The synchronous electric motors present in their design two types of winding, the three-phase winding in the stator and the winding with continuous current in the rotor. One method used to solve the problem of limiting the synchronous motor at start is to make the coupling next to the auxiliary motor and to turn off at 90% of the speed of the rotating field, since after that moment the synchronous motor will be able to reach the synchrony with the magnetic field in the stator.
To operate as an engine we have to apply a three-phase voltage to the stator, which will be responsible for creating a rotating field that holds at speed according to the number of poles of the winding and its frequency of feeding. In the rotor winding is concentrated a continuous resulting voltage, in order to generate a constant magnetic field that will accompany the rotating magnetic field. The synchronous motor is not able to reach the synchronous speed from rest under load without auxiliary methods for starting, since the two poles formed in the rotor are unable to keep up with the speed of the three-phase rotating magnetic field in the stator.