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Alnico remagnitizing question
Alnico remagnitizing question
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Old 27th March 2011, 02:27 PM   #1
woody is offline woody
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Join Date: Jan 2002
Location: Tyrone Ga. U.S.A.
Default Alnico remagnitizing question

In some reading I did I saw it stated that to magnitize alnico a flux of 250KA/M was required. I have a driver using an alnico magnet about
1" in diameter. So just doing the math in my head that would only require
~ 250 turns of wire and 1A of current. Did I miss something there I would
have thought it would need a lot more flux than that?
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Old 27th March 2011, 04:43 PM   #2
Conrad Hoffman is offline Conrad Hoffman  United States
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Join Date: Mar 2007
Location: Canandaigua, NY USA
That just doesn't sound right, though I haven't done the math. I know Alnico is one of the easier ones. Magnetizing even guitar pickups requires more turns and some pretty hefty pole pieces to complete the loop. Larger and more exotic magnets aren't practical for the DIY type because they usually discharge a large bank of very high voltage caps through a big magnetic structure to get enough flux. I'm not saying it can't be done, but when I looked into it, it got scary and expensive very quickly.

I may be barking up the wrong tree, but at least I'm barking!
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Old 27th March 2011, 04:53 PM   #3
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
what are the units KA/M in that value 250KA/M?
regards Andrew T.
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Old 27th March 2011, 08:59 PM   #4
woody is offline woody
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Join Date: Jan 2002
Location: Tyrone Ga. U.S.A.
I found my error. I assumed the 240KA/m refered to the diameter of the magnet
but it applies to the legnth. So for my 1" long magnet it would require 6000 A turns.
I also found that ceramic magnets require 20,000 amp turns per inch. That may be
one reason you don't see long ceramic speaker magnets.
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Old 27th March 2011, 09:11 PM   #5
Don Hills is offline Don Hills  New Zealand
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Join Date: May 2009
Location: New Zealand
K = 1000
A = amps
M = meter.

Take a meter long tube. Wind 1000 turns of wire along it. Pass a current of 250 amps through the wire. The magnetic field near the centre of the tube will be 250KA/M.
(This actually assumes a tube (coil) of infinite length.)

See here for details, and how to convert to oersteds and/or gauss:

Oersted - Wikipedia, the free encyclopedia

The main "problem" with magnetising Alnico or other conductive material is that you need to maintain the magnetising field for a short but significant amount of time, long enough for the eddy currents in the material to die away. It's much easier for ferrite materials, there are no significant eddy currents so you can use a very short pulse generated by a capacitor bank.
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