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If a toroidal transformer has two separate, concentric secondary windings, one inside the primary coil and the other outside, does the force-field which induces a current in the inner secondary, for instance, get totally "consumed" by it, leaving nothing to power the outer secondary, or does this influence somehow get shared between the inner and outer secondaries?

What is the nature of these influencing force-fields, since almost all of the magnetic flux is contained within the primary coil and electric fields can't escape the conducting torus of the primary coil itself (or can they)?

What field, then, powers the outer secondary?

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    \$\begingroup\$ Transformers with more than one secondary are common. The primary satisfies the demand of the secondaries. \$\endgroup\$
    – Chu
    Jun 9, 2017 at 7:18
  • \$\begingroup\$ Thanks. I can see how multiple inner windings, for example, can be "powered" together, but my doubt relates to the particular geometry of a transformer with both inner and outer windings. In fact, there seems to be a prevailing mystery as to how outer windings of a transformer can "detect" changes in the core, as the primary toroid would seem to shield the outer coil from EM influences emanating from the core. \$\endgroup\$ Jun 9, 2017 at 22:17
  • \$\begingroup\$ There's no shielding going on. As long as a secondary conductor loops through and around the toroidal core it gets an induced voltage, and also a current if that loop is connected to a load. There is no 'splitting' of the core flux to deal separately with, say, two secondaries. The primary current creates as much flux as is demanded by the secondaries according to their load demands. That is, the secondary loads control the amount of flux produced by the primary (apart from a small amount required to overcome iron and copper losses) \$\endgroup\$
    – Chu
    Jun 10, 2017 at 6:37

2 Answers 2

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The magnetic field powers both secondaries.

...since almost all of the magnetic flux is contained within the primary coil...

The magnetic flux is contained in the magnetic core, not in the primary coil.

Any loop of wire that links the core will have a voltage induced in it by changes in the core flux. That is, all loops of wire that link the core have exactly the same voltage induced in them. It doesn't matter what order these are put onto the core, they still get the same voltage (to first order, any differences due to leakage inductance will be vanishingly small, parts per ten thousand in typical iron-cored transformers)

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    \$\begingroup\$ Since the answer to the title question is "Yes", i suggest you do not start your answer with "No", it may be confusing. \$\endgroup\$
    – Sclrx
    Jun 9, 2017 at 8:22
  • \$\begingroup\$ @Sclrx thx for the prompt to clarify \$\endgroup\$
    – Neil_UK
    Jun 9, 2017 at 8:29
  • \$\begingroup\$ Do you really mean 'get the same voltage'? \$\endgroup\$
    – Chu
    Jun 9, 2017 at 12:20
  • \$\begingroup\$ @Chu yes, 'get' is a clumsy word, but it's shorter than 'have the same voltage generated in them'. In reality, it's not the same voltage, as leakage inductance alters the voltage very slightly, but we are talking parts per ten thousand here, not even percent difference. It's something that can be safely ignored in most transformer design. The voltage you measure might be percents different, due to current flow in the windings causing a voltage drop, but that's a different and well known effect. \$\endgroup\$
    – Neil_UK
    Jun 9, 2017 at 13:06
  • \$\begingroup\$ If all loops of wire that encircle the core have exactly the same voltage induced in them, that seems to suggest that the total power output of the transformer can be multiplied by two? Or more, if extra sets concentric coils are added? \$\endgroup\$ Jun 9, 2017 at 22:11
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Can a toroidal transformer with two secondaries, inner and outer, simultaneously induce current in both?

Voltage is induced not current. Any current that flows is as a result of the induced voltage and a load connected to the winding where voltage is induced.

The primary creates a changing magnetic field that couples to both secondaries irrespective of where they are sited on the toroid so there is no situation where that field can somehow be consumed by the inner secondary at the expence of the outer secondary.

In other words both secondaries "receive" the same changing magnetic field and both have a voltage induced in their respective windings (subject to Faraday's law of induction) irrespective of the level of currents that might be taken from either of those secondaries. The only assumption here is that there is 100% magnetic coupling and therefore there are no leakage inductances.

If there are leakage inductances (normally and typically on average about 3%) there will be a slight reduction in the terminal voltages of each secondary when current is taken.

What is the nature of these influencing force-fields, since almost all of the magnetic flux is contained within the primary coil and electric fields can't escape the conducting torus of the primary coil itself (or can they)?

With concentric secondaries and primary surrounding a magnetizable core, any changing magnetic field is "felt" by all three. The electric fields are unimportant in the simple analysis of a transformer of the type you are referring to.

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  • \$\begingroup\$ Thanks for that. But I thought that the magnetic field outside an ideal toroid was always zero. (I know, bifiliar coils and "once-around" effects and all that). I still can't imagine how the influence from the changing core flux can be "split" to serve both the inner and outer coils. \$\endgroup\$ Jun 9, 2017 at 22:31
  • \$\begingroup\$ Whatever the position of the winding, the core goes through its middle and affects all windings equally. An inner winding does not act like a shield except in extreme cases of short circuit. \$\endgroup\$
    – Andy aka
    Jun 10, 2017 at 8:17

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