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

The operation of a typical three-phase synchronous motor can be summarized as follows:

• Three-phase 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 does not require the magnetic induction from the stator field for its excitation. As a result, the motor has zero slip compared to the induction motor, which requires slip in order to produce torque.

Synchronous motors are not self-starting and therefore require a method of bringing the rotor up to near synchro nous speed before the rotor DC power is applied. Synchronous motors typically start as a normal squirrel cage induction motor through use of special rotor amortisseur windings. Also, there are two basic methods of providing excitation current to the rotor. One method is to use an external DC source with current supplied to the windings through slip rings. The other method is to have the exciter mounted on the common shaft of the 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 motor operating within the same system. This will produce a leading power factor, canceling out the lagging power factor of the inductive loads. An underexcited DC field will produce a lagging power factor and for this reason is seldom used. When the field is normally excited, the synchronous motor will run at a unity power factor. Three-phase synchronous motors can be used for power factor correction while at the same time performing a major function, such as operating a compressor. If mechanical power output is not needed, however, or can be provided in other cost-effective ways, 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. Such a machine is called a synchronous condenser or capacitor.