Internal emf in phase windings in ac generator is controlled by
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AC Generators AC GENERATOR OPERATIONIf DC field excitation is connectedFigure 6 Simple AC Generator - Rotating Field, StationaryArmatureto the rotor, the stationary coilswill have AC induced into them(Figure 6). This arrangement iscalled a rotating field, stationaryarmature AC generator.The rotating field, stationaryarmature type AC generator isused when large power generationis involved. In this type ofgenerator, a DC source is suppliedto the rotating field coils, whichproduces a magnetic field aroundthe rotating element. As the rotoris turned by the prime mover, thefield will cut the conductors of thestationary armature, and an EMFwill be induced into the armature windings.This type of AC generator has several advantages over the stationary field, rotating armature ACgenerator: (1) a load can be connected to the armature without moving contacts in the circuit;(2) it is much easier to insulate stator fields than rotating fields; and (3) much higher voltagesand currents can be generated.Three-Phase AC GeneratorsFigure 7 Stationary Armature 3f GeneratorThe principles of a three-phase generator are basicallythe same as that of a single-phase generator, exceptthat there are three equally-spaced windings and threeoutput voltages that are all 120° out of phase withone another. Physically adjacent loops (Figure 7) areseparated by 60° of rotation; however, the loops areconnected to the slip rings in such a manner thatthere are 120 electrical degrees between phases.The individual coils of each winding are combinedand represented as a single coil. The significance ofFigure 7 is that it shows that the three-phasegenerator has three separate armature windings thatare 120 electrical degrees out of phase