VOLTAGE REGULATION OF THE ALTERNATOR. 133 



The synchronous reactance curve is determined as follows : The 

 armature is short-circuited * through an ammeter, the machine is 

 driven at normal speed, the field excitation is adjusted until full- 

 load current (or any desired fraction of full-load current) flows 

 through the ammeter and the ammeter reading is taken. The 

 short-circuit is then opened, and, with unchanged speed and field 

 excitation, the voltage across the alternator terminals is measured. 

 The ammeter readings are then plotted as abscissas and the cor- 

 responding open-circuit voltages are plotted as ordinates. Figure 

 1 18 is the synchronous reactance curve of the 2,ooo-kilowatt, 



3-phase alternator which is referred to in connection with Fig. 117 

 above. In Fig. 1 1 8 the abscissas represent amperes in each 

 armature winding (ammeter readings divided by 1/3). 



The reason for calling this curve the synchronous reactance 

 curve is that the voltage corresponding to any given abscissa 



* In a polyphase armature all of the phases must be simultaneously short-circuited. 



