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A synchronous generator is called “synchronous” because the waveform of the generated voltage is synchronized with the rotation of the generator. Each peak of the sinusoidal waveform corresponds to a physical position of the rotor. The frequency is exactly determined by the formula f = RPM x p /120 where f is the frequency (Hz), RPM is the rotor speed (revolutions per minute) and p is the number of poles formed by the stator windings. A synchronous generator is essentially the same machine as a synchronous motor. The magnetic field of the rotor is supplied by direct current or permanent magnets. The output frequency of an asynchronous generator is slightly (usually about2 or3%) lower than the frequency calculated from f = RPM x p /120. If the RPM is held constant, the frequency varies depending on the power level. The peaks of the waveform have no fixed relationship with the rotor position. An asynchronous generator is essentially the same machine as an asynchronous or induction motor. The magnetic field of the rotor is supplied by the stator through electromagnetic induction. The output frequency of a synchronous generator can be more easily regulated to remain at a constant value. Synchronous generators (large ones at least) are more efficient than asynchronous generators. Synchronous generators can more easily accommodate load power factor variations. Synchronous generators can be started by supplying the rotor field excitation from a battery. Permanent magnet synchronous generators require no rotor field excitation. The construction of asynchronous generators is less complicated than the construction of synchronous generators. Asynchronous generators require no brushes and thus no brush maintenance. Asynchronous generators require relatively complicated electronic controllers. They are usually not started without an energized connection to an electric power grid, unless they are designed to work with a battery bank energy storage system. With an asynchronous generator and an electronic controller, the speed of the generator can be allowed to vary with the speed of the wind. The cost and performance of such a system is generally more attractive than the alternative systems using a synchronous generator
Well I have read all the answers ! very good opinions by all experts , But i will add an answer , if someone is being asked this question in a paper or interview , for students and fresh engineers :
Synchronous Generator : in this type the rotor speed is just equal to the flux produce by the stator .
Asynchronous or Induction : in this type Rotor speed is not equal the Flux produced by the stator .
Asynchronous machines act like a donkey following a carrot, there's a phase shift between the speed of rotor and the speed of the rotating magnetic field. In synchronous machines, there's no phase shift, the speed of the rotor is called synchronous speed.
synchronous Gen. no a phase shift between the speed of rotor and the speed of the rotating magnetic field, although it in Asynchronous Gen.