ALTERNATOR REGULATION AND OPERATION 



169 



give any indication as to polarity, as the indications of an 

 alternating-current voltmeter arc independent of its polarity. 



Lamps, however, can be used to determine the correct polarity. 

 Figure 168 shows the connections for phasing a three-phase alter- 

 nator with the bus-bars. A lamp is connected across each pole 

 of the three-pole switch which connects the machine to the lino. 

 The voltage rating of the lamps should be 15 per cent, greater 

 than that of the machine or line. For example, if the system is 

 olts, two 115-volt lamps in series may be used across each 

 pole, although these lamps will be subjected to overvoltage dur- 

 ing a part of the synchronizing period. If the machines are 



i * \ n * i 



O \ \o. Syncbronulng 



]T\ VJi Lnmpi 



A I torn alp r 



ions for " .''. dark" method of synchronizing with lamps. 



properly connected, the three lamps should all become bright and 

 dim together. If they brighten and grow dim in sequence, it 

 means that the phase rotation of the two machines is opposite, 

 so that one phase must I !. 



Th<- l.-imps flicker at :i frequency equal to the difference in the 

 frequencies of the t w<> machines. As the machines approach 

 synchronism l he flicker beOOmefl -lower and dower. When I he 

 lamp- are all dark the .-witch may he closed. The fact that tin- 

 lamps are all dark ind: .at the potential diffei 



