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Toroidal transformers

  1. Feb 1, 2008 #1

    dot

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    This question relates to 50Hz power toroidal transformers.

    Toroidal transformers are well known for their self shielding properties and consequently have a near-zero external field. Most of the magnetic flux lies within the core.
    Yet if one where to thread a good conductor through the centre of the toroid and connect the ends to form a loop, thus creating a one-turn secondary, one would in practice cause a short circuit and a very large current would flow (limited by the resistance of the loop).
    This seems to imply that the loop is experiencing most of the field created by the primary coil. How can this be explained when the wire itself lies completely in a near-zero field region? Seems spooky to me. This phenomenon is not restricted to toroidal transformers but is at the heart of every electrical machine, just that in a toroidal transformer it becomes plain that current theories and textbooks are missing something big time. The standard answer would be that the emf generated in the secondary depends on the amount of flux “linking” the primary and secondary and in this case all of the flux plainly passes through the area enclosed by the secondary circuit. I just cannot see how this “explanation” explains anything. How do the electrons in the wire feel the primary magnetic field?
     
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  3. Feb 1, 2008 #2

    ZapperZ

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    Maybe I don't understand your setup here, but why would "...wire itself lies completely in a near-zero field region..." when a part of it is threading through the center of the solenoid where the field is the largest? One could easily think of this region as the "emf source" for the secondary to cause current to flow.

    Zz.
     
  4. Feb 1, 2008 #3

    pam

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    There is no B field at the wire, but there is an electric field.
    The oscillating magnetic field in the toroid produces an electric field in the wire.
    The integral of this field around the circle equals the rate of change of the magnetic flux.
    That is Faraday's law.
     
  5. Feb 1, 2008 #4

    dot

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    To clarify
    By centre of the toroid I mean not the core of the toroid, but the empty space of the "window in the doughnut" or the hole in a "Polo" mint. This space is also empty of any field as far as I know at least in an ideal toroidal transformer.
     
  6. Feb 1, 2008 #5

    pam

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    The E field is in that "empty space".
     
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