ALTERNATOR RE<H LAT1ON AND OPERATION 



167 



If the machines arc operating under load, 7 merely puts more 

 load on the machine which tends to lead and takes load .off the 

 machine which tends to lag. This last machine will not ordinarily 

 operate as a motor, as it did under no-load conditions, but as its 

 load is reduced its angular position will be advanced. 



Because 7 tends to hold the two machines in synchronism, it 

 is called the synchronizing current. 



It has already been stated that changing the field current does 

 not vary the (list rilmtion of load between two alternators. How- 

 ever, it does affect the current delivered by the two machines. 

 Figure 166 (a) shows the vector diagram for two similar alter- 

 - ha vimr a common terminal voltage V. Both machines are 

 delivering equal currents I\ and 1 2 respectively, which are in 



E, 



I,X< 



I i<,. 166. Vector diagram of currents and voltages when alternators operate 



in parallel. 



phase with the terminal voltage V. The resultant load current 



is their sum /', which is in phase with V. As both machines 



have equal resistances and reactances, their respective internal 



voltages Ei and E 2 are the same. (In this diagram the machines 



an treated with reference to the external circuit in which case 



the voltages and currents are acting in conjunction.) 



Let the field of generator 1 be weakened and that of 2 be 



-thened. It has already been shown that this cannot affect 



the division of the kilowatt load between the machines. When 

 the field of generator 1 is weakened, its internal volt a ut* decreases 



and when the Held of 2 b strengthened, its internal voltage 

 increases. Now both machines must continue to have equal 



terminal volt;iL r <-. It has ahv;idy been shown that if a machine 

 delivers a lending cm: internal voltage is less than when 



the machine delivers ,i current. (See par. 64, page 1-7 



Mori-over, a leading current in an alternator tends to streniit hen 

 the field and ;i lamriiitf cun ken I lie field, through 



arm.-ihire reaction. 



