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Magnetic circuits and Magnetization

  1. Oct 20, 2015 #1
    From my little understanding of magnetism and the magnetization process, in order to magnetize a magnet it is better to have a closed magnetic circuit. But how does an air gap affects the magnetization "strength"?


    Example: Assume that we have a unmagnetized horsehoe magnet with a coil around it for the whole length with a constant current flowing and constrant "coil/length ratio". Will the magnet get more magnetized if we add a keeper to connect the poles?

    And another example, this time with a small coil (same ampere-turn) in the center of the magnet, how will the keeper affect the magnetization process? And how will this compare to the previous example?

    I know the question probably requires a better understanding of magnetism in general but I would be glad to find someone able to explain it to me.
     
  2. jcsd
  3. Oct 20, 2015 #2

    Hesch

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    A magnetic field consists of two fields:

    The H-field ( the magnetic field strength ) which may be compared to electric voltage.

    The B-field ( the magnetic induction ) which may be compared to electric current density.

    The relation between these two fields is: B = μ*H , where μ is the permeance ( may be compared to electric conductivity ).

    An airgap has a low permeance, thus it weakens the B-field ( like a resistor weakens the current ). If you have an electric circuit supplied by 10V and there is a tiny resistance in the circuit, say 0.001Ω, the current will be V/R = 10V / 0.001Ω = 10000A. Likewise you will get a strong B-field if your magnetic circuit has a high permeance ( no airgap ). The permeance is a factor ≈ 1000 greater in iron than in air/vacuum.
    Yes.
    Likewise, no difference. It doen't matter where the coil is placed in the magnetic circuit. You can wind it around the keeper, if you like. A magnetic field is a closed circulating field, no beginning, no end.
     
  4. Oct 20, 2015 #3
    Thankyou very much for your answer, can you check those concepts for correctness:

    1. The H-field is directly proportional to the ampere-turns of the coil.
    2. The B-Field is directly proportional to the H-Field and inveresely proportional to the permeance.
    3. Permeance is like the inverse of the resistance of an electric circuit, (reluctance).
    4. Reluctances adds up for the length of the magnetic circuit so a tiny 1mm air "circuit" is like ≈1m of iron "circuit".
    5. Reluctance of any material becomes high when the B-field reach the saturation level of that material.(?) At that point the B-Field will increase slowly and will expand outside the magnetic circuit.(?)
    6. The "magnetization strength" is directly proportional to the B-Field/surface inside the magnetization target.(?)
    Bear with me and my terminology.
     
  5. Oct 20, 2015 #4

    Hesch

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    Right - wrong - added by me.
     
  6. Oct 20, 2015 #5
    Thank you again for your patience:
    • The B-Field is directly proportioal to the H-Field and directly proportional to the permeance. ( B = μ * H ).
    (this makes perfect sense indeed, I messed up while writing the post )

    • Reluctance of any material becomes low when the B-field reach the saturation level of that material.
    (I do not understand this, why is reluctance decreasing when the path is saturated? )​


    • The "magnetization strength" is directly proportional to the B-Field/surface inside the magnetization target.(?) ( What do you mean ? )
    Well that was a shot in the dark, I turn this into a question:
    When you need to magnetize a permanent magnet in practice, what matters? The H-Field, The B-Field, something else?



     
  7. Oct 20, 2015 #6

    Hesch

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    Typical magnetizing curve:
    mag19.gif
    When the B-field closes up yo about 2 Tesla, the steepnes of the curve, which is μ, decreases.

    μ = dB/dH
    To say that it's the B-field that matters is somehow too easy, because the B-field is a result of a H-field. Is the hen or the egg the first? Anyway, if you create a strong B-field through hard steel, It will be magnetized.

    I will attach a link in a moment, concerning how to magnetize magnets.

    Here it is:
    https://www.physicsforums.com/threads/re-magnetized-alnico500-but-failed.836549/#post-5251785
     
  8. Oct 21, 2015 #7
    Thank you for your clarification.
     
  9. Oct 21, 2015 #8
    I'm a bit confused with this statement:
    • Reluctance of any material becomes low when the B-field reach the saturation level of that material.
    It's the permeance that equals dB/dH. So at saturation of B, the permeance(μ) reduces and reluctance increases since reluctance inverse of permeance.
     
  10. Oct 21, 2015 #9

    Hesch

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    You are right. ( English is not my language ).
     
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