An alternator is a unit or electrical rotating machine by which the mechanical energies (of coal, oil, gas etc.) may be converted into the electrical form. Alternating e.m.f.s. can be induced by simple coils while they rotate in magnetic field. Alternating current is taken out from stator conductors to an external circuit for transmission & distribution.
Alternators can be divided into three types according to the type of drive or prime mover. These are:
(i) Direct Engine-driven (Diesel engines) alternators operate at very low speed. A flywheel is provided with the engine to obtain constant speed
(ii) Steam turbine driven alternators generally run at high speed (750 rpm to 3600 rpm). This type of alternator is of horizontal construction.
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(iii) Water turbine driven alternators run at low speed. These units may be designed in both vertical and horizontal types.
(iv) Nuclear Power Station. Conventional Nuclear Power Station is similar to thermal power station. In place of furnace and steam boiler in thermal power station atomic reactor and heat exchangers are provided.