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Friction with a Magnetic Bearing System?

  1. Feb 27, 2013 #1
    One question that I have been thinking about for a while is if you were to use magnetic bearings (no physical friction) for a type of motor and put energy into it by spinning it and slow it down with another magnet, where does the mechanical energy go? If there is no physical contact with the bearing will it produce heat because of the magnetic friction? or Will the energy be completely destroyed?
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  3. Feb 28, 2013 #2


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    What do you mean by "slow it down with another magnet" ?

    "Slow" implies a load of some sort and that's your answer.
  4. Feb 28, 2013 #3
    "By slowing it down with another magnet," I mean that the magnet is used as a form of friction and prevents the motor for rotating after the work is applied to it.
  5. Feb 28, 2013 #4


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    Magnets don't work by friction. Instead like poles repel each other. If you don't have enough momentum or velocity to overcome this repulsion the motor would simply come to a stop and then be accelerated the other way before coming to a stop again as it approached the magnet once more. In a perfect system with no losses it would simply oscillate back and forth.

    If instead you had enough velocity to overcome the repulsion, the motor would slow down as one of its poles approached the magnet and then accelerate back up to speed once it passed the magnet. No energy loss.

    However, we don't have perfect systems in real life. You will lose energy through friction with the air and with the flexing of the magnet and motor.
  6. Feb 28, 2013 #5


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    I suppose a magnet could slow the motor by causing eddy currents. Bit like a dynamo with the output shorted. In which case the energy would go into heating the motor.
  7. Feb 28, 2013 #6
    I'm sorry, I forgot to mention that the motor is attached to another system (which is a magnetic rotating shaft). You first transfer the energy from the motor to the rotoring shaft then disconnect the motor, so that the shaft is running off momentum. Then have the magnets attract one another. Should the attraction of the magnets slow the staft to a stop?
  8. Mar 8, 2013 #7
  9. Sep 29, 2014 #8
    Energy is never destroyed, but eventually it all ends up as heat as we all know. I believe (and am not sure) that since there is actually 'work' being done that eventually there is energy being spent. Compare for instance a fridge magnet stuck to a door: not doing any 'work', so it will probably hang there forever and not lose it's magnetism, as opposed to your magnetic bearing system where there is continuous motion, resulting in energy being spent. Although there is extremely little friction, there will be energy release in very small quantities over a long period, since the magnet won't lose it's energy quickly (especially if its a rare-earth magnet with very high coercivity). Another consideration: What if you run this bearing in a vacuum, where even the air resistance is removed? I myself have been working on such a concept involving a polar array setup of permanent neo magnets to use as a magnetic bearing - not even sure if it would work!
  10. Sep 29, 2014 #9


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    You haven't really explained what you mean by "have the magnets attract one another" but the answer is probably yes.

    You certainly can't make a permanent magnet motor so just about any arrangement of permanent magnets you add to a spinning shaft will slow it down faster than if they weren't there.
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