Re: Finding 'mu' for magnetics

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

On 9 Dec 2003, at 7:48, Tesla list wrote:

 > Original poster: Matthew Smith <matt-at-kbc-dot-net.au>
 >
 > Hi All
 >
 > For those of us who are saving the planet by using re-cycled materials
 > for our coils and apparatus, I have been wondering how we can
 > determine the factor 'mu' for ferrites and other materials that go
 > into gate drive transformers, flyback transformers, etc.
 >
 > My proposed method:
 >
 > 1) Wind a coil on the core, measure the inductance.
 > 2) Take coil of core, measure inductance again.
 > 3) Calculate mu from ratio.
 >
 > Sounds too simple - but is it?

That gives you the initial relative permeability but says nothing
about the flux density the material will stand before saturating or
the incremental permeability (the shape of the Hanna curve). Core
material is inherently non-linear because as you apply an increasing
magnetizing force, more and more domains line up leaving less and
less to do so and it additionally becomes harder and harder to get
the remaining domains to align (if it wasn't, they'd all align at
once). The result is an asymptotic magnetization curve. Gapping the
core reduces these effects but then allows the core (and gap) to
store a lot of energy which is bad news for a transformer (great for
true flyback topologies though).

Malcolm