THE ALTERNATING CURRENT GENERATOR 271 



If the prime mover of No. 2 is given more steam it 

 will of course try to speed up generator No. 2. But this can 

 occur for only a small fraction of a second, until No. 2 has 

 pulled slightly ahead of No. 1 in phase; the two machines 

 must turn the same number of revolutions per minute 

 (same number of poles on both machines assumed) because 

 they are operating in synchronism with each other, so that 

 the increased speed of No. 2 can only last during that small 

 time while No. 2 is pulling ahead of No. 1 in phase. 



After No. 2 has pulled ahead in phase its e.m.f. vector 

 is represented in Fig. 166 by OEz', which is ahead of its 

 original phase by the angle a. Now the resultant of OE\ 



i 

 \ 



FIG. 166. Vector Diagram to Show Load Division. 



and OE2 is not zero but OR. This voltage acting in the 

 local circuit will force a current through the two armatures 

 and this current is shown in Fig. 166 by 01, lagging behind 

 OR by the angle 0, the angle being the same as it was in 

 Fig. 165. 



The current 01 is nearly in phase with OE2 and nearly 

 180 out of phase with OEi. The result of this current is to 

 add to the load No. 2 was already carrying, a load represented 

 by the current 01, and to decrease the load No. 1 was carrying 

 by the same amount. 



The Load Varies Directly with the Phase Shift. The 

 value of OR (hence of 01) depends upon the value of the 

 angle a (Fig. 166) and this angle depends upon the torque 

 of the prime mover of machine No. 2. To change the load 

 on one of the alternators from zero to full load it is only 



