synchronous generator[′siŋ·krə·nəs ′jen·ə‚rād·ər]
a synchronous machine that operates as a generator. Synchronous generators are usually employed as sources of alternating current of a constant frequency. They are used, for example, in power stations, electrical installations, and transportation systems. Synchronous generators came into use in the 1870’s when P. N. Jablochkov (Iablochkov) invented the arc lamp known as the Jablochkov candle.
Synchronous generators are most often used to produce commercial-frequency current. The rotors of such machines are driven by steam or water turbines. Synchronous generators are also built that are driven by gas turbines, internal-combustion engines, wind engines, and electric motors.
The rotor windings of a synchronous generator are supplied with direct current from rectifying equipment or from a separate generator (seeELECTRICAL MACHINE EXCITER), which is usually mounted on the same shaft as the synchronous generator. When the rotor turns, its magnetic field induces an alternating electromotive force (emf) in the three-phase winding of the stator. The frequency of this emf is f = pn, where P is the number of pole pairs and n is the rotational speed of the rotor. Highspeed synchronous generators (steam- and gas-turbine generators) have a small number of pole pairs: p = 1 or 2. Low-speed generators (water-turbine generators) may have several tens of pole pairs. The magnitude of the emf is regulated by varying the current in the rotor winding.
In some low-power synchronous generators, the AC winding is placed on the rotor, and the field winding on the stator. A special group is formed by generators that are provided with a large number of pole pairs in order to obtain higher-frequency currents (seeELECTROMECHANICAL MEDIUM-FREQUENCY GENERATOR).