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

The operation of a typical three-phase synchronous motor could be summarized as follows:
Three-stage AC voltage is applied to the stator windings and a rotating magnetic field is definitely produced.
DC voltage is put on the rotor winding another magnetic field can be 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 does not require the magnetic induction from the stator field for its excitation. As a result, the engine has zero slip compared to the induction electric motor, 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 normal squirrel cage induction electric motor through use of unique rotor amortisseur windings. Also, there are two fundamental methods of providing excitation current to the rotor. One technique is to use an external DC source with current provided to the windings through slide rings. The other method is to have the exciter mounted on the normal shaft of the engine. This arrangement will not require the usage of slip rings and brushes.

A power system’s lagging power Rotary Piston Vacuum Pump factor can be corrected by overexciting the rotor of a synchronous motor operating within the same system. This will produce a leading power element, 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 utilized. When the field is generally excited, the synchronous engine will operate at a unity power aspect. Three-stage synchronous motors can be used for power factor correction while at exactly the same time executing a major function, such as for example operating a compressor. If mechanical power output is not needed, however, or can be provided in various other cost-effective ways, the synchronous machine remains useful as a “nonmotor” method of con trolling power factor. It does the same work as a bank of static capacitors. This kind of a machine is named a synchronous condenser or capacitor.