm vs standard formula

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- Subject: Re: [femm] Comparing inductance predictions: femm vs standard formula
- From: David Meeker <dmeeker@ieee.org>
- Date: Fri, 26 Dec 2003 13:23:51 -0500
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`Dan Haronian wrote:`

> `Hi David
> I wonder if you could help me with the following question:
> Self inductance of wire with rectangular cross section is given by
> 2*L[Ln(2*L/(w+t)+0.5+0.22*(w+t)/L]
> L is the length of the inductor, W and t are the width and height.
> I was expecting to see the inductance to go to zero when w or t go to zero but this is not the case. Do you have a more accurate equation?
> Thanks
> Dan Haronian
> Israel
> `

`
The inductance of a single wire with no return isn't a well-defined problem. There are some formulas (which I think were cribbed from Grover) at:
http://home.san.rr.com/bushnell/self_inductance.htm
but don't use the one for the various single conductor cases unless you know what you are doing. I also haven't verified that these formulas have been _accurately_ cribbed from Grover....`
`At any rate, one can't model a single-conductor inductance problem by a 2D planar analysis in FEMM. Some return path for the current is always implied by the boundary conditions that you draw for the finite element solution -- the per-unit-length inductance of a single conductor in a truly unbounded 2D planar domain is infinite.`
`Inductance converges to finite, nonzero values if a coil is thin in one dimension. Inductance is infinite for a 2D or axisymmetric point current. If you try to simulate a point current in FEMM, the inductance that you obtain is strongly dependent on mesh density, with the inductance going to infinity as the mesh size goes to zero.`

    --
    David Meeker
    dmeeker@ieee.org
    http://femm.berlios.de/dmeeker

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  - [femm] Comparing inductance predictions: femm vs standard formula
    - From: Dan Haronian <Dan.Haronian@siband.com>

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