My experience with generators tell me that the impedance of the coil will work as a voltage drop source and today I understood it is the same as the leakage inductance of a transformer

Basically the effect is that a greater current at the load reduces the voltage at the secondary

The lower is the coupling the greater will be the leakage inductance... also resistance of the coils limits theirs coupling

Graphs Can be plot with the voltage as function of load and find the leakage

Transformers are normally better coupled than generators so in generators I noticed this effect before

https://en.m.wikipedia.org/wiki/Leakage_inductance