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  • OT Halbach arrays

    I know this is OT, but some of you need a break from the shop-

    I've been looking at halbach arrays and reading about all the great advantages to them. If I follow the field lines I can see where they emerge in strength on one side of the magnetic structure and don't on the other side. What I'm having trouble with is the notion that it's something special- can't it be duplicated simply by magnetizing a ring of magnetic material from one side? Hard drive magnets are made this way- a north and south are 'printed' on one side of the block of material, a horseshoe magnet is essentially the same, so would be an arrangement of magnets on a steel ring such as in an ordinary pm motor of multiple pole construction. What am I missing?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Pretty simple Darryl. The magnets used to make a Halbach array are magnetized in isolation from each other. Because of that the orientation of the flux lines is not affected by opposing flux lines nearby as would be the case in an attempt to create such an array using adjacent magnetized regions.

    When the independent magnets are brought together to form a Halbach array the field lines are strongly compressed by the proximity of the adjacent magnets. You cannot duplicate this by magnetizing regions in place.
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    • #3
      Evan's got it right. I've wondered how entropy plays into the Halbach array - it seems very at odds with the balance that Nature so dearly loves.

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      • #4
        it seems very at odds with the balance that Nature so dearly loves.
        It is. You would never see such an arrangement happen naturally. It takes a great deal of force and manipulation to arrange the magnetic elements in a Halbach array. Normally a special jig is required to hold the magnets in place until they can be glued or wrapped.

        It is possible to reverse entropy locally in any system including a completely closed system at the expense of the overall entropy of the system. All it takes is an energy input from sources of energy that exist in the system. There is a large amount of potential energy stored in the compressed fields of a Halbach array. It is exactly analogous to a series of compressed springs.
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        • #5
          Originally posted by dp
          Evan's got it right. I've wondered how entropy plays into the Halbach array - it seems very at odds with the balance that Nature so dearly loves.
          Huh??? It's just common 2-pole magnets aligned so that the magnetic flux on one side cancels.

          Just get a couple of magnets and align them like this, and you'll see how it' works:

          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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          • #6
            Originally posted by lazlo
            Huh??? It's just common 2-pole magnets aligned so that the magnetic flux on one side cancels.

            Just get a couple of magnets and align them like this, and you'll see how it' works:

            Magnets in close proximity to each other affect permanently each others' magnetic strength. They are less powerful when separated than they were before being placed in close proximity.

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            • #7
              Try placing some supermagnets in that configuration and you will find it's a lot easier said than done.

              Magnets in close proximity to each other affect permanently each others' magnetic strength. They are less powerful when separated than they were before being placed in close proximity.
              The strength of the magnet doesn't change at all. The flux is compressed giving a higher flux density but less volume. Same strength.
              Last edited by Evan; 12-10-2009, 10:42 AM.
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                Thanks, Evan . That makes total sense, so it clears it up for me. It seems likely then that if you were 'building' a magnet by gluing a few together side by side to form a curved section, you could add some magnets between these built-up poles to enhance the strength of the poles without changing the spacing of the poles.
                Last edited by darryl; 12-10-2009, 08:54 PM.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Magnet FAQ: http://www.magnetsales.com/Design/FA...mes/FAQs_2.htm

                  How permanent is a magnet's strength?

                  If a magnet is stored away from power lines, other magnets, high temperatures, and other factors that adversely affect the magnet, it will retain its magnetism essentially forever.

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                  • #10
                    Yeah, I also wonder about the re or demagnetizing effect when the magnets are placed like that. I suppose when you build the array, any change in the magnetic field of an individual magnet would happen, then stabilize roughly at that state. I can see how the magnets may not be the same as they were originally if you separated them and tested- you would obviously need before and after test results to compare.

                    A really easy way to check the effect of magnetization, or specifically demagnetization, is to take a neo magnet to a ferrite magnet, and see how easy it is to demagnetize the ferrite.

                    In any event, enhancing the strength of a magnetic pole by using the halbach array method is going to have a significant impact on any device you build using the magnetic structure. In a motor for example, you would expect a higher generated voltage in a winding that is moving past the magnetic poles. That would mean less turns required for the same back emf, which in turn means lower winding resistance, more power output from the motor, etc. One limit though is going to be rpm per volt, since the lowest number of turns you can have is one.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                    • #11
                      You can have a half turn, which is very inefficient. That's how a clamp on meter works.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Boy am I dumb. I thought a Halbach array was the in-haul for high-lead logging. Or a place where they perform music written in 18th Century Leipzig by a famous Baroque compoer. Or the act of dragging back an escaped manta. Or the gizmo that tensions an airplane on the catapult prior to launch. Or pulling back Tom Malgiazzi's brother from the cliff edge.

                        Am I close?

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