Editing Electric generator (section)

===Self-excitation===
{{main|Excitation (magnetic)}}
As the requirements for larger scale power generation increased, a new limitation rose: the magnetic fields available from permanent magnets. Diverting a small amount of the power generated by the generator to an electromagnetic [[field coil]] mounted on the rotor allowed the generator to produce substantially more power. This concept was dubbed [[self-excitation]].

The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of [[remanent magnetism]] present in the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field which generates a larger armature current. This "bootstrap" process continues until the magnetic field in the core levels off due to [[saturation (magnetic)|saturation]] and the generator reaches a steady state power output.

Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger. In the event of a severe widespread [[power outage]] where [[islanding]] of power stations has occurred, the stations may need to perform a [[black start]] to excite the fields of their largest generators, in order to restore customer power service.