The three-phase synchronous electric motor is a unique and specialized motor. As the name suggests, this motor runs at a constant rate from no load to full load in synchronism with range frequency. As in squirrel-cage induction motors, the quickness of a synchronous electric motor is determined by the number of pairs of poles and the collection frequency.
The operation of the three-phase synchronous motor can be summarized the 
following:
Three-phase AC voltage is put on the stator windings and a rotating magnetic field is produced.
DC voltage is applied to the rotor winding another 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 for its excitation. As a result, the motor has zero slip compared to the induction electric 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 close to synchro nous speed before the rotor DC power is applied. Synchronous motors typically begin as a normal squirrel cage induction motor through use of unique rotor amortisseur windings. Also, there are two basic methods of providing excitation current to the rotor. One technique is to use an external DC source with current supplied to the windings through slip rings. The other method is to have the exciter mounted on the normal shaft of the engine. This arrangement does not require the use of slip bands and brushes.
An electrical system’s lagging power factor could be corrected by overexciting the rotor of a synchronous motor operating 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 factor and for this reason is seldom used. When the field is normally excited, the synchronous electric motor will run at a unity power element. Three-stage synchronous motors can be utilized for power aspect correction while at the same time performing a major function, such as for example working a compressor. If mechanical power output isn’t needed, however, or can be provided in various other cost-effective methods, the synchronous Gear machine continues to be useful as a “nonmotor” means of con trolling power element. It can the same work as a financial institution of static capacitors. Such a machine is called a synchronous condenser or capacitor.