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Re: Magnet Distance

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  • g.gibbes
    Hi All, I m a new member here, glad to join the group. Just thought I could help with this question. The magnetic flux across the air gap depends on the
    Message 1 of 17 , Jan 14, 2013
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      Hi All,

      I'm a new member here, glad to join the group. Just thought I could help with this question.

      The magnetic flux across the air gap depends on the reluctance of the complete magnetic circuit.

      the formula is:
      flux = MMF / circuit_reluctance

      MMF is fixed according to the magnet grade and thickness. The circuit reluctance depends mainly on how far the magnetic flux has to travel through low permeability material (air, copper and magnet). Interestingly, the permeability of a Neodymium permanent magnet is similar to air.

      So the relationship would be flux = k/g
      where "k" is a constant and "g" is the total gap between the iron rotor plates. (not the gap between the magnets).

      So for instance, if the magnet on each side is 10mm thick, and the gap between the magnets is initially 10mm, then the total "air" gap is 30mm. If you increase the gap between the magents to 20mm, then the total "air" gap becomes 40mm. The amount of magnetic flux in the this new configuration will be about 30/40 = 75% compared to the initial case.

      Therefore the induced voltage (at any given RPM) will be about 75%. (assuming you don't change the number of turns in the stator)

      hope this helps,
      Greg


      --- In axialflux@yahoogroups.com, Henry Davis wrote:
      >
      > That's kind of what I assumed, the same as how light works. The
      > difference that has me unsure is that with light you have to get very
      > far away for it not to be seen at all but with a magnet you don't have
      > to get very far away at all for its effect to be very near zero.
      >
      > If I get some extra time I going to experiment with the distance thing.
      >
      > Henry
      >
      >
      > On Dec 22, 2012, at 10:46 AM, magn0mite wrote:
      >
      > > Hi All
      > >
      > > If I am not mistaken, the inverse square law applies. If you double
      > > the distance apart, you halve the force.
      > > If you clamp a magnet to the table and aproach it with a like pole
      > > of a similar magnet you will feel when the forces begin to take
      > > effect.
      > >
      > > magn0mite
      > >
      > > --- In axialflux@yahoogroups.com, Henry Davis wrote:
      > > >
      > > > Without going into too many specifics, is there an estimated (a
      > > rough
      > > > percentage) for loss as the distance from magnet to coil increases?
      > > > It must vary with materials but I'm just curious about the typical
      > > > magnets and coils used for the homebuilt Hugh P type axial.
      > > >
      > > > In other words, what's the largest gap where it first begins to
      > > produce?
      > > >
      > > > Henry
      > >
      >
    • Henry Davis
      Thank you for that Greg. Henry On Jan 14, 2013, at 9:28 AM, g.gibbes wrote:
      Message 2 of 17 , Jan 14, 2013
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        Thank you for that Greg.

        Henry


        On Jan 14, 2013, at 9:28 AM, g.gibbes wrote:
        <Snip>

         



        Hi All,

        I'm a new member here, glad to join the group. Just thought I could help with this question.

        The magnetic flux across the air gap depends on the reluctance of the complete magnetic circuit.

        the formula is:
        flux = MMF / circuit_reluctance

        MMF is fixed according to the magnet grade and thickness. The circuit reluctance depends mainly on how far the magnetic flux has to travel through low permeability material (air, copper and magnet). Interestingly, the permeability of a Neodymium permanent magnet is similar to air.

        So the relationship would be flux = k/g
        where "k" is a constant and "g" is the total gap between the iron rotor plates. (not the gap between the magnets).

        So for instance, if the magnet on each side is 10mm thick, and the gap between the magnets is initially 10mm, then the total "air" gap is 30mm. If you increase the gap between the magents to 20mm, then the total "air" gap becomes 40mm. The amount of magnetic flux in the this new configuration will be about 30/40 = 75% compared to the initial case.

        Therefore the induced voltage (at any given RPM) will be about 75%. (assuming you don't change the number of turns in the stator)

        hope this helps,
        Greg


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