SYNCHRONIZATION 141 



is first started, the e.m.f. in the secondary 89 is very feeble and of low 

 triMjuency, and the lamp L is barely aglow. As the amount and 

 frequency of the e.m.f. in Sa gradually increase, very rapid fluctuations 

 in the light of the lamp become noticeable, and as synchronous speed 

 is approached, the frequency of the fluctuations is reduced, the lamp 

 slowly passing from a brilliant glow to complete darkness, and then 

 slowly lighting up again. These fluctuations are easily explained by 

 the fact that when the frequency of the e.m.f.'s in Si and S a is nearly, 

 but not quite, the same, one of the e.m.f. vectors (in the vector 

 diagram of e.m.f.'s) slowly gains on the other, gradually passing from 

 coincidence of phase to phase opposition, and then again to coincidence 

 of phase, and so on. The resultant e.m.f., represented by the diagonal 

 of the parallelogram constructed on the two e.m.f. vectors as sides, 

 will thus slowly fluctuate between a maximum and a zero value. 

 When the pulsations are as slow as it is practicable to make them, 

 and the lamp is at its maximum brilliancy, the main switch is 

 closed. 



The lamp L forms a convenient visual signal for the engine-driver. 

 In order to enable the switchboard attendant to ascertain more 

 accurately the correct moment of closing the switch, a synchronizing 

 voltmeter is frequently provided in addition to the lamp, the two 

 being connected in parallel ; the switch is closed when the voltmeter 

 gives its maximum reading. 



In Fig. 104 the primary PI of the synchronizing transformer is 

 shown permanently connected across the terminals of the incoming 

 machine. In a large generating station it would not, of course, be 

 necessary to provide a synchronizing transformer for each machine, 

 as the same synchronizer could be used for several machines in 

 succession as required, the transformer being switched off as soon as 

 the main switch has been closed. In order to enable the same 

 synchronizing transformer to be used for all the generators, a set of 

 " synchronizer 'bus-bars " is frequently provided. 



It is evident that instead of using two distinct transformers as in 

 Fig. 104, the primaries PI and P a may be wound on a common core, 

 the secondaries Si and S a becoming a single secondary winding. The 

 arrangement is thereby rendered cheaper and more compact. This 

 is the usual practice when the simple form of synchronizer just 

 described is employed. 



Such a synchronizer might also be used in connection with three- 

 phase machines, the primary PI of the synchronizing transformer 

 being connected across two of the 'bus bars, and the primary P a across 

 the two corresponding terminals of the incoming machine. 



A synchronizer of this form, although indicating any difference of 

 frequency or phase, is incapable of showing whether the incoming 

 alternator is running above or below the speed of synchronism. 



