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I think the whole question lies on the title. I can't understand how the material put inside the coil affects the total current drawn.

muyustan
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    Just think of it as a transformer. Do you know how a transformer core heats up? Or how a transformer core affects the frequency response of the transformer (thereby affecting the current drawn)? Same thing. It's because transformer cores aren't perfect and have currents circulating inside them due to the changing magnetic field around them. – DKNguyen Oct 29 '19 at 18:11
  • Are you using a commercial induction heater or home built? – Marla Oct 29 '19 at 20:15
  • @Marla I don't have one, just watched some videos and in all of them the current was increasing when the material is placed into the coil. – muyustan Oct 29 '19 at 20:17

2 Answers2

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Changing the current in an inductor causes a changing magnetic field around the inductor. The changing magnetic field induces a voltage opposing the changing current in the inductor, thus limiting the peak AC current. Place certain materials inside the inductor, and the changing magnetic field will induce eddy currents in that material which will reduce (rob from) the magnetic flux, which will decrease the back EMF on the inductor coil, thus allowing the current in the inductor to change more easily up to a higher AC maximum.

The resistive losses of the eddy currents will both heat something up, and allow the eddy currents to continue “robbing” even more of the energy from the magnetic field, further reducing back EMF on the coil, which then can allow even more current to flow.

hotpaw2
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  • thanks for the answer, but when I think like the wat below, it gets complicated. When I place a core as a replacement to air, then the inductance of the coil definetely increases(due to higher permeability). Considering the phasor resistance of the inductor is jwL, the resistance of the coil/inductor should also be increased resulting in a decrease in the current. However, it is not the case. – muyustan Oct 29 '19 at 20:57
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    Yes, the reactance of the inductor can increase, but the reduction in reverse EMF can increase even more. – hotpaw2 Oct 29 '19 at 21:50
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With no material on the inside, the coil is basically an air-core inductor, but when the material is placed inside the coil, there are induced eddy currents in the material, just as in the secondary coil of a transformer. You can view the coil with no material as a transformer with the secondary open (or missing) and when you place the conductive material inside, the analogy is a transformer with a shorted secondary. The resistivity of the material combined with the current causes the heating.

John Birckhead
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