132 



ELEMENTS OF ELECTRICAL ENGINEERING. 



machines, however, this direct method is not usually feasible and 

 it is therefore customary to calculate the voltage regulation of 

 large machines in terms of certain easily obtained data. The data 

 which are used for this purpose are included in what are called 

 the saturation curve and the synchronous reactance curve of the 

 machine. 



The saturation curve. The saturation curve of an alternator 

 is a curve of which the abscissas represent the values of the field 

 current and the ordinates represent the corresponding observed 

 values of the terminal voltage of the machine at zero load. Thus, 

 Fig. 117 shows the saturation curve of a certain 2,000 kilowatt, 



A-connected, 3 -phase alternator. The ordinates of this curve 

 represent the observed voltages across any two collector rings 

 of the machine, and the abscissas represent the corresponding 

 values of field current in amperes. 



