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Rotating magnetic field theory

  1. Jul 11, 2011 #1
    Hey fellas am having some trouble understanding the concept behind a rotating magnetic field. Looked up the wiki entry and a couple of other webpages, but only ended up getting more confused. I am well versed with all EM fundamentals, but still...:confused:

    OK so in a basic alternator, we have a magnet rotating inside a conducting loop, or vice versa, as the requirement is relative motion between a conductor and a magnetic field so that the flux lines are cut and an EMF is induced. I get it up to this point.

    Now I consider the stator with 3 coils, i.e a 3-phase system. So here does the rotating magnetic field refers to the field by the rotor(which is a permanent or electro magnet) since its the only thing that's rotating? If I power the coils with a 3-phase supply, then a magnetic field is set up in each of the coils and is 120 degree out of phase with each other, but how is this field rotating? Because in the wiki entry it says that the rotor will end up chasing this field as it will try to align the N or S pole of its magnetic field with that of the vector sum of the stator's field. I understand the mechanism behind that, but I dont see how the stator's field will rotate, and if this is the rotating magnetic field?

    Also, if the rotor does move and ends up chasing the field of the stator, then how do I get 3 phase AC from this movement? And if the coils are to be driven by a 3 phase system in the 1st place, then that means you require 3phase current to make 3 phase current?

    Any help would be appreciated. Thanks in advance.
     
  2. jcsd
  3. Jul 12, 2011 #2

    Drakkith

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    In the 3 phase system, the coils have their poles switched back and forth to cause the motion in the rotor as its poles try to align. The field from each coil itself isn't rotating, nor does it need to, as the rotors field is the one rotating. It is a CHANGING magnetic field that causes current in the alternator. The rotation is the cause of the changing field.
     
  4. Jul 12, 2011 #3
    OK so the poles keep changing coz of the AC being applied to each coil. So is that why the rotor constantly moves to align itself?
    Also, what about the fact that 3 phase current is required in the coils to produce 3 phase current again? I mean, is there no other way to do that?
     
  5. Jul 12, 2011 #4

    Drakkith

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    As the poles change they repel or attract the poles of the rotor.
    The 3 phase current in the coils is merely one way to run a motor. It doesn't have to be 3 phase. And in an alternator there is no 3 phase in the coils initially, only when the magnet on the rotor is turning is any current being induced in the coils. (If an alternator even produces 3 phase, I don't know)
     
  6. Jul 12, 2011 #5
    Alternator does produce 3 phase, according to Wikipedia ("The rotating magnetic field induces an AC voltage in the stator windings. Often there are three sets of stator windings, physically offset so that the rotating magnetic field produces a three phase current, displaced by one-third of a period with respect to each other").
    So what other ways to we have to run a motor?
    Also, say if I use a 3 phase system for the windings, and I take the output from separately each of the coils, then shouldnt the o/p be the sum of the current induced in the windings and the current already in the windings?
     
  7. Jul 12, 2011 #6

    Drakkith

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    Look up Electric Motor in wikipedia and you can find plenty of info!

    In an alternator there is no current in the windings already. You apply current to them if it is a motor to form the magnetic fields.
     
  8. Jul 12, 2011 #7
    You misunderstand me. I mean that suppose I use a 3 phase system to create the magnetic field. Now there is some current already flowing in the windings since I am supplying each coil from a 3 phase supply. As the rotor moves, EMF is again induced into the same coils. My question is: If the O/P is taken from these coils, then it should be the sum of the EMF already in them(from the 3-phase system) and the EMF induced, isn't it?
     
  9. Jul 12, 2011 #8

    Drakkith

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    I actually don't know, sorry!
     
  10. Jul 12, 2011 #9
    @Drakkith: That's ok....thanks for all your help. Much appreciated!
    Though I really hope somebody clears that up for me.
     
  11. Jul 13, 2011 #10
    C'mon guys...someone help me out here. :redface:
     
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