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When we connect a bigger resistance to a transformer, the current decreases both in the primary and secondary. To be able to maintain the same current, we'd have to increase the voltage.

Now imagine you have an induction coil that is supplied by a constant voltage of variable frequency, and you are heating some metal trough induction. The material has permeability equal to that of air.

If we increase the frequency, the eddy currents in the metal increase. Now increase in currents by previous explanation should increase the primary current, which means that impedance of the system has decreased.

But that would mean that the resistance most probably decreases, which is not what literature say. They say that any increase in eddy currents is seen as an increase in resistance. What gives?

The answer that explains what the magnetic field of eddy currents do to the primary coil to change the emf would be great. (Yes I know that it interacts with it, but I want to know exactly how and why).

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  • $\begingroup$ Why do we expect an increase in frequency to increase the eddy currents? That would lessen the skin depth in the metal, and the effective resistivity of the metal would go up. $\endgroup$
    – Whit3rd
    Commented Dec 12 at 3:27

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Eddy current increases the impedance because of the direction for which they are turning. Always keep in mind the right hand rule. Wrap your right hand around the eddy currents inside the material and trace this direction through the coil. Then wrap your hand around the coil to find the direction of this magnetic field. You'll find that the two oppose each other which means the currents inside the wire your using oppose each other so the impedance increases.

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  • $\begingroup$ Doesn't the current in a secondary coil (If we had a normal transformer) also have the same effect? So why does then the current in a normal transformer increase with secondary current increase? Also, what increases? Ohmic resistance or reactance? As far as I know, ohmic resistance does increase. $\endgroup$
    – MaDrung
    Commented Nov 13, 2017 at 6:53

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