Experiment_12_Alternator_Power - Experiment 12 Alternator...

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12 1 Experiment 12 Alternator Power OBJECTIVE To observe the effect of DC excitation upon the power delivered by an alternator. To observe the effect of power delivered by an alternator upon the toque of the prime mover. DISCUSSION Apart from portable or mobile engine driven AC alternator operating in remote areas or for emergency use, most AC generators feed into large electrical distribution networks where the frequency and voltage have been established by other generators operating in the system. Alternators are then said to feed into an “infinite bus”, meaning literally, a large electrical distribution system of tremendous power. The existing frequency and voltage of this infinite bus cannot be altered by the addition of any incoming alternator. The incoming alternator will have a constant flux in its air-gap because of the fixed frequency and voltage of the “infinite bus” it is connected to. The flux is normally produced by the DC rotor current and/or the AC currents in the stator. Should the DC current be less than that needed to produce the required flux, then the stator must supply the difference by drawing lagging reactive power from the infinite bus. Conversely, if the DC motor current is larger than required, the stator will draw leading reactive power from the infinite bus, and the alternator looks like a capacitor. Changing the DC excitation of an alternator that is “tied” into an infinite bus can only cause it to exchange more or less reactive power with the infinite bus. An alternator can only deliver active power (watts) to an infinite bus by forcing its motor to move ahead of its normal no-load position. Mechanical torque must be exercised to attain and keep this advanced rotor position. The torque multiplied by the speed is a measure of the mechanical power which the alternator receives and, therefore, the electric power it delivers. It is clear then, that the prime mover must apply torque to the rotor. The more torque applied the
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Experiment_12_Alternator_Power - Experiment 12 Alternator...

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