The three-phase synchronous electric motor is a distinctive and specialized motor. As the name suggests, this motor operates at a constant velocity from no load to complete load in synchronism with line frequency. As in squirrel-cage induction motors, the acceleration of a synchronous motor is determined by the number of pairs of poles and the line 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 can be produced.
DC voltage is put on 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 will not require the magnetic induction from the stator field because of its excitation. Because of this, the motor has zero slip compared to the induction electric motor, which requires slip to be able to produce torque.
Synchronous motors are not 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 definitely used. Synchronous motors typically begin as a Sprial Gear Reducer standard squirrel cage induction engine through use of special 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 slide rings. The other technique is to have the exciter installed on the common shaft of the motor. This arrangement will 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 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 create a lagging power factor and for this reason is seldom utilized. When the field is generally excited, the synchronous motor will operate at a unity power aspect. Three-stage synchronous motors can be used for power element 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 various other cost-effective methods, the synchronous machine continues to be useful as a “nonmotor” method of con trolling power aspect. It does the same job as a financial institution of static capacitors. This kind of a machine is named a synchronous condenser or capacitor.