The Operation Of The Synchronous Motor

       A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it. Synchronous motor can be contrasted with an induction motor, which must slip in order to produce torque. It operates synchronously with line frequency. It is available in sub-fractional self-excited size to high-horsepower direct-current excited industrial size.
       The operation of a synchronous motor is simple to imagine. The armature winding, when excited by a poly-phase (usually 3-phase) supply, creates a rotating magnetic field inside the motor. The field winding, which acts as a permanent magnet, simply locks in with the rotating magnetic field and rotates along with it. During operation, as the field locks in with the rotating magnetic field, the motor is said to be in synchronization.
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 i.e., the applied load is large enough to pull out the field winding from following the rotating magnetic field. The motor immediately stalls after it falls out of synchronization.
       The synchronous motor has the following advantages over non-synchronous motors:

1. Speed is independent of the load, provided an adequate field current is applied.
2. Accurate control in speed and position using open loop controls.
3. It will hold its position when a DC current is applied to both the stator and the rotor windings.
4. Its power factor can be adjusted to unity by using a proper field current relative to the load.
5. Its construction allows for increased electrical efficiency when a low speed is required.
6. It runs either at the synchronous speed or they do not run at all.