Synchronous motors. A synchronous electric motor is one where the rotor normally rotates at the same rate as the revolving field in the machine. The stator is similar to that of an 
induction machine comprising a cylindrical iron body with windings, usually three-phase, located in slots around the internal periphery.
usually, synchronous motors are used for applications where precise and constant speed is necessary. Low power applications of these motors include positioning devices. These are also applied in robot actuators. Ball mills, clocks, record player turntables also make use of synchronous motors.
The principle of operation of a synchronous engine could be understood by taking into consideration the stator windings to be linked to a three-phase alternating-current supply. The result of the stator current can be to establish 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 swiftness. If the rotor acceleration 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 one another. As mechanical load is applied, the rotor slips back several degrees with respect to the rotating field of the stator, developing torque and ongoing to end up being drawn around by this rotating field. The angle between the fields raises as load torque can be increased. The maximum available torque is accomplished when the angle by which the rotor field lags the stator field can be 90°. App of more load torque will stall the engine.
One benefit of the synchronous motor is that the magnetic field of the machine can be produced by the immediate current in the field winding, so that the stator windings need to provide just a power Auto Chain component of current in stage with the applied stator voltage-i.e., the motor can operate at unity power factor. This condition minimizes the losses and heating in the stator windings.