The three-phase synchronous engine is a unique and specialized engine. As the name suggests, this motor operates at a constant velocity from no load to complete load in synchronism with line frequency. As in squirrel-cage induction motors, the quickness of a synchronous motor is determined by the amount of pairs of poles and the collection frequency.
The operation of a typical three-phase synchronous motor can be summarized as follows:
Three-phase AC voltage is put on the stator windings and a rotating magnetic field is certainly produced.
DC voltage is applied to the rotor winding and a second magnetic field is usually 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 will not require the magnetic induction from the stator field because of its excitation. Consequently, the engine has zero slip compared to the induction motor, which requires slip to be able to produce torque.
Synchronous motors aren't self-starting and therefore need a method of bringing the rotor up to near synchro nous speed before the 
rotor DC power is applied. Synchronous motors typically begin as a standard squirrel cage induction motor through use of unique rotor amortisseur windings. Also, there are two simple methods of offering excitation current to the rotor. One method is to use an external DC source with current provided to the windings through slip rings. The other technique is to really have the exciter installed on the common shaft of the electric motor. This arrangement does not require the usage of slip rings and brushes.
An electrical system's lagging power factor can be corrected by overexciting the rotor of a synchronous motor operating within the same system. This will produce a leading power aspect, canceling out the lagging power aspect of the inductive loads. An underexcited DC field will produce a lagging power element and because of this is Sprocket seldom used. When the field is generally excited, the synchronous engine will operate at a unity power factor. Three-phase synchronous motors can be used for power factor correction while at the same time carrying out a major function, such as working a compressor. If mechanical power output is not needed, however, or could be provided in various other cost-effective methods, 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.