why the stator frame of a 3-phase generator is made up of cast iron?
The stator frame supports the stator and the end brackets. It must be strong and accurately machined to that the stator is concentric with the rotor. It can be made of cast aluminum, cast iron or fabricated steel plates. The material is selected based on cost, heat transfer characteristics and manufacturing capability considering the size of the machine. Synchronous generators range in size from about 0.5 kW for a small automotive alternator or portable generator to nearly 2000 MW for large power station generators.

The armature of a synchronous generator is the stator. The armature core is made of silicon steel formulated for high magnetic permeability and low magnetic hysteresis. It is laminated to reduce eddy-current losses.

The field of a synchronous generator is mounted on the rotor. The field core needs to have low magnetic permeability, but it is provided with DC excitation, so hysteresis and eddy-currents are not as as much concern as in the armature. It may have either solid or laminated construction.