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alternating-current generator[¦ȯl·tər‚nād·iŋ ¦kər·ənt ′jen·ə‚rād·ər]
a machine for converting mechanical rotational energy into AC electric power. A distinction is made between synchronous and asynchronous generators. The asynchronous type, generally used only in independent electrical supply systems, had been almost entirely replaced by synchronous generators by the 1970’s.
Three-phase AC generators are the most common; single-phase AC generators are not widely used because their design and operating qualities are considerably inferior to the three-phase type. High-output AC generators are used in electric power plants. AC generators with relatively low output are used in independent power-supply systems and in frequency converters.
A machine that converts mechanical power into alternating-current electric power. Almost all electric power is produced by alternating-current (ac) generators that are driven by rotating prime movers. Most of the prime movers are steam turbines whose thermal energy comes from steam generators that use either fossil or nuclear fuel. Combustion turbines are often used for the smaller units and in cases where gas or oil is the available fuel. Where water power is available from dams, hydroelectric ac generators are powered by hydraulic turbines. Small sites may also use diesel or gasoline engines to drive the generator, but these units are usually used only for standby generation or to provide electric power in remote areas. See Diesel engine, Gas turbine, Hydraulic turbine, Hydroelectric generator, Internal combustion engine, Steam electric generator, Steam turbine
Alternating-current generators are used instead of direct-current (dc) generators because ac power can easily be stepped up in voltage, by using transformers, for more efficient transmission of power over long distances and in larger amounts. Similar transformers step the voltage down again at the utilization site to levels that are safer and more convenient for general use. See Direct-current generator, Electric power systems, Transformer
Most ac generators are synchronous machines, that is, the rotor is driven at a speed that is exactly related to the rated frequency of the ac network. Generators of this type have a stationary armature with three windings that are displaced at regular intervals around the machine to produce three-phase voltages. These machines also have a field winding that is attached to the rotor. This winding provides magnetic flux that crosses the air gap and links the stator coils to produce a voltage according to Faraday's law. The field winding is supplied with direct current, usually through slip rings. See Armature, Slip rings, Windings in electric machinery
Induction generators, based on the principle of the induction motor, have been used in a few remote applications where maintenance of the excitation system is a problem. These units are essentially like induction motors, but are driven by a prime mover at speeds slightly above synchronous speed, forcing the unit to generate power due to the reverse slip. The units draw reactive power from the system and are not as efficient as synchronous generators. See Induction motor
High-frequency single-phase generators have been built as induction alternators, usually with twice as many stator poles (teeth) as rotor poles, and with a constant air-gap flux supplied from a homopolar field coil in the center of the machine, pushing flux into the stator at one end and out at the other. Their effectiveness is lower than that of the synchronous machine because the flux is a pulsating unidirectional field, rather than an alternating field. See Alternating current, Electric rotating machinery, Generator