Synchronous motors. A synchronous engine is one where the rotor normally rotates at the same acceleration as the revolving field in the machine. The stator is comparable to that of an induction machine consisting of a cylindrical iron frame with windings, generally three-phase, situated in slots around the internal periphery.
usually, synchronous motors are used for applications where precise and continuous speed is required. Low power applications of the motors include positioning devices. These are also used in robot actuators. Ball mills, clocks, record player turntables also utilize synchronous motors.
The principle of operation of a synchronous electric motor can be understood by considering the stator Leaf Chain windings to be connected to a three-phase alternating-current supply. The effect of the stator current can be to determine a magnetic field rotating at 120 f/p revolutions per minute for a frequency of f hertz and for p poles. A direct current in a p-pole field winding on the rotor will also create a magnetic field rotating at rotor acceleration. If the rotor velocity is made equal to that of the stator field and there is absolutely no load torque, these 
two magnetic fields will tend to align with each other. As mechanical load is usually used, the rotor slips back a number of degrees with respect to the rotating field of the stator, developing torque and continuing to become drawn around by this rotating field. The position between the fields improves as load torque is definitely increased. The utmost available torque is attained when the angle where the rotor field lags the stator field is 90°. Application of more load torque will stall the electric motor.
One advantage of the synchronous electric motor is definitely that the magnetic field of the machine can be made by the immediate current in the field winding, so that the stator windings have to provide only a power component of current in phase with the applied stator voltage-i.e., the electric motor can operate at unity power factor. This problem minimizes the losses and heating system in the stator windings.