AC GENERATOR THEORYAC GeneratorsThe frequency of the generated voltage is dependent on the number of field poles and the speedat which the generator is operated, as indicated in Equation (10-1).f = (10-1)NP120wheref = frequency (Hz)P = total number of polesN = rotor speed (rpm)120 = conversion from minutes to seconds and from poles to pole pairsThe 120 in Equation (10-1) is derived by multiplying the following conversion factors.x60 seconds1 minute2 polespole pairIn this manner, the units of frequency (hertz or cycles/sec.) are derived.LossesinanACGeneratorThe load current flows through the armature in all AC generators. Like any coil, the armaturehas some amount of resistance and inductive reactance. The combination of these make up whatis known as the internal resistance, which causes a loss in an AC generator. When the loadcurrent flows, a voltage drop is developed across the internal resistance. This voltage dropsubtracts from the output voltage and, therefore, represents generated voltage and power that islost and not available to the load. The voltage drop in an AC generator can be found usingEquation (10-2).(10-2)Voltage dropI_{a}R_{a}I_{a}X_{La}whereI_{a}= armature currentR_{a}= armature resistanceX_{La} = armature inductive reactanceES-10 Page 6 Rev. 0

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