In this Master´s thesis is told basic principles of synchronous generator and how different loads affects generators. This thesis studies the structure of stator, by telling the structure of the core iron and by explaining the meaning of the stator winding and its insulation. Rotating magnetized rotor generates three phase electricity to stator winding. Maintenance assures generators production to its whole lifecycle. In this thesis is told operations and inspection that should be done in generator overhaul. The purpose of this thesis is to create maintenance plan to power plant generator units and ensure units use without disruption.