Inducing Voltage in a Transformer: How Does It Work?

[Elquery]

TL;DR Summary

A changing current in a transformer primary produces a changing magnetic field, which induces a voltage in the secondary, but if no circuits are closed on the secondary, there's no current in the primary. How is there measurable voltage on the secondary?

A changing current in a transformer primary produces a changing magnetic field, which induces a voltage in the secondary (correct?), but if no circuits are closed on the secondary, there's no current in the secondary (and therefore primary as well). So how is this voltage induced?

 

[cabraham]

If the secondary is unloaded, the impedance is infinite in the ideal case, so that a secondary voltage & zero current occur.
In real world transformers, the secondary winding has a capacitance. This capacitance charges & discharges every ac half cycle. The secondary current is not zero, due to charging & discharge action.

 

[DaveE]

There is always some (often small) amount of primary current in a transformer. We call this the magnetizing current and it is essentially the same as if the transformer primary was a simple inductor. In fact most physics classes are taught with "coupled inductors" as opposed to "transformers". It is important to understand that a transformer is just an inductor with extra windings (the secondaries). You can see more of the details in this older post, although you may find it a bit complex. The point is that every transformer model should have an large inductor (the magnetizing inductance) shunting one of the windings. This models the impact of the magnetic core on the windings, this is the inductor that is coupled to the other windings.

 

[Baluncore]

A changing current in a transformer primary produces a changing magnetic field, which induces a voltage in the secondary (correct?), but if no circuits are closed on the secondary, there's no current in the secondary (and therefore primary as well). So how is this voltage induced?

The voltage in the secondary is induced independent of any secondary current to a load.

The primary is an inductor, so a reactive current flows that magnetises the core. That reactive current is in quadrature with the primary voltage and represents idle energy circulating in the supply and primary winding, not real power.

If a real resistive load is connected to the secondary, in-phase currents will flow in the secondary and the primary. Those in-phase currents represents real energy being transferred from the supply to the secondary load.