A.C Generators are usually called Alternators. They are also called Synchronous Generators. Rotating machines that rotate at a speed fixed by the supply frequency and the number of poles are called Synchronous Machine.
An alternating voltage is generated in a single conductor or an armature coil rotating in a uniform magnetic field with stationary field poles. The voltage is also generated in stationary armature conductors when the field poles rotate the conductor. The voltage will be generated in the armature conductors as long as there is a relative motion between the armature conductors and the field flux.
In DC generators, the field poles are stationary, and the armature conductors rotate. The voltage generated in the armature conductors is of the alternating nature. This generated alternating voltage is converted to a direct voltage at the brushes with the help of the commutator.
A Synchronous generator is a machine for converting mechanical power from a prime mover to AC electric power at a specific voltage and frequency. A synchronous machine rotates at a constant speed called synchronous speed. A synchronous motor is a synchronous machine that converts electric power into mechanical power.
Synchronous generators are usually of 3 phase type because of the several advantages of the 3 phase power generation, transmission and distribution. Large synchronous generators are used to generate bulk power at thermal, hydro and nuclear power stations. Synchronous generators with power ratings of several hundred MVA are very commonly used in the generating stations.
The biggest size of synchronous generators used in India has a rating of 500 MVA. They are used in super thermal power stations. Synchronous generators are the primary sources of the world’s electric power system. For the power generation in bulk, stator windings of the synchronous generators are designed for voltage ranging from 6.6 kV to 33 kV.