210 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



ampere-turns, for zero-load, A , and, therefore, the shunt and series 

 windings 



Let e and s' represent the voltage-values corresponding to these 

 two load-conditions, according to the excitation-curve. The lag- 

 lines corresponding to all the intermediate values are parallel, and are 

 comprised between two lines which are determined by Eq. (29) when the 

 right-hand member is made equal to A\ AQ and to A', respectively. 

 These lines pass through P and P' respectively. 



In order to obtain effective overcompounding, i.e., to have s' > e, it 

 is necessary that the point O should not be very far distant from P 

 on the right line of reference B Y. This means, simply, that it is nec- 

 essary to have either a powerful series-winding, or else a large reactive 

 current with zero-load. The first of these two ways appears to be the 



JL. 



Fig. 25. 



preferable one; but inasmuch as it is always desirable to increase the 

 inclination of the right line BY to the right in order to decrease s, it is 

 necessary to resort to both plans to obtain overcompounding. 



Regulation of Supply E.M.F. by Compounding of the Generator. 

 As already stated, it is possible, theoretically, to do away with the reac- 

 tive current at all loads by using, as the source of current-supply for the 

 converter, a suitably compounded alternator, giving an E.M.F. E, 

 which increases with the load in such a way that the point C is always 

 displaced on the right line of reference BY, while e remains constant 

 or else increases, according to the results desired. It is interesting to 

 know how the excitation should be effected in such a case. 



If it be required to have the direct-current voltage, e, constant, the 

 excitation will evidently be that of a simple shunt-winding; and its 

 value is easy to determine by the condition that it must give exactly the 

 voltage e, with zero-load, since there is not to be any reactive current. 



