a synchronous machine will generally be smaller than for an induction machine of similar rating. When the rotor is turning at the same speed as the stator's revolving magnetic field, no current is induced into the squirrel-cage windings and the windings will have no further effect on the operation of the synchronous motor at steady-state. The squirrel cage winding in some machines provides a damping effect for load or system disturbances, and in this role may be designated as an amortisseur windings. Large machines may only have amortisseur bars in the individual pole faces, not interconnected between poles. Because the squirrel cage winding is not large enough to dissipate the heat of continuous operation, large synchronous machines often have protective relays to detect when the machine has fallen out of synchronization with the supply voltage. Induction generators Three phase squirrel cage induction motors can also be used as generators. For this to work the motor must see a reactive load, and either be connected to a grid supply or an arrangement of capacitors to provide excitation current. For the motor to work as a generator instead of a motor the rotor must be spun faster than its stator's synchronous speed. This will cause the motor to generate power after building up its residual magnetism. See also AC motor Induction motor Mikhail Dolivo-Dobrovolsky References Induction motors