68 SYNCHRONOUS ALTERNATORS. [Exp. 



is used as load; with constant armature current, readings are 

 taken of terminal voltage for different field currents, and plotted 

 as in Fig. 2. 



For the first reading, adjust the field rheostat to its maximum 

 resistance ;* with field circuit open, reduce the load resistance to 

 zero (i. e., short-circuit the armature through the ammeter) ; 

 close the field circuit and adjust the field rheostat until the de- 

 sired value of armature current is obtained. For each succeed- 

 ing reading, increase the load resistance by a small step and re- 

 adjust the field rheostat until the desired value of armature 

 current is again obtained, taking care that the increase or de- 

 crease in excitation is continuous. 



10. In Fig. 2, the excitation data are as follows : 



A comparison of the no-load and full-load saturation curves, 

 Fig. 2, shows the following : 



At constant excitation, the difference in the ordinates of the 

 two curves (their distance apart vertically) shows the difference 

 in terminal voltage of the alternator at no load and at full load. 



At constant terminal voltage, the difference in the abscissae of 

 the two curves (their distance apart horizontally) shows the 

 difference in excitation (magnetomotive force) required at no 

 load and full load in order to maintain the voltage constant. 



At constant excitation, a voltage of 575 at full load increases 

 to 627 when the load is thrown off, giving a regulation of 9 per 



curves of Fig. 2. To take these is unnecessary, unless some special object 

 is in view. For inductive load, the full-load saturation curve will be lower 

 than with non-inductive load (as shown in Fig. I, Exp. 3-B, for zero power 

 factor). For different power factors, see 13, and take data as in 14. 



* This resistance should be sufficient to reduce the field current to but a 

 small fraction of its normal value. 



