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Alnico Magnets pole reversal

  1. May 28, 2009 #1
    An cylindrical alnico magnet is 1/2 inch diameter and 1 inch long. It is magnetized through its length. I realize that this magnet can be magnitized over and over again. But can the poles be reversed if subjected to a machine that magnetizes and de-magnitezes magnets? Also, where can one purchase such a machine? What are they called? Are there small table-top units availabe for home experiments?
  2. jcsd
  3. May 28, 2009 #2
    Suppose you wanted to build a dc magnet that could go to 1.6 Tesla, and re-magnetize your Alnico magnet. Suppose the gap was 0.025 meters, and the remainder of the path was 1 meter in iron ( "C" magnet) with a permeability u of 5000. The total amp turns you will need is

    NI = 1.6 x 0.025/u0 + 1.6 x 1.0/u u0 = 32,000 + 260 = 32,260 amp turns, where u0 = permeability of free space ( 4 pi x 10-7 Henrys per meter).
    So you will need say 460 turns at 70 amps (peak).

    See http://www.gmw.com/electromagnets/dipole/5403/5403_Specs.html

    [Edit] Here is a sheet of Alnico properties:
    http://www.dextermag.com/uploadedFiles/Alnico_Data_Sheet.pdf [Broken]
    Some Alnico's have a residual magnetization of 1.35 Tesla, so you probably want to push Bmax up to 1.6 Tesla. You can do it with a pulse several seconds long, so you can use an air-cooled magnet much smaller than the one shown in the GMW web page. You can use a capacitor bank to provide the pulse. The voltage required depends on the resistance of the wire coils (e.g., 10 Ga formvar wire), and the pulse length on the RC time constant. If your iron is solid, the pulse length must be long enough for the eddy currents to die out.
    Last edited by a moderator: May 4, 2017
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