Generators are intended to operate with balanced three-phase loading, but exposure to unbalanced currents is inescapable. Unbalances arise from many sources: untransposed transmission line construction, unbalanced loads, faults and open phases. These unbalances appear as negative sequence current in the generator leads. By definition, negative-sequence quantities have a rotation opposite that of the power system. This reversed rotating stator current induces double frequency currents in rotor structures. The resulting heating can damage the rotor very quickly.