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Magnetic Force and Torque Question

  1. Sep 30, 2015 #1

    I'm new to the forums but I had a problem I was trying to solve that I could use some help. Hoping someone here could point me in the right direction.

    Imagine you have two (2) cylindrical magnets that are diametrically magnetized. Both magnets are of equal size and magnetic strength. Magnet (A) is stationary and held place by an external force. Magnet (B) is stationary and held in place by an external force. However magnet (B) can rotate along it's axis. The magnets are located near each with their cylindrical axis in parallel. At their starting state the magnets have their opposite poles facing each other (North to South).

    The question is how much torque would be needed to rotate magnet (B) so that the poles the same poles are facing each other (North to North... or South to South).

    Greatly appreciate any help or assistance.

    - Kevin
  2. jcsd
  3. Sep 30, 2015 #2


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    Is there only one pole-pair as per magnet ?
  4. Sep 30, 2015 #3
    Yes, there is one pair per magnet.
  5. Sep 30, 2015 #4


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    I think that will be very hard to calculate. The torque is due to magnetic energy in the airgap, and you can approximately calculate such a torque when the width of the airgap is small, compared to the size of the magnet, when the magnetic field is homogenous inside the airgap and so on.

    It could be calculated numerically ( by computer program ).

    Some information is missing, for example the relative permeability as for the magnets, how exactly are the magnets magnetized and of course all mechanical dimensions.

    But I will for sure follow your thread. :smile:
  6. Sep 30, 2015 #5
    Thank you Hesch!

    Currently I'm trying to determine the equations I were to use and not a numerical value for the torque. Depending how much torque is required (relative to magnetic size) will determine what size magnets, air gap, and field strength I decide to use.

    I wouldn't mind writing a code to determine the value. I'm decent enough of a programmer that I could write something and let it run overnight. Probably wouldn't very accurate but would work for purpose.

    Do you know the general concept you would use to approach this situation?
  7. Sep 30, 2015 #6


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    Well, actually I tried to sketch the magnetic field on a drawing, but it's rather difficult to sketch a field, which must be a circulation field and must have the correct direction as to the north/south poles.

    I must try again later.

    Anyway you will get a maximum torque when the fields from the magnets are perpendicular to each other ( like in a synchronous motor ).
  8. Sep 30, 2015 #7
    That helps a lot. I didn't think about doing a finite analysis of this problem with a program. To get a good answer I think that's the only way.

    The magnetic poles are in the same orientation as this magnet.


    I'm turning the magnet with a servo motor and not sure how powerful of servo would be needed.
  9. Sep 30, 2015 #8


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    Well, here it is attached ( the drawing ).
    The "diameters" are some coils that I have wound to be able to calculate with Biot-Savart. But you see that the magnetic fields will be completely unhomogenious.
    The drawing just shows a slice of the magnets with the thickness dL. Having found the torque from a slice, you can just integrate/sum the torques along the cylinder.

    Attached Files:

  10. Sep 30, 2015 #9
    Thank you Hesch! I understand what you are showing and the technique. I'll wrote a code later this week to figure out what I need.
  11. Sep 30, 2015 #10


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    When you have made a program that calculates the B-field, you must test that the curves are (nearly) correct closed.
    If not, your program is calculating wrong.
    And yes, as you said: Calculate overnight. Maybe you should use assembler programming? :smile:
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