<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2010) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2010) (Learn how and when to remove this template message) </Td> </Tr> <P> An electric generator or electric motor consists of a rotor spinning in a magnetic field . The magnetic field may be produced by permanent magnets or by field coils . In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor . The process of generating a magnetic field by means of an electric current is called excitation . </P> <P> For a machine using field coils, which is most large generators, the field current must be supplied, otherwise the generator will be useless . Thus it is important to have a reliable supply . Although the output of a generator can be used once it starts up, it is also critical to be able to start the generators reliably . In any case, it is important to be able to control the field since this will maintain the system voltage . </P>

Magnetic field in a dc generator is produced by