I've got an RLC meter which I can set so it's a fixed AC voltage amplitude source. I'm using a planar coil of 25 turns and in theory, when a secondary resistance increases, so should the primary. But this is not what I observe.
When the coil primary coil is isolated, the resistance is just the resistance of the wire, the reactance is high, so the current flowing is comparatively small.
But when I add a second planar coil on top of it, it decreases reactance, because the current in the secondary is flowing in such a way to create a magnetic field that is in the opposite direction from the primary, which reduces total magnetic field, reducing the reactance felt by the primary, which is why the current increases.
This is all fine, but I don't understand what is happening with resistance. Because it increases. And if I look at equations, it says that increase in secondary resistance should create an increase of primary resistance. Now if I add a resistor in series with the secondary coil, the reactance increases (less current is flowing which makes sense), but the problem is that resistance felt by the primary actually decreases. So adding resistance to the secondary actually DECREASES primary resistance felt, not increase it.
But in induction heating if we increase the temperature of the load (effectively increasing resistance) the resistance felt by the primary also increases. So why do 2 different experiments give 2 different results?
EDIT: I've done some more testing and found out that the resistance of the primary actually increases if I add resistors to the secondary to about the same resistance that the primary had on it's own. Any more resistance, and the primary resistance starts to fall. It has something to do with the power transfer but I don't know exactly how. And I'd like to find out.