198 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



turns because, in general, the series-winding must work concordantly 

 (or cumulatively) with the shunt-winding, as will be seen later. 



The unknown constants, r and , of the windings, are easily deter- 

 mined when the E.M.F., SQ for zero-load, and the E.M.F. e m for the 

 mean load (corresponding to the current I m , with zero-lag) are known. 



The excitation-curve (Fig. 15) gives the ampere-turns, A , A nt , 

 corresponding, respectively, to the E.M.F. 's SQ and e m . For the 

 normal load-condition we will have 



jo) 



mm m 



S' r V 2 



For the no-load condition we will have 



KN'. 



........ (22) 



\ 2 



As already seen, the value of the reactive current corresponding 

 to zero-load cannot fall below a certain minimum. If a minimum 

 value be assumed for this current, Eq. (22) can be used to determine 

 the conductance of one turn of the shunt winding. From Eq. (22), 



KN'i \ . . . 



(23) 



solving for , we have 

 r 



From Eq. (21), solving for n, and substituting for the value given 



r 



in Eq. (23), we have 



4 ^ m I A ^"* A I*/ I ^"IfATV 



A m r- (A m A )\ / 2-\ KN'tQ 



$r / \ Q I Q 



=-T77 rrV 2 =^ ' -^ . . (24) 



nk(I m -jo) xk(I m -jo) 



It is seen that the shunt ampere-turns, which are proportional to , 



are less than the total ampere-turns AQ (on the assumption that 

 >M> o)) and that they decrease when the reactive current IQ increases; 

 whereas the series ampere-turns, which are proportional to n(I n >jo) 

 increase with the reactive current i$. 



Determination of Reactive Current as a Function of the Excita- 

 tion, when the Active Current is Constant, then when the Power 



