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# Re: [femm] Permanent Magnet Models.

- To: femm@egroups.com
- Subject: Re: [femm] Permanent Magnet Models.
- From: dcm3c@aol.com
- Date: Wed, 26 Apr 2000 15:53:51 EDT

    In a message dated 4/26/00 9:27:51 AM Eastern Daylight Time, sje@tinet.org
    writes:

    > Hi all:
    > In my permanent magnet provider catalog I have Permeability in
    > different
    > units, for:
    > Ferrite magnets: 1.35 mT/(kA/m) and 1.10 G/Oe
    > Neodimium: 1.36 mT/(kA/m) and 1.08 G/Oe
    > Samarium: 1.32 mT/(kA/m) and 1.05 G/Oe
    >
    > What unit system use FEMM for Permeability?

    When femm asks for permeability, it is usually after relative permeability.
    Relative permeability is the permeability of the material divided by the
    permeability of free space, in whatever units you want. The units cancel
    out. It just so happens that in CGS (that is, Gauss and Oersted units), the
    permeability of free space is 1 G/Oe, so any permeability in these units is
    numerically the same as relative permeability. Therefore, use 1.10, 1.08,
    and 1.05 respectively as the relative permeabilites for your magnets.

    > For Coercivity, in the catalog I have Hcb (B=0) and Hcj (j=0). The
    > correct value for the model is Hcb?

    It sounds the value you want is denoted Hcb in your catalog. At any rate,
    when are presented with Coercive Force (often represented as Hc) and
    Intrinsic Coercive Force (often represented as Hci), the easy way to remember
    it is that femm always wants the smaller of the two values (that is, Hc).
    Anyhow, Hc is sort of a measure of the ``strength'' of the magnet (sort of
    how many amp-turns the material apparently has), whereas Hci is more like a
    measure of what it takes to permanently demagnetize the material (sort of how
    hard you can push it without functionally destroying it).

    Dave
    --
    http://members.aol.com/dcm3c

- Follow-Ups:
  - Permanent Magnet Motor.
    - From: Sebastia Jardi Estadella <sje@tinet.org>
  - Permanent Magnet Models Library.
    - From: Sebastia Jardi Estadella <sje@tinet.org>

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