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# Re: [femm] Re: Incorrect force direction

- To: <femm@egroups.com>
- Subject: Re: [femm] Re: Incorrect force direction
- From: "Steven Stretz" <stretzsj@execpc.com>
- Date: Thu, 4 Jan 2001 12:03:18 -0600
- References: <5b.fefc9e5.27823eac@aol.com> <000901c07451$cd9c00e0$52e78ec6@ibmbn145hv> <3A534275.E66BF094@foster-miller.com> <000901c075ce$f329f5a0$a2e78ec6@ibmbn145hv>

    Comparison is a very good idea however I have had such terrible luck with
    IES software I would not trust it any farther than I could throw the CD.
    Boundary elements are great for far field effects but historically their
    attempts at handling near fields, nonlinearities, induced currents, etc. has
    been at best questionable. I'll trust your judgement and hope the software
    has improved but be careful using it. I leased the software for a year and
    returned in after several months and asked for my money back, which I never
    received.
    ----- Original Message -----
    From: "Greg Watson" <gowatson@optusnet.com.au>
    To: <femm@egroups.com>
    Sent: Wednesday, January 03, 2001 3:48 PM
    Subject: Re: [femm] Re: Incorrect force direction

    > ----- Original Message -----
    > From: "David Meeker" <dmeeker@foster-miller.com>
    > To: <femm@egroups.com>
    > Sent: Thursday, January 04, 2001 1:47 AM
    > Subject: Re: [femm] Re: Incorrect force direction
    >
    >
    > > Anyhow, I modelled your problem on Amperes by IES, which is a rather
    expensive
    > > commercial 3-D magnetostatic solver based on boundary elements, rather
    than
    > finite
    > > elements. This ought to give a good idea of how things would run in a
    real
    > 3-D test
    > > rig, assuming everything is carefully aligned. I arbitrarily made the
    machine
    > 1"
    > > deep in the "into the page" direction, for the purposes of making a 3-D
    > geometry. I
    > > chose a relatively fine density for the surface elements, and then set
    it to
    > > evaluate the forces on the magnet at a bunch of different axial
    positions.
    > What it
    > > predicts is that the centring force peaks when the front edge of the
    magnet is
    > just
    > > past the iron, with the centring force dropping to near zero when the
    magnet
    > is
    > > completely between the iron bars. femm predicts the same sort of
    behaviour,
    > except
    > > that femm predicts the peak force to be more like 15 Newton/inch, rather
    than
    > 9 N.
    > > The force from the 3-D geometry is smaller than in the 2-D model because
    some
    > of the
    > > magnet's flux leaks into the sides of the bars--this is an intrinsically
    3-D
    > effect
    > > that can't be modelled by a 2-D solver.
    >
    > Hi David,
    >
    > Thanks for the effort.
    >
    > My concern was not that FEMM should report a large centring force but that
    a
    > program which I normally have found to be very realistic would report a
    > repulsive & not a centring force, no matter how fine the mesh or how much
    care I
    > took in making sure the geometry of the mesh and / or of the model didn't
    effect
    > the result.
    >
    > My duplication involved a round Neo, 4 on point right angle plastic guides
    hot
    > glued to the vertical faces of 2 right angle steel pieces (two guides per
    face,
    > 1 top, 1 bottom). Alignment isn't too critical. Although the plastic
    guides
    > aren't ideal, you can get a feel for the forces by the amount of effort
    needed
    > to roll the magnet back & forth between the guides.
    >
    > Greg
    >
    >
    >
    >

- Follow-Ups:
  - Re: [femm] Re: Incorrect force direction
    - From: Dave Squires <djsquires@plix.com>

- References:
  - Re: [femm] Re: Incorrect force direction
    - From: dcm3c@aol.com
  - Re: [femm] Re: Incorrect force direction
    - From: "Greg Watson" <gowatson@optusnet.com.au>
  - Re: [femm] Re: Incorrect force direction
    - From: David Meeker <dmeeker@foster-miller.com>
  - Re: [femm] Re: Incorrect force direction
    - From: "Greg Watson" <gowatson@optusnet.com.au>

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