284 ELECTRICAL ENGINEERING 



The component of /' in phase with E r , that is, the power com- 

 ponent does not represent a power transfer, but the reactive com- 

 ponent, which is in quadrature behind E' } transfers the power 

 required to keep the machines in step. This lagging component 

 of current is proportional to the reactance of the two armatures 

 and therefore a fairly high synchronous reactance or armature 

 reaction is required in alternators and synchronous motors to 

 give a good synchronizing power. If the synchronous reactances 

 are too large the circulating current will be limited and the syn- 

 chronizing power reduced. 



Take the case where one machine falls behind the other in 

 phase by angle a = 20; the circulating current is 



, _ ^.ou^y _ ai68 , 



V (n + r,y + ( Xl + x*)* V (n + rtf + ( Xl + x 2 ) 2 



O TyT 



but , l is the average short-circuit current of 



the two alternators with full-field excitation, and this is about six 

 times the average full-load current; thus the circulating current is 



/' = 0.168 X 6 X full-load current 

 = full-load current, approximately. 



These power cross currents, if of great magnitude, tend to 

 tear the machines out of synchronism and also produce fluctua- 

 tions of the voltage. 



If the speed characteristics of the prime movers are not the same 

 and the speed of one machine tends to fall below the other as the 

 load on the system is increased, then the machine driven by the 

 prime mover of closer speed regulation takes more than its share of 

 the load and so relieves the other machine and keeps its speed up. 



Thus to insure a proper division of the load between alter- 

 nators operating in parallel, it is necessary that their prime 

 movers shall have similar speed characteristics, that is, that their 

 speed shall fall under load by the same amount and in the same 

 manner. It is therefore preferable that the prime movers should 

 have drooping speed characteristics. 



The voltage characteristics of the alternators have no effect on 

 the division of the load, but they do affect the amount of the 

 wattless cross currents between the machines. 



173. Effect of Difference of Wave Form. If two machines in 

 parallel are adjusted to give the same effective value of voltage 



