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How does a magnetic coupling work?

  1. Apr 11, 2014 #1
    Is it just a series of attracting magnets in a circle?
     
  2. jcsd
  3. Apr 11, 2014 #2

    Baluncore

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    It rather depends on just what you mean by “a magnetic coupling”.
    It could be a series of magnets or it could be constructed more like an electric motor.
    We need more context before we can provide a more definitive answer.
    Can you provide an example application or a web link to the particular device ?
     
  4. Apr 12, 2014 #3
    Basically what i have is a hermetically sealed vacuum chamber but i need to transmit torque via a shaft to and from a flywheel inside. The 2 ways i can see to do this is just having a seal on the shaft which might leak and cause a lot of friction or transmitting the torque through the wall of the chamber via a magnetic coupling but i dont fully understand how they work or whether they are available to buy. could i just construct my own with some neodymium magnet do you think? Thanks for the reply :)
     
  5. Apr 12, 2014 #4

    Q_Goest

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    You're correct, a magnetic coupling transmits torque by using magnets on both sides of some pressure boundary.

    The magnets can either be set up around a cylinder:
    mag_coupling_3d.jpg

    Or around a flat disk:
    magnetic-coupling.jpg

    Obviously there is a non-magnetic pressure boundary that will be placed between the two. The distance between the magnets will dictate how much torque you can transmit for any given application but if you're only transmitting through a wall that has a differential pressure of one atmosphere, you won't have any problems. I've seen them used in applications where the pressure is many hundreds of psi.

    You can purchase them off the shelf so I wouldn't try and design your own. Just do a search on magnetic couplings and you'll find all sorts of suppliers.
     
  6. Apr 12, 2014 #5
    That great thanks mate :)
     
  7. Apr 12, 2014 #6

    Baluncore

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    The internal flywheel bearings are going to be a problem. Rolling bearings fail very rapidly in a vacuum. Lead dust or silver powder has been used to lubricate ball bearings for rotating anodes in X-ray tubes.
    http://en.wikipedia.org/wiki/X-ray_tube#Rotating_anode_tube
    Maybe you can combine magnetic bearings with your magnetic coupling.
     
  8. Apr 12, 2014 #7
    Thats a good point, i hadn't thought of that. So are you saying the coupling would support it one end and then another magnetic bearing at the other to support that? The system would be in use on a car would using magnetic bearings not lead to the flywheel wobbling after bumps in the road? I was planning to have magnetic bearings central to support the weight of the flywheel without much friction and then ordinary bearings to the sides to stabilise the flywheel but theres still the vacuum problem would ceramic bearings work in a vacuum?
     
  9. Apr 12, 2014 #8

    Baluncore

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    There will need to be some form of variable torque to RPM transformation in a flywheel energy storage system. That may best be done using electrical coupling. By placing a motor/generator inside the vacuum chamber you then only need to get the electrical conductors or the magnetic field through the wall.

    By trying to guess what I meant, you may come up with a new and better solution.
    I have no simple answer. I can only apologise for playing the Devil's advocate here.
    Technology advances when new combinations of ideas are implemented.
     
  10. Apr 12, 2014 #9

    Q_Goest

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    Baluncore makes a good point about grease in a vacuum but I guess that begs the question, what vacuum level are you trying to attain? There are a lot of greases that have very low vapor pressure to avoid outgassing. Try google.
     
  11. May 19, 2015 #10
    yes,
    As our experience, it is not easy to do the magnetic coupling, by your own, because of the maching and manufacturing tools. anyway, there are so many supplier online offer neodymium manget and the shaft or steel shell. you can search it online.
     
    Last edited: May 19, 2015
  12. May 19, 2015 #11
    the pictures is very
    the pictures are vivid and lively.
     
  13. May 21, 2015 #12
    I was looking into this myself some time ago, and having two stators of magnets seems odd to me, I was thinking more about how it is done as magnetic damping (dampening).

    If you have two aluminium discs with magnets fitted to one of the discs on the fact, you rotate one, and the other will follow it, due to the eddy currents generated in the disc.

    I have also seen this used for high speed disc breaking in fast cars, as you don't get friction heat, and the higher the speed (rate of change) the more effect happens.

    This youtube shows this effect.

     
  14. May 21, 2015 #13

    Baluncore

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    You have reinvented the induction motor where a conductive rotor is dragged by a rotating magnetic field. You can achieve the same by using a three phase field winding rather than permanent magnets. With 3 phase AC you can better control the speed and do not need external moving parts.

    The energy released while braking still appears as heat in the conductive disc. That is because eddy currents induced in the disc release heat proportional to I2R. The advantage of eddy current braking is that there is no friction material to wear out or burn. The disc can be more efficiently air cooled if there are no hot friction pads insulating it from the air. Changes in disc surface due to contamination, water or changes in the metal are also largely eliminated.
     
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