292 



SYNCHRONOUS MOTORS. 



alternator voltages and the local current circulating in the 

 circuit joining them. The current supplied to the external 

 circuit will consequently not be shown on the diagram, 

 and will only affect it by introducing an increased voltage 

 drop in the armature voltage of the machines. 



Effect of Variation of Excitation of Synchronous Motor. 

 We are now in a position to consider somewhat more in 

 detail the causes of the special form of the curves shown in 

 Fig. 130. 



If we neglect the losses in the machine, the curves in 

 Fig. 130 show that for a given load there are two values of 

 the excitation, for which the load will be overcome with a 

 certain current in the armature. The two points correspond- 

 ing to a fixed armature current and load indicate that the 

 resultant voltage in the circuit is the same for both points, 

 because the resultant voltage determines the value of the 

 current. Further, the value of the power supplied to the 

 motor must be equal in both cases, and consequently the 

 angle of phase difference between current and applied voltage 

 is the same for both points. The difference between the 

 points is due to the fact that in one case the current is in 

 advance of the applied voltage, while in the other case it lags 

 behind it by an equal angle. 



Fig. 139 is drawn to show that these conditions can be 

 produced by variation of the motor excitation. Thus for a 

 certain excitation the phase relations of the circuit are shown 



FIG. 139. 



Fl(i. 140. 



EFFECT OF VARIATION OF EXCITATION IN CASE OF Two SYNCHRONOUS MOTORS 



WITH DIFFERENT ARMATURE SELF-INDUCTION. 



