202 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



These ampere-turns are determined for each value of the alternat- 

 ing current / by measuring on the diagram (Fig. 13), the correspond- 

 ing segment CD, which indicates the reactive current 7<f, and subtract- 

 ing from this value the ampere-turns corresponding to zero-load which 

 ran give the E.M.F. e and the magnetizing ampere-turns produced 

 by the armature, namely, 



A i KN/ 



A a =( = 



\ V 2 



using the + or the sign according to whether /</ is positive or 

 negative, i.e., according to whether the point C be at the right or at 

 the left of D. 



FIG. 19. 



Taking the values of the excitation A , thus determined, as abscissas, 

 and the corresponding values of / as ordinates, we obtain a V-curve 

 such as shown in Fig. 19, and of which the " bend " corresponds to 

 the load-condition for which there is no lag of the current. If the cor- 

 responding values of E are also plotted as ordinates, a second V-curve 

 will be obtained, whose bend is much farther to the right, so that, 

 in the useful portion of the diagram, it consists only of the descending 

 portion of the curve. These curves would be prolonged to the left 

 if we were to consider negative excitations. 



A similar pair f V-curves will be obtained for each value of the 

 power-output of the converter. 



Upper Limit of Reactive Current. It has already been seen that 

 there is an inferior limit to the reactive current. It also has an upper 

 limit, according to Eq. (23), namely, that value which makes the shunt- 



