The core became a magnet

Discussion in 'General Electronics Chat' started by atferrari, Feb 6, 2012.

  1. atferrari

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    To experiment in repelling a magnet I winded a 3-layers inductor around a piece of a common nail (soft-iron?)

    Initially it seemed to work OK but later I found that, after cutting the current, the magnet was atracted by the core which started to behave as a magnet. Not what I wanted, precisely.

    Just to be sure I verified that a paper clip is attracted by the core.

    I did not foreseen that. How to avoid it? Not using a core? Or is this the wrong material?

    Now that I think of it this would be good if I wanted to attract the magnet. But I want just the opposite.
     
  2. praondevou

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    I guess there is no way to use an iron core for an inductor and NOT magnetize it, at least a little bit. In order not to magnetize a ferromagnetic material you will need to avoid aligning the magnetic domains, i.e. the magnetic field applied to the core would have to be very weak. (see also Barkhausen effect)

    I don't know if there is any material that concentrates flux but cannot be magnetized. Isn't that a contradiction?

    If you want to be absolutly sure that the core is demagnetized after the the inductor is deenergized you could apply a decaying AC current to it.


    Does that make sense for your application?

    What about an air core?
     
  3. atferrari

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    Air core. I actually know I have to resort to it but resisted the idea. To get the equivalent effect I will have to make the coil bigger.
     
  4. Ron H

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    From Wikipedia:
    Apparently, "soft" refers to its magnetic properties, not its physical hardness.
     
    Last edited: Feb 7, 2012
  5. crutschow

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    The core material of transformers is magnetically soft and retain a minimum of magnetism (small hysteresis loop) thus you might try using part of an old transformer core for your electromagnet.
     
  6. praondevou

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    That's the problem with internet sources. contradicting informations.
    IMO even if the material has very low coercivity it can still remain slightly magnetized. Apparently the OP wants zero rest-magnetism of the core after the power to the electromagnet is switched off.
     
  7. Ron H

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    Junk relays, solenoids, motors, transformers, etc., are probably the best sources of "free" soft iron. As praondevou pointed out, they won't have zero residual magnetism, but they should be pretty good.
     
  8. crutschow

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    "Zero" is relative to the task. I believe he just wants it low enough so it doesn't noticeably attract magnetic materials. That may be obtainable with typical transformer/motor/solenoid magnetic material.
     
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