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Consider a transformer. I put a constant sine wave AC voltage through the primary circuit and a resistor on the secondary circuit. I know that the secondary voltage is proportional to change in magnetic flux which is proportional to the maximum flux. But the amount of flux generated by a coil is proportional to the current.

This means that if I increase the resistance on the secondary circuit, the secondary current decreases, the primary current decreases, the magnetic flux decreases, so the secondary voltage decreases?

This obviously can't be right. What am I missing?

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  • \$\begingroup\$ Have a look at my answer to a similar question here. Basically, magnetisation doesn't change with load current. \$\endgroup\$
    – Rohat Kılıç
    Commented Aug 21 at 6:50

2 Answers 2

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You should make a clear distinction between magnetization flux (a nearly constant AC quantity made by the primary) and, the naturally cancelling fluxes from the primary and secondary windings (that are due exclusively to secondary load current).

In effect, the cancelling load fluxes er... cancel out and, the only flux remaining in the core (the flux that causes induction) is magnetization flux. Magnetization flux is due to the primary voltage and the inductive reactance of the primary and, is largely unaffected by secondary load currents.

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  • \$\begingroup\$ So am I right in understanding that the secondary coil produces a magnetic flux in the opposite direction to the primary coil which cancels it out? Also, what is magnetisation flux? I can't find any information on it \$\endgroup\$
    – EnderShadow8
    Commented Aug 19 at 13:54
  • \$\begingroup\$ @EnderShadow8 Have you tried searching on Google? “magnetisation flux” gives a few million results. Link: googlethatforyou.com/?q=magnetisation%20flux \$\endgroup\$
    – winny
    Commented Aug 19 at 14:08
  • \$\begingroup\$ @winny All the results concern magnetic flux not magnetisation flux \$\endgroup\$
    – EnderShadow8
    Commented Aug 19 at 14:27
  • \$\begingroup\$ @EnderShadow8 In your transformer example, the resulting flux is only the magnetization one. \$\endgroup\$
    – winny
    Commented Aug 19 at 14:30
  • \$\begingroup\$ @EnderShadow8 with the secondary unloaded, the primary current is purely magnetization current and, it produces magnetization flux which, in turn, induces the secondary voltage. When secondary current flows, the the primary current is the original magnetization current plus a current due to the secondary load. For a 1:1 transformer this extra primary current is equal and opposite to the secondary load current hence, those two fluxes cancel with. The remaining flux being the magnetization flux (still inducing the same secondary voltage). \$\endgroup\$
    – Andy aka
    Commented Aug 19 at 15:48
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The magnetisation flux is due to the sum of the ampere-turns in primary and secondary. That means it's easiest to think about it when the secondary current is zero, and the magnetising current is simply the primary current.

The changing core flux induces a voltage in both primary and secondary. In an ideal, zero winding resistance transformer, it's this induced primary voltage that balances the applied primary voltage across the primary (across, not through, as in your question), so setting the proportionality between applied primary voltage, and rate of change of core flux.

If you now load the secondary, a secondary current will flow, but a cancelling current will also flow in the primary. The sum of these load currents, multiplied by their respective turns, will be zero in an ideal transformer. Leaving the magnetising flux essentially unchanged.

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