· A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current.

· The rotation period is exactly equal to an integral number of AC cycles.

· Synchronous motors contain multiphase AC electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current.

· The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field of any AC motor.

· A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.

· The synchronous motor and induction motor are the most widely used types of AC motor.

· The difference between the two types is that the synchronous motor rotates at a rate locked to the line frequency since it does not rely on current induction to produce the rotor's magnetic field.

· The induction motor requires slip: the rotor must rotate slightly slower than the AC alternations in order to induce current in the rotor winding.

· Small synchronous motors are used in timing applications such as in synchronous clocks, timers in appliances, and precision servomechanisms in which the motor must operate at a precise speed; speed accuracy is that of the power line frequency, which is carefully controlled in large interconnected grid systems.

· Synchronous motors are available in self-excited sub-fractional horsepower sizes to high power industrial sizes.

· In the fractional horsepower range, most synchronous motors are used where precise constant speed is required.

· These machines are commonly used in analog electric clocks, timers and other devices where correct time is required.

· In higher power industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting AC energy to work. Second, it can operate at leading or unity power factor and thereby providing power-factor correction.