Why a synchronous motor cann start by itself?
A synchronous motor may be started by a DC motor on a common shaft. When the motor isbrought to synchronous speed, AC current is applied to the stator windings. The DC motor nowacts as a DC generator and supplies DC field excitation to the rotor of the synchronous motor.The load may now be placed on the synchronous motor. Synchronous motors are more oftenstarted by means of a squirrel-cage winding embedded in the face of the rotor poles. The motoris then started as an induction motor and brought to ~95% of synchronous speed, at which timedirect current is applied, and the motor begins to pull into synchronism. The torque required to pull the motor into synchronism is called the pull-in torque. The synchronous motor rotor is locked into step with the rotating magneticfield and must continue to operate at synchronous speed for all loads. During no-load conditions,the center lines of a pole of the rotating magnetic field and the DC field pole coincide . As load is applied to the motor, th
