The three-phase synchronous motor is a unique and specialized motor. As the name suggests, this motor runs at a constant swiftness from no load to complete load in synchronism with collection frequency. As in squirrel-cage induction motors, the quickness of a synchronous motor is determined by the number of pairs of poles and the series frequency.
The operation of a typical three-phase synchronous motor can be summarized the following:
Three-stage AC voltage is applied to the stator windings and a rotating magnetic field is certainly produced.
DC voltage is put on the rotor winding and a second magnetic field can be produced.
The rotor then acts just like a magnet and is attracted by the rotating stator field.
This attraction exerts a torque on the rotor and causes it to rotate at the synchronous speed of the rotating stator field.
The rotor does not require the magnetic induction from the stator field because of its excitation. As a result, the electric motor has zero slip when compared to induction engine, which requires slip to be able to produce torque.
Synchronous motors are not self-starting and therefore require a method of bringing the rotor up to near synchro nous speed prior to the rotor DC power is usually used. Synchronous motors typically begin as a normal squirrel cage induction electric motor through use of particular rotor amortisseur windings. Also, there are two basic methods of offering excitation current to the rotor. One method is by using an external DC source with current provided to the windings through slide rings. The other technique is to have the exciter installed on the normal shaft of the electric motor. This greenhouse motor arrangement will not require the use of slip rings and brushes.
An electrical system’s lagging power factor can be corrected by overexciting the rotor of a synchronous engine operating within the same system. This will create a leading power factor, canceling out the lagging power factor of the inductive loads. An underexcited DC field will create a lagging power factor and for this reason is seldom used. When the field is normally excited, the synchronous engine will run at a unity power aspect. Three-phase synchronous motors can be utilized for power element correction while at exactly the same time carrying out a major function, such as for example operating a compressor. If mechanical power output is not needed, however, or could be provided 
in additional cost-effective ways, the synchronous machine continues to be useful as a “nonmotor” means of con trolling power factor. It can the same work as a financial institution of static capacitors. Such a machine is named a synchronous condenser or capacitor.