ALTEK.\'.\rnK HKcri.ATION AND OPERATION 



137 



(See Fig. 138.) When the current is in phase with tin 

 voltage, the armature innif. is along A 2 , because the terminal 

 voltage lags the induced emf. by an angle a. (See Fig. 1 1"> (6).) 

 If the current lags the terminal voltage by degrees, its 

 innif. must act along A, 6 degrees behind A 2 . Combining 

 the mint'.. -1, with the impressed mmf., FI, gives the resultant 

 innif., F. 



Under the usual conditions of operation, the power-factor is 

 neither unity nor zero. Hence, the armature reaction may 

 strengthen the field or may weaken it, according as the current 

 leads or lags, and at the same time may distort it. 



64. Armature Impedance Drop. In a direct -current generator, 

 the induced armature voltage is obtained by adding numtricallu 

 the Il-i drop in the armature and the terminal voltage. In the 

 alternator, the armature reactance drop as well as the armature 



(A) 



Fio. 144. Alternator vector diagram for unity power-factor. 



drop must be added to the terminal voltage in order to 

 obtain the induced armature voltage. These voltage drops must 

 be added vector fall if to the terminal voltage, in order to obtain the 

 induced electromotive force. That is, the emf. induced in an 

 alternator armature is the terminal voltage plus the armature 

 lance drop, this addition being performed vectorially. 



// Tirmimil \'<>lt<i<H. Figure 144 (a) sh>\\- 



nditinii- exiMinsi when the load power- factor is unity. V 



tar terminal Voltage and / is the armature runvni 



in phase with F. The //,' drop in the armature is in pha-e with 



;! /. /,' beiO ' he armature. 



Th<- 7.V drop lead- lip' current by ( .)(f and i< laid off at the end 



'ini iif these t wn give* the // drop in the 



armature. Tin- .mpedance drop when added vectorially to he 

 terminal voltage V give- th. /.'' induced in 



