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The synchronous motor is a type of alternating current motor. Like an induction motor, it has a stator and a rotor. Its stator winding closely resembles that of an induction motor, and it, too, receives AC power from the power source to drive the connected load.
Synchronous motors are available with various rotor designs to fit different applications. In one type, for example, the rotor is constructed somewhat like a squirrel-cage rotor. In addition to rotor bars, it has coil windings for providing direct-current (DC) excitation, as shown in Fig. 1. The coil windings are connected to an external DC power supply by slip rings and brushes. Like a squirrel-cage motor, a synchronous motor is started by applying AC power to the stator; however, DC power then is applied to the rotor coils after the motor reaches maximum speed. This produces a strong, constant magnetic field in the rotor, which locks in step with the rotating magnetic field of the stator. Because the rotor turns at the same speed as synchronous speed (speed of the rotating magnetic field), there is no slip. The speed of rotation of the motor is constant in a synchronous motor, and does not vary with load, as in an induction motor.
Synchronous motors are designed to operate at unity (1.0) power factor or 0.8 leading power factor. By varying the DC excitation of the motor, the power factor of the motor can be varied widely. Overexcited synchronous motors operate at leading power factor and provide reactive kVAR-like capacitors. This yields an improved power factor for the power-supply system. Because most utility companies bill their industrial customers on the basis of their kVAR use, rather than kW, an improved power factor provides large savings for the customer.
Synchronous motors initially were used as a way to raise the power factor of systems that have larger induction-motor loads; now, however, they are used because they can maintain the terminal voltage on a weak power system, are lower cost, and are more efficient than equivalently sized induction motors.
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