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# Re: drag force with FEMM

- To: femm@yahoogroups.com
- Subject: Re: drag force with FEMM
- From: "tano1938" <rgarritano@comcast.net>
- Date: Thu, 22 May 2003 21:23:55 -0000
- In-reply-to: <3ECB84F8.3080906@ieee.org>
- User-agent: eGroups-EW/0.82

    Thanks Dave, you're the greatest!!! We are all very fortunate to
    have you as a moderator of this group.

    Ron

    --- In femm@yahoogroups.com, David Meeker <dmeeker@i...> wrote:
    > tano1938 wrote:
    >
    > > Is it possible with FEMM to get the drag force on a copper or
    > > aluminum cylinder in a voice coil type magnetics design. In other
    > > words, if I replace the coil in a linear voice coil motor (which I
    > > have already modeled in FEMM ) with a solid cylinder of the same
    size
    > > as the coil, I would like to find the drag force on the cylinder.
    >
    > Yes, you can look at this sort of problem with femm. In an
    axisymmetric
    > problem, the magnitude of the currents that you induce due to
    motion are
    > J=conductivity * v cross B = conductivity*velocity*Br
    > where "Br" denotes the radial component of the field and "velocity"
    > denotes the axial velocity of the coil. The force on a
    differential
    > volume of your cylinder is then:
    > dF=J cross B = -velocity*conductivity*Br^2
    > Now, we could make two key assumptions: The frequency of the
    motion is
    > "low enough" so that the contribution of the reaction currents in
    the
    > coil to the Br field are negligible; and the amplitude of the
    motion is
    > small. Then, we could get a single damping coefficient by
    integrating
    > conductivity*Br^2 over the volume of your cylinder:
    > Damping Coefficient = Integral[conductivity*Br^2] over coil volume
    >
    > Unfortunately for you, the volume integral of Br^2 isn't one of the
    > "canned" volume integrals. You could be lazy and just do the
    integral
    > of B over the block to get the average Br, which you could then
    square
    > and multiply by the coil volume--this would be OK if your coil is
    > immersed in a relatively homogeneous field. Otherwise, you could
    do a
    > more detailed quadrature, e.g. draw a line down the center of the
    coil,
    > dump the B.n line plot values to disk, and do the Br^2 integral "by
    hand".
    >
    > However, there are some caveats. The frequency has to be "low
    enough"
    > for this approximation to be valid. You could get an idea of what
    is a
    > "low enough" frequency by treating your coil as a 1-turn coil
    carrying a
    > constant current density and computing the R and L of the coil.
    Your
    > frequency should then be low compared to Sqrt[R/L]--maybe at least
    an
    > order of magnitude less, also remembering that the units of Sqrt
    [R/L]
    > are rad/sec and not Hz. If this condition is not met, you then
    have to
    > account for the inductances of the current loops in the cylinder,
    which
    > is possible, but requires a lot more work.
    >
    > Dave.
    > --
    > David Meeker
    > email: dmeeker@i...
    > www: http://femm.berlios.de/dmeeker

- References:
  - Re: [femm] drag force with FEMM
    - From: David Meeker <dmeeker@ieee.org>

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