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Subject: iron and ferrite
From: sean@...........
Date: Mon, 20 Nov 2000 14:45:56 -0600 (CST)


Regarding the use of ferrite or soft iron for magnet structures.
There is considerable information on ferrites and transformer 
laminate alloys in the "Reference Data for Radio Engineers".

A first point is that steel as in carbon steel is not used for 
magnetics: various "soft iron" alloys are used. The most common
are the silicon (4% Si/Fe) alloys with permeabilities to 30,000.
Then there are the Cobalt and Nickel alloys with higher flux 
densities, and then there are the Permalloy series, which includes
Mumetal, with permeabilities from 100,000 to over 600,000. These
are also expensive, since they can be up to 80% nickel.

The tradeoff the other way are that the nickel alloys saturate at 
much lower levels (10 to 16k gauss) than the soft silicon types 
(20k gauss), with the Cobalt alloys having the highest saturation 
at 24k gauss.

Ferrites are a ceramic made with iron oxide (Fe,2/O,4) and trace
elements. Their permeabilities run 3000 to 5000. They are used
primarily for high frequency transformers and inductors where
eddy current losses in iron materials would be excessive. Like 
for the flyback transformer in a TV set, the 16 khz drive would 
require laminates 0.0001" thick, so a cast ferrite core is used.
They are used for switching power supplies because the switching
frequency is very high, and for radio frequency work where high
inductances are not needed but low loss is essential.

Regarding my description of the magnet assembly for the feedback
transducer: I was shopping for hardware I could buy that would 
create a coil/magnet with an adequate generator constant, hence
the assembly of "bought" parts that have not been modified other
than filing edges. Its main advantage is that it is repeatable.
I am sure that a commercial iron assembly could do much better 
with the same rare-earth magnet, but I have not explored the options.
As for the application, there is little to be gained by increasing
the feedback transducer constant other than extending the high
frequency response, which is about 30 hz in the present design.
A more compact transducer would help in a more portable design,
and higher flux density would allow for a larger annular gap so
as to provide better coil clearance over a shorter arc range of motion.


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Larry Cochrane <cochrane@..............>