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

The operation of a typical three-phase synchronous motor can 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 and a second magnetic field is definitely 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 because of its excitation. Because of this, the electric motor has zero slip when compared to induction motor, which requires slip to be able to produce torque.
Synchronous motors aren’t self-starting and therefore need a method of bringing the rotor up to close to synchro nous speed prior to the rotor DC power is certainly used. Synchronous motors typically start as a standard squirrel cage induction engine through use of unique rotor amortisseur windings. Also, there are two fundamental methods of providing excitation current to the rotor. One technique is by using an external DC source with current provided to the windings through slide rings. The other method is to have the exciter installed on the common shaft of the electric motor. This arrangement will 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 create a leading power aspect, canceling out the lagging power aspect of the inductive loads. An underexcited DC field will produce a lagging power aspect and for this reason is seldom utilized. When the field is normally excited, the synchronous motor 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 working a compressor. If mechanical power output is not needed, however, or can be provided in various other cost-effective methods, the synchronous machine remains useful as a “nonmotor” means of con trolling power factor. It does the same work as a financial institution of static capacitors. This kind of a machine is called a synchronous condenser or capacitor.