The three-phase synchronous engine is a distinctive and specialized motor. As the name suggests, this motor operates at a constant rate from no load to full load in synchronism with series frequency. As in squirrel-cage induction motors, the velocity of a synchronous engine is determined by the number of pairs of poles and the line frequency.

The operation of the three-phase synchronous motor could be summarized as Screw Vacuum Pump follows:
Three-stage AC voltage is put on the stator windings and a rotating magnetic field is usually produced.
DC voltage is put on the rotor winding and a second magnetic field can be 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 for its excitation. As a result, the electric motor has zero slip compared to the induction 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 applied. Synchronous motors typically begin as a standard squirrel cage induction electric motor through use of special rotor amortisseur windings. Also, there are two simple methods of providing excitation current to the rotor. One technique is to use an external DC resource with current provided to the windings through slip rings. The other technique is to have the exciter mounted on the common shaft of the engine. This arrangement does not require the use of slip rings and brushes.

An electrical 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 factor, canceling out the lagging power element of the inductive loads. An underexcited DC field will produce a lagging power factor and because of this is seldom used. When the field is generally excited, the synchronous engine will operate at a unity power factor. Three-stage synchronous motors can be used for power factor correction while at the same time executing a major function, such as operating a compressor. If mechanical power output is not needed, however, or could be provided in additional cost-effective methods, the synchronous machine remains useful as a “nonmotor” means 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.