loss of magnetism in permanent magnets

Discussion in 'General Electronics Chat' started by strantor, Aug 27, 2011.

  1. strantor

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    I had an idea when I was a kid about making a non-electric motor out of nothing but permanent magnets. I went on a forum (actually, a news group) and asked about it and was told that I was not the first one to come up with that design and that it was the stuff of fantacy, in the realm of overunity and such.

    It was explained to me that the design I had drawn, if by some act of magic were to work, it would only work for a little while because permanent magnets are like batteries, and if held in opposition, they will rapidly deplete. That is the understanding I have gone forward with until this point.

    Now I am curious how the magnet in a permanent magnet motor does not deplete. I'm sure it may deplete a little bit over time, but not rapidly as my understanding of magnets dictates that it should. I also have a memory fragment of reading that permanent magnet motors will lose their magnetism if you remove the magnet for too long.

    so, my questions are:
    1. Will a magnet indeed lose it's magnetism if held in opposition to another magnet?
    2. If so, Why does a permanent magnet motor not lose it's magnetism?

    The sad thing is, I've been over this before and figured it out, but now I can't remember.
     
  2. kubeek

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    Magnets don´t loose magnetism like that. They maybe loose some fraction of a percent over a year if held in a magnetic field, but for sure it is not rapid.
     
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  3. shortbus

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    In a permanent magnet motor, the rotor in the field of the magnets gives a return path for the flux. The modern rare earth magnets don't have much problem with reduced field strength, not like the old ferric and Alnico types.

    The older Alnico magnet motors would lose power if the rotor was removed for a length of time.
     
  4. SgtWookie

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    I ran across some documentation on pre-WWII era magnetos; in the manual they specifically warned against removing or even disturbing the magnets, as they would lose some of their magnetism and weaken the resulting spark.
     
  5. Adjuster

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    Magnetic materials can differ enormously in their coercivity (the "fatness" of the BH loop), making them more or less vulnerable to being demagnetised in use, and for that matter more or less difficult to magnetise in the first place. http://en.wikipedia.org/wiki/Coercivity

    As SgtWookie points out, some very "hard" magnets are now available, but even with these you would not be able to produce a motor which could run continuously. The most you could expect would be something a bit like a spring, where magnets might initially be put into position to repel each other. If the magnets were then released, they would only move until they took up the most stable position that the mechanics of the system would allow.
     
  6. strantor

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    Ok, I'm attempting to wrap these ideas up into an understanding:
    So the older permanent magnets were more likely to lose magnetism presumably because the technology was not around to make them very "hard"

    Another question has come to my mind; what causes magnets to deplete faster? having no return path for the flux, or being held in opposition, I.E. "working"?

    Thanks everybody
     
  7. shortbus

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    I'm pretty sure that having no return path is worse. When in 'opposition ' they do have the return path. Have you seen the 'horse shoe' type magnet with a keeper across the poles? The keeper is to provide the return path.
     
  8. bertus

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  9. SgtWookie

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    Yes, the old Alnico magnets came with a steel "keeper" across the poles. If you didn't leave them with the keeper attached, they'd lose strength over time more rapidly.
     
  10. THE_RB

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    I would argue that, the second part.

    When i was a child the school science bar magnets came 2 to a box, with instructions to place the 2 magnets in N-S. Of course we used to put them away N-N (well the bad kids did, not me!) and as you can guess half the magnets in the classroom had barely any magnetism left so the kids would argue over who got the good magnets. :)

    I can't think of ANY motor or electrical device I have seen over the years that ever had permanent magnets that were forced to oppose each other (forced "like poles"). Only motors etc that had perm magnets with iron poles acting as keepers when the motor was off.
     
  11. shortbus

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    RB - I took Strantors opposition to mean like in a motor with the rotor in place.
     
  12. strantor

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    I meant held with like poles together. RB's example of putting the magnets in the box backwards matches what I was trying to say.

    I still can't understand why the magnet in a permanent magnet motor does not get depleted.

    I was thinking that the reason is because in the motor, the magnet's forces of attraction are exploited rather than repulsion.

    Then I started to think that the attractive and repulsive forces of the magnet are one in the same. So, if you were to design a permanent magnet motor that worked on repulsion rather than attraction, it should still work.

    so if a permanent magnet DC motor could work on repulsion then would the permanent magnet still not deplete rapidly?

    Whether the stator attracts or repulses, it seems to me that in some way, some power is being drawn from the permanent magnet itself. so it should become depleted. but it doesn't. so I have a flawed understanding somewhere.
     
  13. steveb

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    I can think of an example. Linear motors often use this approach. The use of NN and SS alternating magnets allows a sinusoidally varying field to be set up over the length.
     
  14. steveb

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    By the way, I didn't see anyone mention the fact that banging a magnet can cause it to demagnitize.
     
  15. bertus

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    Hello,

    I have given two links in post #8.

    In one of the links this is stated:

    Bertus
    [/SIZE]
     
  16. THE_RB

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    Hmm, but that doesn't mean the motor moves by "like poles pushing each other away"?

    From what I understand a linear motor (or a stepper motor) works as the rotor N pole stays very closely positioned to the rotating stator field S pole. So it's linked by attraction as the N moves until it's at the closest possible distant to the S then moves at the same speed as the S moves although load torque will cause some corresponding deflection it is still "unlike poles" pulling each other around.

    Could you even make a motor like that? I think if you tried to turn it it would very quickly "flip" until the unlike poles were at the shortest distance from each other? Basically the same as if you try to push a barmagnet across the table with another barmagnet.

    I'm not sure you can make a "pusher" motor electrically unless you (say) had only N poles facing out on the rotor, and managed to generate only N poles on the stator poles. It might be possible but would have low efficiency and the rotor magnets would demagnetise pretty quick.
     
  17. strantor

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    I really wish I had the grapics software and skills to make animations. Picture this:
    [​IMG]
    I'm not sure what A,B,H, or Sc mean in this picture, so I'm ignoring them (and ignore the arrow at the top). Also, keep in mind as I continue, I am describing things as I understand them so that maybe you can pick out where exactly my misunderstanding is; I have no intention to lecture you on how a motor works. This picture shows the force of attraction between the N & S poles, and in the picture is looks as if the rotor is moving counterclockwise, and just about to line up with the permanent magnet poles on the stator. As soon as this happens, the DC will shift to a different set of windings via the brushes/commutator and the poles will shift, again attracting, and a never ending cycle of attract/shift/attract will occur.
    Now, using this same picture, imagine you reverse poles on the rotor and the direction of the flux arrows so that the poles are opposing each other and it should spin the opposite direction via oppositional forces rather than attractive forces

    ...Actually, when I say it that way, all that would be required is to swap polarity of the DC. When you do that, you swap rotation direction, which checks out in real life. now when I look at that picture I see oppositional forces already at work. check it out:
    [​IMG]

    So in the motor N is repelling N at the same time as S is attracting N!
    Am I right? I think I'm getting somewhere.
     
  18. THE_RB

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    Well, actually that diagram shows the stator as the inside, with the coils, and the moving rotor is outside.

    And as I said, the motive force is provided by attraction on the unlike poles, as the magnetic field that produces the most easily harnessed force is the unlike poles N and S attracting each other.

    Anyway I'm losing track of what we were supposed to be talking about! ;)
     
  19. strantor

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    Yes, I couldn't find a good picture of what I was trying to talk about, so I tried to make that picture work. That's why I said to ignore the directional arrow. I found a better picture, read on...
    Yes, I agree, but I think I'm also seeing a repulsive force at work. in the picture below (left), the rotor's north pole is at about 1 o'clock. Wouldn't the North pole of the stator's permanent magnet be pushing the rotor's north pole until it got to 3 o'clock, at which point the rotor's south pole would take over by attracting it? Wouldn't that mean that during operation the permanent magnets are held in opposition for 50% of the time?
    [​IMG]
    I'm still just trying to figure out why the magnets in a permanent magenet motor do not deplete. I think now that when held in opposition, the magnet is drained, and when held in attraction it is charged. if this is true and it is also true that it is held in opposition for 50% of the cycle as I propose, then there is a balance between the attractive and repulsive forces which means it would never deplete, except for environmental causes (banging, heat, etc.). Does this all sound like poppycock?
     
  20. strantor

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    should this be posted in physics or what?
     
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