148 



ELEMENTS OF ELECTRICAL ENGINEERING. 



of <=o or 360 in Fig. 127, P f and P" are both positive 

 and each is more than 1 500 kilowatts, although the machines to 

 which Figs. 127 and 128 refer, are each of about 100 kilowatts 

 rating. The excessive values of P 1 and P" in the neighbor- 





Fig. 129. 



hood of 



o or 360 is due to the fact that when < is zero 



each machine forms a short-circuit for the other, whereas in the 

 neighborhood of < = 180 the two electromotive forces A and 

 B oppose each other and the current is small. 



70. Necessity of exact synchronism for the operation of an 

 alternator as a motor. Suppose that machine B runs continu- 

 ously a little slower or a little faster than machine A, then the 

 value of the angle </> will change continuously, and the point 

 P, Figs. 125 and 126, will move steadily around the circle in 

 one direction or the other. Under these conditions the power 

 outputs P 1 and P" of the two machines pass repeatedly 

 through all of the values shown in Fig. 127, and, on the average, 



