ALTERNATING-CURREM 7.VN7 'AT MKXTS 71 



of phase difference of their currents, the instrument indicates 

 correctly. 



If the angle between the two coils MM' be made 120, as 

 shown in Fig. 67, the instrument can be made to indicate three- 

 phase power-factor, if the system is balanced. Non-inductive 



ances R, R are now connected in series with each of the 

 moving coils. The fixed coil is connected in one line of the thror- 

 phase system and the common terminal of the two moving coils 

 connects to this same line. The other terminal of each of the 

 moving coils connects to one of the other two lines of the three- 

 phase system, as shown in Fig. 67. This is the scheme of con- 

 nections for the power-factor indicator of the General Electric 

 Co., so often soon on switchboards. The instrument indicates 

 the three-phase power-factor if the system is very nearly bal- 

 anced. If the system is unbalanced, the reading has little 

 significance. 



40. Synchroscope. Before connecting an alternator to the 

 bus-bars and in parallel with other alternators, it is necessary not 

 only that its voltage be the same as that of the bus-bars but that 

 it be in phase opposition as well. This corresponds to having 

 direct-current generators of the same polarity before connecting 

 them in parallel. 



A synchroscope is an instrument for indicating when machines 

 are in the proper phase relation for connecting in parallel, and 

 at the same time for showing whether the incoming machine 

 is running fast or slow. This type of instrument is based on 

 the principle of the power-factor indicator. A diagram of one 

 type of -ynchroscope is shown in Fig. 08. A horse-shoe magnet ic 

 circuit is excited by a winding which conned- to the incoming 

 machine, usually through a potential transformer. Tin- moving 

 coiU are the same as i hose of the Tuma phase-meter, except that 



onnection- are made through slip-rings. This allows the 

 coiK to revolve freely. The moving element is connected I 

 the bus-ban, u-ually through potential transformers. If the 

 incoming machine has the >ame frequency as the bu<-bai>. 

 tin- pointer remains Stationary. When the machines are in the 

 proper phase relation for closing the switch, the poi- 

 index nn the di: 1. thi- position being shown in Fig. 0'.). The 

 direction of rotation of the pointer shows whether the incoming 



