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How exactly does a transformer work?

  1. Jun 12, 2012 #1
    Hi, the explanation I have been given for how a transformer works is this:

    The current flows through the primary coil, hence generating a magnetic field around the coil. Imagine that the coil is wrapped in the anticlockwise direction around the soft iron core (and hence current flows anticlockwise), then by the Right Hand Grip Rule, the north pole would point down. Since the soft iron core is square/rectangular, then by extending the magnetic field induced in the core around the core, one can see that the magnetic field now within the core runs anticlockwise as well.

    By Lenz's Law, the current in the secondary coil will then flow so as to oppose this magnetic field which produced it in the first place. So the coil is wrapped in the clockwise direction (and so current flows clockwise around the core), and the magnetic field generated by the secondary coil now has north pole pointing down. So the magnetic field it induces in the core is clockwise, and is opposed to that generated by the primary coil.

    That's great and all, but I don't really see how this can be explained using Fleming's Right Hand Rule? I see the magnetic field generated by the primary coil at a particular instant when the current is pointing in that direction to have an upwards direction since it is anticlockwise around the core. But the relative motion of the coil to the magnetic field is downwards.

    So, by Fleming's Right Hand Rule, doesn't this mean the direction of magnetic field is PARALLEL to the direction of relative motion? So how does this induce an emf in the secondary coil in the first place? Or is there any other way to explain besides Lenz's Law?

    Sorry if this was damn long.... thanks!!!
     
  2. jcsd
  3. Jun 12, 2012 #2

    Drakkith

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    What do you mean by "the relative motion of the coil to the magnetic field is downwards"? What motion of the coil?
     
  4. Jun 12, 2012 #3
    Like, when using FRHR, we need to know the direction of current through the coil, the direction of external magnetic field, and the direction of relative motion of the coil. Meaning how the coil is cutting across the magnetic field lines.

    In this case, the coil itself isn't moving, but the magnetic field should be when it's expanding or collapsing? So I guess the RELATIVE motion is opposite to that of direction magnetic field is moving.

    Like, if A is stationary and B is moving, then if you take B as stationary, RELATIVE to B, A is moving.
     
  5. Jun 12, 2012 #4

    Drakkith

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    Ah ok I think I see what you are asking. The field should be moving or changing in a parallel direction to the coils as if they were moving and the fields were static. Is that correct?
    Edit: If so, what's the problem? The field lines still intersect the coil at a 90 degree angle at every point in the wires.
     
    Last edited: Jun 12, 2012
  6. Jun 12, 2012 #5

    sophiecentaur

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    Why invoke Fleming's rule? That is about induction of a voltage due to movement. There are no moving parts in a transformer. To discuss how a transformer works you need to look at the rate of change of magnetic Flux.
     
  7. Jun 12, 2012 #6

    krd

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    There's an alternating current passing through the primary coil - so the flux is constantly changing - this induces a current in the secondary coil.

    One of the most important aspects of how a transformer works, is the current needs to be alternating - alternating current = alternating magnet flux. It doesn't work if the current is DC.

    With an electric generator/motor, you change the magnetic flux by moving the shaft of the motor - current flows through the coil. With the transformer, you change the magnetic flux.

    With the transformer you don't have to worry about the right hand rule - the coils just need to be close enough together.
     
  8. Jun 12, 2012 #7
    For a typical transformer, the input current in phase with the input voltage produces the output power, while the input current 90 degrees out of phase with the input voltage produces the reactive (inductive) excitation (V = L dI/dt).
     
  9. Jun 13, 2012 #8

    sophiecentaur

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    This just HAS to be, because a resistive load on the secondary appears as a resistance across the primary. V and I have to be in phase for the primary.

    I can't see why the RH rule ever came into this. RH rule involves Movement.
     
  10. Jun 14, 2012 #9
    From Bob S
    I am just pointing out that a transformer has two components of input current, both real power and reactive power components, and these two input currents are in phase quadrature.
     
  11. Jun 14, 2012 #10

    sophiecentaur

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    Oh yes, of course but the quadrature can be made vanishingly small. The real is as a result of actual power transfer.
     
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