3-A] CHARACTERISTICS. ^5 



4. Characteristics. Four characteristics are to be taken : 



One no-load characteristic the no-load saturation curve. 



Three load characteristics the external characteristic, the full- 

 load saturation curve, and the armature characteristic. 



These characteristics are similar to the corresponding char- 

 acteristics of a direct current generator, Exp. i-B. 



5. No-load Saturation Curve.* This curve shows the ter- 

 minal voltage for different values of field current, the machine 

 being driven at constant speedf without load. The connections 

 are shown in Fig. I. Data are taken 

 in the same way as for the no-load 

 saturation curve of a direct current 

 generator (5-10, Exp. i-B) ; the 

 alternator, however, is necessarily 

 separately excited. RHEOSTAT 



The field current can be variedj by FlG * ' Connections for n - 



load saturation curve. 

 a field rheostat in series, as in Fig. I 



(with a second rheostat in series with it to give greater range, 

 if necessary) ; or by an arrangement of resistances as in Figs. 

 4 and 5, of Exp. i-A. 



Voltage readings are corrected by proportion for any varia- 

 tion of speed, and plotted as in Fig. 2. Descending, as well as 

 ascending, values may be plotted when desired. The saturation 

 factor and per cent, saturation are determined as in Fig. 2, of 

 Exp. i-B. 



6. External Characteristic. This curve shows the variation 

 in terminal voltage with load. The alternator is driven at nor- 



* (Sa). If the alternator is motor-driven, it is commercial practice to 

 determine its core loss and friction at the same time that the no-load 

 saturation curve is taken. See 15. 



f (5b). Speed and frequency are proportional; with a good frequency 

 meter at hand, it may be more convenient to observe frequency than 

 speed. If the speed can be varied, note that voltage is proportional to 

 speed. 



$(5c). As in all such curves, the variation should be made continu- 

 ously and no back steps should be taken (7, Exp. i-A). 

 6 



