ALTERNATORS. 227 



(a) Determination of Characteristic Curve on Inductive Loads. 



An inspection of the characteristic of the alternator 

 on non-inductive load showed that four readings taken 

 at about 4, 8, 12, and 16 amps, respectively, would be 

 sufficient to enable the curves for other power factors to be 

 plotted. 



Accordingly, a series of readings, each corresponding to 

 a fixed value of the current, but each being taken with 

 a different proportion of inductive and non-inductive 

 resistance in the circuit, was taken. 



The four series of readings were then plotted with voltage 

 horizontal and power-factor vertical, as shown in Fig. 105. 

 The actual values of the currents, for which these curves 

 were drawn, were respectively 4-17, 8-28, 12-8, and 17-2, as 

 these currents corresponded to exact readings on the 

 ammeter of 4, 8, 12, and 16 amps., the exact value being sub- 

 sequently obtained from the calibration curve of the instru- 

 ment. From the curves obtained in this manner, the lower 

 characteristics shown in Fig. 107 for power-factors of -9, -8, 

 and -6, were plotted, each curve in Fig. 105 giving a single 

 point on each characteristic. The general similarity between 

 the curves in Fig. 104, and those drawn for a similar purpose 

 in Fig. 96, comparing power-factor and excitation, will be 

 noticed. 



This was to be expected, since the variation in exciting 

 current to maintain a given voltage must bear a fairly constant 

 ratio to the drop of voltage, with a constant exciting current. 

 The curves are, consequently, very similar in their significance. 



(b) Determination of Characteristic Curve on Capacity Loads. 



The characteristic curves obtained in Fig. 107, which lie 

 above the characteristic for non-inductive load, were obtained 

 with currents leading instead of lagging behind the voltage. 

 Such curves may be obtained by choosing as load a circuit 

 possessing considerable capacity, such as a long concentric 

 cable or a number of condensers. The curves shown were 

 determined in a manner which is usually more convenient 

 than this, viz., by connecting the alternator to a syn- 

 chronous motor, the fields of which were excited so that the 

 motor voltage was higher than the alternator voltage. It 

 will be explained later in the section devoted to synchronous 

 motors that over-excited synchronous motors produce a 

 leading current hi the circuit supplying them, and thus 

 have the same effect as condensers or cables with capacity. 



