180 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



We also find (from the triangle CBH, in Fig. 3), 



Z cos 7-= /?= resistance, 

 Z sin 7-= X= reactance. 



Substituting these values in the last term, in (i), we have 



... (2) 



This equation enables the reactive current, I d , to be calculated, 

 as a function of the load, i.e., of the active current /,; because, on 

 solving with respect to ZI d , we have 



sn 



cos r =o. 



Solving this quadratic equation for I d , we have 



=^ -csi 



(3) 



This expression will be put into another form later. 



Application of the Diagram. Representation of Converter 

 Operation with Constant Potential at Primary Terminals and at 

 Brushes. If all that is desired is to maintain the potential constant 

 at the terminals of the con- 

 verter, the segment OB= e 

 remains constant, and, in 

 order to predetermine the 

 different conditions of 

 operation it is sufficient 

 to let the point C, which 

 defines the conditions of 

 operation, describe a circle 

 around O as a center, 

 with the constant radius 

 OCE. For each posi- 

 tion of C the load is FIG. 4. 

 measured by dD and the 



reactive current by CD. It is thus seen, in Fig. 4, that the reactive 

 current, whicn is positive for light loads (since the characteristic 

 point C is to the right of the line BY), diminishes as the load 

 increases from the maximum value F (secondary load alone) to zero, 

 at G; and it then changes in sign, and begins again to increase. 



