Explaining Toroidal Transformers & Their Self-Shielding Properties

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Discussion Overview

The discussion revolves around the self-shielding properties of toroidal transformers, particularly focusing on the implications of threading a conductor through the center of the toroid and the resulting electromagnetic effects. Participants explore the relationship between magnetic fields, electric fields, and induced currents in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that toroidal transformers are known for their near-zero external magnetic field and questions how a one-turn secondary loop threaded through the toroid can experience significant current despite being in a low-field region.
  • Another participant challenges the initial claim by suggesting that the wire does not lie entirely in a near-zero field region, as part of it is within the solenoid where the magnetic field is strongest, potentially serving as an emf source.
  • A different participant asserts that while there may be no magnetic field at the wire, an electric field is present, induced by the oscillating magnetic field, which can generate current according to Faraday's law.
  • A clarification is made regarding the "center of the toroid," specifying that it refers to the empty space within the toroid, which is believed to be devoid of any field in an ideal transformer.
  • Another participant reiterates that the electric field exists in the "empty space" of the toroid.

Areas of Agreement / Disagreement

Participants express differing views on the presence and role of electric and magnetic fields in the context of the toroidal transformer. There is no consensus on how the phenomena described can be reconciled with existing theories.

Contextual Notes

Participants reference Faraday's law and the behavior of fields in toroidal transformers, but there are unresolved assumptions regarding the ideal conditions of the transformer and the nature of the fields in the specified regions.

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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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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.
 
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.
 
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.
 
The E field is in that "empty space".
 

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