Electric generators are made up of a rotor spinning in a magnetic field created by an arrangement of either permanent magnets or electromagnets (field coils).. With permanent-magnet generators (PMGs), excitation - where a magnetic field is generated by an electric current - is provided by permanent magnets, so-called because once magnetised, they retain a persistent magnetic field.
In PMGs, the rotor structure can be built around a ring of magnetic materials with magnets made of rare-earth-based alloys such as neodymium-boron-iron or the more expensive samarium-cobalt..
