THE SYNCHRONOUS MOTOR. 



147 



The variation of P ff with </> may be shown in greater detail 

 as follows : Draw a line ss f through 0, Fig. 125, at right angles 

 to ef t as shown in Fig. 1 29. Consider that if machine A runs 

 momentarily faster than B or, in other words, if machine B 

 runs momentarily slower than A, then the angle (f> increases, 

 and the point P, Fig. 1 29, moves around the circle in the direc- 

 tion of the curved arrow. So long as OQ is towards f the 



\ 



00 



60 



ZOO 



100 



zo% 



-100 



-200 



A =. i, 100 volts. ^ = l.oo ohm. 



B = 1,000 volts. uL =0.58 ohm. 



Fig. 128. 



value of P" is positive and machine B gives out power ; when 

 the point P, Fig. 129, is at s or s f , however, the value of OQ 

 is zero and machine B neither gives out nor takes in power ; 

 and when the point P is anywhere in the region sMs* the value 

 of OQ is negative, and machine B takes in power. This re- 

 gion in which OQ, Fig. 129, is negative is the region in which 

 P" is negative in Figs. 127 and 128. 



It is to be particularly noted that when the value of (/> is 

 nearly zero, that is, when the electromotive forces A and B are 

 nearly in phase with each other, the power output of both ma- 

 chines is excessively large. Thus, in the neighborhood of 



