THREE-PHASE SYNCHRONIZER 147 



were running at a speed either slightly below or slightly above 

 synchronism, the lamps would periodically and simultaneously brighten 

 up and grow dark. 



Let us next suppose that one lamp is connected between A and 

 A', another between B and C', and a third between C and B', as 

 shown in Fig. 109. For the sake of simplicity, we may suppose the 

 neutral points and 0' of the two secondaries connected together. 

 Let in each case the instantaneous value of the e.m.f. be reckoned 

 positive when acting away from the neutral point. Then for any 

 phase relation of the secondary e.m.f.'s, the e.m.f. acting around the 



Fio 109. Three-phase Synchronizer. 



entire circuit which contains any lamp is clearly the vectorial 

 difference of the e.m.f.'s in the two phases of the star-connected 

 windings on either side of the lamp ; thus, the e.m.f. acting around 

 OAA'0'0, the circuit which contains the lamp LI, is the vectorial 

 difference of the e.m.f.'s in OA and O'A' ; similarly for the remaining 

 lamps La and La- Consider now the instant at which all the e.m.f.'s 

 are in phase, as shown by the vector diagrams (a) and (b) in Fig. 110. 

 The vector difference of OA and O'A' is clearly zero, so that LI is 

 dark. The vectorial difference of OB and O'C' is OLa, as shown 

 in the vector diagram (c), and the vectorial difference of 00 and O'B' 

 is OLa i n diagram (d). It is thus evident that since OLj = OLa, the 

 lamps La and La will be partly incandesced. Thus at the proper 

 instant for closing the switch L! is dark, while La and LS are equally 

 bright, but duller than when fully incandesced. 



Let us now consider the changes which take place when the 

 incoming machine is running below the speed of synchronism, so 

 that the three vectors of the diagram (6), Mg. 110, gradually lag 

 behind those of diagram (a). It is evident that as the lag increases* 



* The vectors are supposed, as usual, to revolve counter-cJoektoite, so that a lag of (6) 

 relatively to (a) implies u clockwise rotation of (6) relatively to (a). 



