Consider an ideal transformer connected to a real AC voltage source (with output resistance Rg).
Now consider to apply the Thevenin theorem between the output terminals of the secondary circuit. I was told that, when we evaluate the Thevenin voltage, it is equal to Vg because:
The current in the secondary is 0 -> the current in the primary is 0 because of the transformer equation (Is = Ns/Np Ip), in which we put Ip = 0. So we get Vp = Vg and so Vthevenin= Vs = Vg.
I did not understand the physical meaning of that thing: if I have 0 current at the secondary, why should I have 0 current at the primary, which is a closed circuit? It is simply a normal circuit with an inductor.
Moreover, if it is true that Ip = 0, the transformer is off since there is not the magnetic coupling between the inductors: so how can we say that Vs = Vg?