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# Re: inductance - axisymmetric model

- To: femm@yahoogroups.com
- Subject: Re: inductance - axisymmetric model
- From: tborzic@yahoo.com
- Date: Tue, 13 Nov 2001 22:50:40 -0000
- In-reply-to: <F33ZPnjkzr0FNacww5q0001160c@hotmail.com>
- User-agent: eGroups-EW/0.82

    --- In femm@y..., "Jim in_Dallas" <jim271@h...> wrote:

    > >
    > If we use the axisymmetric model for the coil, the FEMM results is
    very
    > close to the value calculated using the formula for a current loop.
    >
    > Example:
    >
    > current loop: L=N^2*R*mu0*Ln(R/r)
    > where R=radius of loop
    > r=radius of wire

    Jim,in Your smallcoil femm file r is not 0.0005cm it is d,(d=2r)
    so r=0.000025
    so this formula works only for ....Ln(R/d)?
    because you draw d=0.0005 not r (r is a half of diameter)

    >
    > Using your dimensions:
    > R=4cm
    > r=0.0005cm
    > 4 turns
    > then L = 167.2nH
    >
    > An alternative formula found in some references is: L=N^2*r*mu0*[Ln
    (8*R/r)
    > -2]
    >
    > Since the Ln(8) approx = 2, these formulae give similar results.
    >
    > Using FEMM:
    > Let J=1.0 then
    > integral(A.J)=6.236e-9
    > I = 0.1954 Amps/turn
    >
    > L=163.4nH which differs by less than 3 percent. I've attached my
    fem file.
    >

    when i changed gap between wires to something smaller result is 167.x
    nH. Only thing that bothers me is that radius in formula..
    So, now i will bite my nails all night (it is night in Croatia,
    actually midnight) hoping that r in formula is actually d.
    Or I made mistake when mesuring dimensions in smaillcoil file?
    Hope You will soon read this:)))
    Thanks again
    tomislav

- References:
  - Re: [femm] inductance - axisymmetric model
    - From: "Jim in_Dallas" <jim271@hotmail.com>

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