. 1 ///'A: K\.\Ti> If ItM.'l'LAT/ON AND (>1'KKATH>\ 



L65 



Ko.S 



characteristics of tho prime movers must be changed. If the 

 alternators are driven by shunt motors, the speed-load character- 

 of the motors may be changed by adjusting the motor 

 field rheostats. It will be noted, in Fig. 163, that the loads of 

 the two machines are equal at one frequency only. 



If the prime movers had flat 

 :-load characteristics, tho 

 operation of the alternators would 

 l>e unstable. That is, very small 

 disturbances or changes of fre- 

 quency would cause very large 

 fluctuations in the kilowatt load 

 delivered by each machine. This 

 condition would result in serious 

 operating difficulties. 



It has been shown that direct- 

 current shunt generators operating FIO. 1 63. Speed-load curves of 

 in parallel are in stable equilibrium alternators in parallel effect of 



changing governor control. 



Vol. 1, page 373, par. 235). 



That is, any circumstance which tends to throw machines 

 out of parallel is counteracted by reactions opposing this 

 tendency. In the same way, any action tending to throw 

 alternators out of parallel is opposed by reactions which 

 tend to prevent the alternators pulling out. This is most 

 clearly illustrated by the conditions existing when neither 

 alternator is supplying external load. If the two alternators am 

 considered as a local series circuit, their voltages are in opposition. 



voltages are represented in Fig. 164 by E\ and / 

 tively. K\ and A'- : are equal and opposite, so that the net 



Load- 



164. Synchronizing urn-nf >f ftltofUaton in parallel. 



ge acting in the local circuit of the t wo alternate 



lore, there i> no current flowing between thealtera 



just as theiv is no current circulating between two lotteries 



baving equal emfs. and connected with terminals of like polarity 



ner. 



