Alnico remagnitizing question - diyAudio
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Old 27th March 2011, 02:27 PM   #1
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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
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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.

Conrad
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Old 27th March 2011, 04:53 PM   #3
AndrewT is offline AndrewT  Scotland
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what are the units KA/M in that value 250KA/M?
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Old 27th March 2011, 08:59 PM   #4
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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
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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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