Synchronous motor operation

The squirrel-cage Amortisseur winding in the rotor produces Starting Torque and Accelerating Torque to bring the synchronous motor up to speed. ?
? When the motor speed reaches approximately 97% of nameplate RPM, the DC field current is applied to the rotor producing Pull-in Torque and the rotor will pull-in -step and “synchronize” with the rotating flux field in the stator. The motor will run at synchronous speed and produce Synchronous Torque.
? After synchronization, the Pull-out Torque cannot be exceeded or the motor will pull out-of-step. Occasionally, if the overload is momentary, the motor will “slip-a-pole” and resynchronize. Pull-out protection must be provided otherwise the motor will run as an induction motor drawing high current with the possibility of severe motor damage.
? The rotation of a synchronous motor is established by the phase sequence of the three-phase AC applied to the motor stator. As with a three-phase induction motor, synchronous motor rotation is changed by reversing any two stator leads. Rotor polarity has no effect on rotation. ?
? Synchronous motors are often direct-coupled to the load and may share a common shaft and bearings with the load. ?
? Large synchronous motors are usually started across the line. Occasionally, reduced voltage starting methods, such as autotransformer or part-winding starting, may be employed. ?