In Isolated Transformer, the primary and secondary are separated yet when there is a demand for current in the secondary, the primary can produce current and transfer it via fluxes. I understood this a bit although I'd appreciate it if someone can share a youtube video of the principle because of this finding I had where when I used gaussmeter to measure the magnetic field, there was no difference between no load and with load as if the magnetic field was already maximum at the start.

Does this also occur in Autotransformer? I'm asking this because when I used a gaussmeter to measure magnetic field of an autotransformer, the no load and load magnetic field was the same. I was expecting the magnetic field to increase with load because there is more current and in an autotransformer, the secondary is just tap of different point of the continuous wire.. unless it's the same principle as the nonlocal flux transfer in isolated transformer? How?

  • \$\begingroup\$ You would have to measure the flux inside the core -- I'm not at all sure what you'd read outside the core, but I doubt it'd be representative of what's inside. \$\endgroup\$ – TimWescott Oct 29 '18 at 23:24
  • \$\begingroup\$ At a distance of 16 inches, 20 inches, half meter, one meter. I'm measuring the same magnetic field whether it's no-load or there is load. It is constant. If there is changes in the magnetic of the core.. it should reflect outside. \$\endgroup\$ – Samzun Oct 29 '18 at 23:27
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    \$\begingroup\$ Those measurements would be useful if what you cared about was the degree to which the transformer was leaking flux (which does matter, in some circumstances). But it's not going to be of help for knowing what's going on inside the core. \$\endgroup\$ – TimWescott Oct 29 '18 at 23:42

Do this thought experiment: take a normal transformer. Connect the non-dot wire of the secondary to the dot wire of the primary. You now have an autotransformer.

So yes, the mechanism is the same.

I suspect that the reason your flux readings don't change is because you were not reading the flux inside the core. Even if you were able to do so, in a reasonably efficient transformer that's operating even remotely normally* the magnetizing flux is going to remain substantially the same.

* "remotely normally" in this context means it's not bursting into flames.

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    \$\begingroup\$ Simple , yet right to the point. And correct. \$\endgroup\$ – Marla Oct 29 '18 at 23:32
  • \$\begingroup\$ Do you or does anyone have youtube video of the flux dynamics in isolated transformers (and also autotransformers)? It's cool that fluxes alone can seemingly transfer current non-locally. I wonder if some taps can also be made to the vacuum and acquire current non-locally too. \$\endgroup\$ – Samzun Oct 30 '18 at 0:21
  • \$\begingroup\$ @Samzun Yes, that's called an antenna. \$\endgroup\$ – Hearth Oct 30 '18 at 0:42
  • \$\begingroup\$ TimWescott. So in an autotransformer. When load was put in the tap.. it can set up current and back EMF that can make the voltage source produce more current (for the flux to be stabilized)? But note in autotransformer the tap is in series. Even without any flux or back EMF.. won't it take in current as well just like any normal circuit? So the back emf or flux is just redundant? \$\endgroup\$ – Samzun Nov 2 '18 at 7:24

Go read this answer; I discuss the transformer as a feedback system.

Transformer magnetizing inductance


In Isolated Transformer, the primary and secondary are separated yet when there is a demand for current in the secondary, the primary can produce current and transfer it via fluxes.

No, that is not how a transformer works. The primary voltage sets up a current in the primary winding inductance and this sets up the flux in the core. That flux RMS value remains pretty much constant under loading and no-load conditions.

If it didn't remain constant then the output secondary voltage would be very uncontrolled rather than be defined by the turns ratio. And, of course, the turns ratio and Faraday's law of induction is what produces the secondary voltage.

It's no different to an auto transformer except that the secondary voltage is either propped-up by the primary voltage due to there being a series connection OR the secondary voltage is tapped off directly from the primary at some lower level (potential divider action).

Regards sourcing current from the secondary, if you have a secondary voltage and a load, then by simple ohms law you have secondary current.


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