60 



SYNCHRONOUS MOTORS 



[Since there is no term of the first degree, these ellipses have the origin of the 

 co-ordinates for their center. If we take as ordinates the product Z7, the 

 inclination of their axes is constant and equal to 



ZI 



=tan45=i. 



Lj 



If, instead of the product (ZI), we take, as ordinates, the values of 7, the inclina- 

 tion of their axis becomes 



This inclination increases inversely as the reactance X, i.e., a given variation 

 of the exciting current produces a current-variation which is all the greater the 

 higher the inductance of the motor. This conclusion is supported by other con- 

 siderations, as we have already seen.] 



The conditions for which the current is a minimum are those for 

 which the phase is zero with respect to E\. They are obtained by tak- 

 ing, on the diagram of Fig. 27, all the values situated on the right 

 line A\N. For these points the triangle OA\A^ gives 



- 2E\ZI cos 7- 



