The three-phase synchronous electric motor is a unique and specialized motor. As the name suggests, this motor operates at a constant rate from no load to complete load in synchronism with series frequency. As in squirrel-cage induction motors, the rate of a synchronous engine is determined by the amount of pairs of poles and the range frequency.
The operation of a typical three-phase synchronous motor could be summarized as follows:
Three-phase AC voltage is put on the stator windings and a China Pulley rotating magnetic field is certainly produced.
DC voltage is applied to the rotor winding another magnetic field is 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. Because of this, the engine 
has zero slip when compared to induction engine, which requires slip to be able to produce torque.
Synchronous motors aren’t self-starting and therefore need a approach to bringing the rotor up to close to synchro nous speed prior to the rotor DC power is certainly applied. Synchronous motors typically start as a standard 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 to use an external DC source with current provided to the windings through slide rings. The other method is to have the exciter mounted on the normal shaft of the engine. This arrangement will not require the utilization of slip bands and brushes.
An electrical system’s lagging power factor could be corrected by overexciting the rotor of a synchronous electric motor operating within the same system. This will produce a leading power element, canceling out the lagging power element of the inductive loads. An underexcited DC field will create a lagging power element and because of this is seldom utilized. When the field is normally excited, the synchronous engine 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 for example operating a compressor. If mechanical power output isn’t 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 element. It can the same job as a financial institution of static capacitors. Such a machine is named a synchronous condenser or capacitor.