Since rotating magnetic fields are necessary for generation of electricity how an AC generator starts to produce electricity? is there a battery or AC supply externally to produce magnetic fields?
For the synchronous generators in large power stations, the machine rotor contains a winding that is externally energized by DC excitation. This ‘field’ winding is the electromagnet that rotates in the presence of the stator phase windings to generate AC.
On some machines, there is a small permanent magnet generator that powers the excitation circuit, through diodes and a control mechanism, which makes these generators capable of ‘black start’.
For induction generators, like what is used in the large wind turbines, you’ve noted a big problem. They require a connection to the AC power system for their excitation. If the grid voltage dips too much while they are operating, they will trip offline, and make a small grid problem even worse.
Latest update as of March 2008