THE SYNCHRONOUS MOTOR. 167 



\ \ 



J 1 



Fig. 142. 



82. To find value of B for which the machine B may take in the greatest 

 possible power from A ; A, X and R being given. Equation (i), Art. 80, 

 expresses the greatest intake of B for given values of A, B, X and R. It is 

 required to find the value of B which will make this greatest intake a maximum. 

 This value of B must render B 1 cos 6 AB [the numerator of right-hand member 

 of equation (i), Art. 80], a maximum. Differentiating this expression with respect 

 to B and placing the differential coefficient equal to zero we have 



2B cos A = O 

 or 



= 2~COS0 



Remark. A comparison of equations (i), Art. 81 and (i) of this article shows 

 that the value of B for greatest possible intake of machine B is half the greatest 

 value of B for which machine B can act as a motor at all. This is also the case 

 with a direct- current motor. The greatest electromotive force such a motor can have 

 is the electromotive force of the dynamo which drives it, and the value of its electro- 

 motive force to permit the greatest possible intake is one-half the electromotive force 

 of the dynamo which drives it. 



83. Excitation characteristics. With given load on a synchronous motor (given 

 value of / >// ) its electromotive force B may be changed by varying its field exci- 

 tation, and for each value of B there is a definite value of the current /. Thus the 

 abscissas of the curves, Fig. 143, represent values of /, and ordinates represent 



