168 ALTERNATING CURRENTS 



For generator 1 to operate with a reduced internal voltage it 

 must deliver a leading current, making E\, shown in Fig. 166 

 (6), less than its previous value shown in Fig. 166 (a). On the 

 other hand E 2 , Fig. 166 (b), is greater in magnitude than in Fig. 

 166 (a) because generator 2 now delivers a lagging current. 

 Also the leading current in generator 1 tends to strengthen its 

 field and the lagging current in generator 2 tends to weaken its 

 field, through armature reaction. In both cases, the change of 

 flux produced by change in field current is opposed by armature 

 reaction. The load current /' cannot change in phase or in mag- 

 nitude, as the phase and magnitude of /' is determined entirely 

 by the character of the load which is connected to the system. 

 Therefore, since Ii and 1 2 are equal, they must make equal angles 

 with V so that their resultant /' will still lie along V. 



It will also be observed that each machine is carrying a larger 

 current than it did before and yet the kilowatt output of each 

 has not changed. This means that the heating (PR) loss in 

 each machine has been increased without any compensating ad- 

 vantages. Therefore, this is not the best condition of operation. 

 Figure 167 shows the diagram of Fig. 166 (b) with the voltage 

 drops eliminated. E Q is now the difference of EI and E^, and /o, 

 the circulating current, lags E by nearly 90 as in Fig. 165. It 

 will be observed that /o is nearly in quadrature with the terminal 

 voltage V so that it transfers practically no power from one 



machine to the other. This 

 substantiates what has al- 

 ready been demonstrated, 

 that changing the field cur- 

 rent cannot transfer appre- 

 FIG. 167. Vector diagram showing ciable load from one machine 



effect of excitation upon alternator circu- 4- Q fUg other 

 latory current. 



74. Synchronizing. Before 



direct-current generators can be safely thrown in parallel, two 

 conditions must be fulfilled. The two terminal voltages must be 

 equal, or substantially so, and the proper polarity must be observed. 

 These same two conditions must be fulfilled when alternators 

 are connected in parallel. The equality of voltages can be readily 

 determined by connecting a voltmeter first to one machine and 

 then to the other. The voltmeter, when so connected, does not 



