The three-phase synchronous electric motor is a distinctive and specialized motor. As the name suggests, this motor runs at a constant velocity from no load to complete load in synchronism with range frequency. As in squirrel-cage induction motors, the acceleration of a synchronous electric motor is determined by the amount of pairs of poles and the range frequency.
The operation of the three-phase synchronous motor could be summarized the following:
Three-stage AC voltage is applied to the stator windings and a rotating magnetic field is usually produced.
DC voltage is put on the rotor winding another magnetic field is produced.
The rotor then acts such as 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 engine has zero slip compared to the 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 near synchro nous speed prior to the rotor DC power is certainly applied. Synchronous motors typically start as a normal squirrel cage 
induction engine through use of particular rotor amortisseur windings. Also, there are two simple methods of providing excitation current to the rotor. One method is by using an external DC resource with current supplied to the windings through slip rings. The other technique is to really have the exciter mounted on the common shaft of the motor. This arrangement will not require the use of slip bands and brushes.
A power system’s lagging power factor could be corrected by overexciting the rotor of a synchronous engine 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 utilized. When the field is generally excited, the synchronous engine will run at a unity power aspect. Three-phase synchronous motors can be used 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, Torque Arm however, or can be provided in various other cost-effective methods, the synchronous machine continues to be useful as a “nonmotor” method of con trolling power factor. It does the same job as a bank of static capacitors. Such a machine is called a synchronous condenser or capacitor.