The three-phase synchronous motor is a distinctive and specialized motor. As the 
name suggests, this motor runs at a constant acceleration from no load to full load in synchronism with range frequency. As in squirrel-cage induction motors, the velocity of a synchronous engine is determined by the amount of pairs of poles and the collection frequency.
The operation of the three-phase synchronous motor can be summarized the following:
Three-stage AC voltage is greenhouse reducer applied to the stator windings and a rotating magnetic field can be produced.
DC voltage is applied to the rotor winding another 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 because of its excitation. As a result, the electric motor has zero slip compared to the induction engine, which requires slip in order to produce torque.
Synchronous motors aren’t self-starting and therefore require a approach to bringing the rotor up to near synchro nous speed before the rotor DC power is usually applied. Synchronous motors typically begin as a standard squirrel cage induction electric motor through use of particular rotor amortisseur windings. Also, there are two simple methods of providing excitation current to the rotor. One technique is by using an external DC source with current supplied to the windings through slip rings. The other technique is to really have the exciter installed on the normal shaft of the motor. This arrangement will not require the use of slip rings and brushes.
A power 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 factor of the inductive loads. An underexcited DC field will create a lagging power element and because of this 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 used for power aspect correction while at the same time carrying out a major function, such as working a compressor. If mechanical power output isn’t needed, however, or could be provided in other cost-effective methods, the synchronous machine remains useful as a “nonmotor” method of con trolling power factor. It does the same job as a bank of static capacitors. This kind of a machine is called a synchronous condenser or capacitor.
