The correct answer is: D. saturation of iron.
A shunt generator is a type of DC generator that uses a shunt field winding to create the magnetic field that induces an emf in the armature winding. The shunt field winding is connected in parallel with the armature winding, which means that the same current flows through both windings.
As the current through the shunt field winding increases, the magnetic field strength increases. This increase in magnetic field strength causes an increase in the emf induced in the armature winding. However, as the magnetic field strength increases, the iron core of the generator becomes saturated. This saturation limits the further increase in magnetic field strength, and therefore limits the further increase in emf induced in the armature winding.
The other options are incorrect because:
- Speed limitation: The speed of a shunt generator is not a factor that limits the voltage build-up. The voltage build-up is limited by the saturation of the iron core.
- Armature heating: Armature heating is a factor that can limit the output power of a generator, but it does not limit the voltage build-up.
- Insulation restrictions: Insulation restrictions are a factor that can limit the output current of a generator, but they do not limit the voltage build-up.