Synchronous motors. A synchronous engine is one in which the rotor normally rotates at the same rate as the revolving field in the machine. The stator is comparable to that of an induction machine consisting of a cylindrical iron body with windings, usually three-phase, positioned in slots around the inner periphery.
usually, synchronous motors are used for applications where precise and constant speed is required. Low power applications of the motors include positioning machines. These are also used in robot actuators. Ball mills, clocks, record gamer turntables also make use of synchronous motors.
The principle of operation of a synchronous engine can be understood by considering the stator windings to be linked to a three-phase alternating-current supply. The result of the stator current is definitely to establish a magnetic field rotating at 120 f/p Auto Chain revolutions per minute for a frequency of f hertz and for p poles. A primary current in a p-pole field winding on the rotor will also create a magnetic field rotating at rotor speed. If the rotor rate is made equal to that of the stator field and there is no load torque, these two magnetic fields will tend to align with one another. As mechanical load is definitely applied, the rotor slips back numerous degrees with regards to the rotating field of the stator, developing torque and ongoing to become drawn around by this rotating field. The position between the fields improves as load torque is certainly increased. The maximum available torque is achieved when the angle by which the rotor field lags the stator field is certainly 90°. App of more load torque will stall the motor.
One advantage of the synchronous motor can be 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 component of current in stage with the applied stator voltage-i.e., the electric motor can operate at unity power element. This problem minimizes the 
losses and heating in the stator windings.