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Electromagnetic induction; bike dynamo

  1. Oct 9, 2009 #1

    I am trying to learn more about electromagnetic induction because I want to build a generator for a wind turbine. I am a bit confused about something, hopefully someone out there can help me!

    I recently opened a bike dynamo. It contains a permanent magnet that can rotate, and the magnet has 4 north poles and 4 south poles (I figured that out using another magnet); see attached drawing. A single coil of copperwire is surrounding the magnet. Now what I understood is that if a south pole is moved inside a coil, it should give rise to a current that has a direction opposite to a current initiated by the same movement of a north pole. That would mean that at any given timepoint, there should always be an equal amount of current trying to flow from both directions, resulting in no net current. Yet it works! Can anybody explain why??

    Attached Files:

  2. jcsd
  3. Oct 11, 2009 #2
    Xalt, I'm sure that you have not got the orientation of the coil and the magnet poles correct.

    The coil will be wound on an iron former in vertical 'segments', like eight coils in series. These are wound alternately in opposing directions so that the currents caused by motion of north and south poles past each coil add.

    Since you have it apart, look carefully at the winding, unwind it if you have to so that you can see how it is arranged.
  4. Oct 12, 2009 #3
    Hi Pogo,

    thanks for your reply. I checked it again, and it is like I drawed it before; I unwound the whole thing, and it is a single coil of copperwire surrounding the magnet as in my previous post. I did not mention that there are 8 iron "clamps" that hold the coil in place (see attachment for another of my great drawings). I think they also serve as cores to increase magnetic flux. However, since there are 8 of them (equally spaced), it still doesn't tackle my point as far as I can see...


    Attached Files:

  5. Oct 12, 2009 #4
    Thanks for the updated drawing, Xalt. The 8 clamps are part of the magnetic circuit, and couple the changing magnetic field (as the rotor rotates) to the winding. I'll check the details with someone I know who is more involved in machines, but I think that the structure changes the rotating flux to a radially alternating flux. that alternation induces an alternating current in the winding.
  6. Oct 13, 2009 #5
    Yes to all, except the rotating magnet induces a voltage (not a current) in the stator coil. The operation is based on Faraday's law. You will get the highest voltage out when the dynamo output (an ac voltage) is not attached to anything.
    Bob S
  7. Oct 13, 2009 #6
    Hi Xalt.

    My motors man sent me this: "It is like a car claw-pole alternator except the rotor uses magnets and the claw-pole is used on the stator (instead of the usual copper-wound field coil on the rotor in a car alternator). Alternate fingers of the claw capture flux from all poles of one sign, e.g. all the N poles, then all the Sth poles, etc - giving alternating flux linking the coil (hence ac voltage).

    My clumsy explanation of coupling the rotating magnetic field to a radial pulsating field is right though. I hope that it helps you understand.

  8. Oct 14, 2009 #7
    Thanks for your answers Pogo and Bob! And thanx for asking to the motor man too... :)

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