Re: [femm] Permanent Magnet Models.

[dcm3c@xxxxxxx]

In a message dated 4/26/00 9:27:51 AM Eastern Daylight Time, sje@xxxxxxxxx 
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
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