3 phase synchronous motor

The three-phase synchronous motor is a distinctive and specialized motor. As the name suggests, this motor runs at a constant rate 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 number of pairs of poles and the series 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 produced.
DC voltage is applied to the rotor winding and a second magnetic field is 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 will not require the magnetic induction from the stator field for its excitation. As a result, the electric motor has zero slip when compared to induction engine, which requires slip to be able to produce torque.
Synchronous motors aren’t self-starting and therefore require a approach to bringing the rotor up to near synchro nous speed prior to the rotor DC power is applied. Synchronous motors typically start as a normal squirrel cage induction motor through use of unique rotor amortisseur windings. Also, there are two basic Taper Pulleys methods of offering excitation current to the rotor. One method is by using an external DC supply with current provided to the windings through slide rings. The other method is to have the exciter installed on the normal shaft of the electric motor. This arrangement does not require the use of slip bands 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 factor, canceling out the lagging power aspect of the inductive loads. An underexcited DC field will create a lagging power element and for this reason is seldom used. When the field is normally excited, the synchronous motor will run at a unity power factor. Three-stage synchronous motors can be used for power factor correction while at the same time carrying out a major function, such as operating a compressor. If mechanical power output isn’t needed, however, or can be provided in various other cost-effective methods, the synchronous machine continues to be useful as a “nonmotor” means of con trolling power aspect. It can the same job as a financial institution of static capacitors. This kind of a machine is called a synchronous condenser or capacitor.