Editing Synchronous motor (section)

===Amortisseur winding===
{{main|Damper winding}}
Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the breakdown load, the motor falls out of synchronization and the rotor no longer follows the rotating magnetic field.

Since the motor cannot produce torque if it falls out of synchronization, practical synchronous motors have a partial or complete squirrel-cage damper called an ''amortisseur'' winding to stabilize operation and facilitate starting.

Because this winding is smaller than that of an equivalent induction motor and can overheat on long operation, and because large slip-frequency voltages are induced in the rotor excitation winding, synchronous motor protection devices sense this condition and interrupt the power supply (out of step protection).<ref name="IEEE">IEEE Standard 141-1993 ''Recommended Practice for Electric Power Distribution for Industrial Plants'' pages 227-230</ref>