The three-phase synchronous motor is a distinctive and specialized electric motor. As the name suggests, this motor operates at a constant rate from no load to complete load in synchronism with range 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 could be summarized the following:
Three-phase AC voltage is applied to 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 certainly produced.
The rotor then acts 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 for its excitation. As a result, the electric motor has zero slip when compared to induction motor, which requires slip to be able to produce torque.
Synchronous motors aren’t self-starting and therefore require a approach to bringing the rotor up to near synchro nous speed prior to the rotor DC power is certainly applied. Synchronous motors typically start as a normal squirrel cage induction electric motor through use of special rotor amortisseur windings. Also, there are two basic methods of offering excitation current to the rotor. One method is by using an external DC supply 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 arrangement does not require the use of slip rings and brushes.
A power system’s lagging power factor can be corrected by overexciting the rotor of a synchronous engine operating Water-lubricated Air Compressors within the same system. This will create a leading power aspect, canceling out the lagging power element of the inductive loads. An underexcited DC field will produce a lagging power element and for this reason is seldom used. When the field is normally excited, the synchronous motor will run at a unity power factor. Three-stage synchronous motors can be utilized for power aspect correction while at the same time carrying out a major function, such as operating a compressor. 
If mechanical power output isn’t needed, however, or can be provided in various other cost-effective methods, the synchronous machine remains useful as a “nonmotor” means of con trolling power element. It does the same job as a financial institution of static capacitors. This kind of a machine is called a synchronous condenser or capacitor.