Re: Finding 'mu' for magnetics

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

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 07:48 AM 12/9/2003 -0700, you 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?
>
>Cheers

Exactly how to go about doing it...  There are a few tricks, though... 
consider that mu might be several hundred, so you're comparing two numbers 
that are orders of magnitude different.

There's also the issue of leakage flux, and so forth.

You might want to compare the measured (with core) L against what you'd 
compute for the same physical test coil (based, on, e.g. Wheeler) , rather 
than trying to measure a small coil in free space.  With a reasonably high 
mu, the inductance of the (with core) coil will dominate any stray lead 
inductance, etc.


Watch out for frequency dependent effects!  All those random ferrite mixes 
out there have very different properties.  There's also the saturation 
thing, for which you need a bit more sophisticated measuring technique (two 
coils on the material one driven, one not, and an oscilloscope, so you can 
look at the B/H curve)

http://home.earthlink-dot-net/~w6rmk/ferrite.htm has some discussion of 
identifying mystery ferrites.