The three-phase synchronous motor is a distinctive and specialized engine. As the name suggests, this motor operates at a constant swiftness from no load to complete load in synchronism with series frequency. As in squirrel-cage induction motors, the velocity of a synchronous motor is determined by the amount of pairs of poles and the range 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 definitely 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 for its excitation. Consequently, the engine has zero slip compared to the induction motor, which requires slip in order to produce torque.
Synchronous motors are not self-starting and therefore need a method of bringing the rotor up to close to timing pulley synchro nous speed before the rotor DC power is usually applied. Synchronous motors typically start as a standard squirrel cage induction motor through use of particular rotor amortisseur windings. Also, there are two fundamental methods of providing excitation current to the rotor. One method is by using an external DC resource with current supplied to the windings through slide rings. The other technique is to have the exciter mounted on the common shaft of the electric motor. This arrangement does not require the use of slip rings and brushes.

An electrical system’s lagging power factor can be corrected by overexciting the rotor of a synchronous engine operating within the same system. This will produce a leading power factor, canceling out the lagging power aspect of the inductive loads. An underexcited DC field will produce a lagging power element and for this reason is seldom utilized. When the field is generally excited, the synchronous motor will run at a unity power aspect. Three-phase synchronous motors can be utilized for power factor correction while at exactly the same time carrying out a major function, such as for example working a compressor. If mechanical power output is not needed, however, or can be provided in other cost-effective methods, the synchronous machine remains useful as a “nonmotor” means 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.