AC GENERATOR OPERATIONAC GeneratorsParallelingACGeneratorsMost electrical power grids and distribution systems have more than one AC generator operatingat one time. Normally, two or more generators are operated in parallel in order to increase theavailable power. Three conditions must be met prior to paralleling (or synchronizing) ACgenerators.Their terminal voltages must be equal. If the voltages of the two AC generators are notequal, one of the AC generators could be picked up as a reactive load to the other ACgenerator. This causes high currents to be exchanged between the two machines, possiblycausing generator or distribution system damage.Their frequencies must be equal. A mismatch in frequencies of the two AC generatorswill cause the generator with the lower frequency to be picked up as a load on the othergenerator (a condition referred to as "motoring"). This can cause an overload in thegenerators and the distribution system.Their output voltages must be in phase. A mismatch in the phases will cause largeopposing voltages to be developed. The worst case mismatch would be 180° out ofphase, resulting in an opposing voltage between the two generators of twice the outputvoltage. This high voltage can cause damage to the generators and distribution systemdue to high currents.During paralleling operations, voltages of the two generators that are to be paralleled areindicated through the use of voltmeters. Frequency matching is accomplished through the useof output frequency meters. Phase matching is accomplished through the use of a synchroscope,a device that senses the two frequencies and gives an indication of phase differences and arelative comparison of frequency differences.TypesofACGeneratorsFigure 5 Stationary Field, Rotating ArmatureAC GeneratorAs previously discussed, there are two types ofAC generators: the stationary field, rotatingarmature; and the rotating field, stationaryarmature.Small AC generators usually have a stationaryfield and a rotating armature (Figure 5). Oneimportant disadvantage to this arrangement is thatthe slip-ring and brush assembly is in series withthe load circuits and, because of worn or dirtycomponents, may interrupt the flow of current.ES-10 Page 10 Rev. 0
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