Electrical Engineering ⇒ Topic : Open-Circuit Characteristics (O.C.C.)
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Erick
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The circuit diagram for determining the open circuit characteristic (0.C.C.) of a dc shunt generator is shown in Figure (a). The circuit is very similar to the one shown in Figure, and the motor circuit is not shown here since it is the same as the one given in Figure. In Figure (a), an additional voltmeter is connected across the field winding. The procedure for determining the
FIGURE (a) Circuit for deternnining the open-circuit characteristic of a shunt generator O.C.C. is exactly the same as the one described in Sec. 9.9.1.1 for a separately excited generator. The generator is run at its rated speed and the field current If is increased from zero value to a definite value in steps. At each value of If, the readings of the two voltmeters Vf and V1 are noted down.The field current is increased until the voltmeter V1 reads at least 25% morethan the rated voltage. After measuring the maximum value, the field current is now decreased in steps and the corresponding readings of the voltmeter V1 are again noted down. It may be noted that the voltmeter V1 reads higher values for decreasing values of the field current due to the hysteresis of the generator core of the field system. The average of the readings of the voltmeter V1 corresponding to the ascending and descending values of field current are taken for drawing the open circuit characteristic as shown in Figure (b).
FIGURE (b) Open-circuit characteristic (0.C.C.) of a shunt generator | |
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Maninder
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Internal and External characteristics (or load characteristics). Fig. (a) Note. The great advantage of separate excitation over all other forms of excitation is that the current is entimly independent of the load current in the armature. It is, however, rather inconvenient to have to depend upon a separate source of supply and, therefore, the method is used only in special cases, where the generator has lo operate over a wide range of terminal voltage. Figure (a) Open-circuit characteristic of a separately excited generator | |
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