Under the load condition, the terminal voltage of alternator is less than the induced e.m.f. Eph. So if load is disconnected , Vph will change from Vph to Eph, if flux and speed is maintained constant. This is because when load is disconnected, Ia is zero hence there are no voltage drops and no armature flux to cause armature reaction. This change in the terminal voltage is significant in defining the voltage regulation.
Note : The voltage regulation of an alternator is defined as the change in its terminal voltage when full load is removed, keeping field excitation and speed constant, divided by the rated terminal voltage.,
So if Vph = Rated terminal voltage
Eph = No load induced e.m.f.
the voltage regulation is defined as,
The value of the regulation not only depends on the load current but also on the power factor of the load. For lagging and unity p.f. conditions there is always drop in the terminal voltage hence regulation values are always positive. While for leading capacitive load conditions, the terminal voltage increases as load current increases. Hence regulation is negative in such cases. The relationship between load current and the terminal voltage is called load characteristics of an alternator. Such load characteristics for various load power factor conditions are shown in Fig. 1.
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| Fig. 1 Load characteristics of an alternator |
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