FIELD-EXCITATION OF ROTARY CONVERTERS 205 



Therefore, to each value of s there corresponds a right line PQ, which 

 is more or less distant from the point d; but all these right lines are 

 parallel to each other, having the same angular coefficient, p, which 

 depends on the windings only. The smaller the number of turns 

 of the series-winding, the farther, evidently, the point Q will be from 

 the point d, on the right line BY. When there is no series- winding 

 (in the case of a shunt-wound rotary converter), the right line PQ 

 becomes a right line parallel to QY, drawn through P. Conversely, 

 the decrease of the number of turns in the armature displaces the point 

 P toward the right, on the perpendicular dP. 



Finally, increasing or decreasing the shunt ampere-turns decreases 

 or increases the difference A Ad, and, consequently, brings the 

 right line PQ nearer to or farther from the point d. This line passes 

 through the point d when Ad=A. 



Effective Characteristic of Rotary Converter under Load. 

 Following the methods just described, it is easy to study, on the diagram, 



a' 



o'~'o & B l "--1^ 



/ ?*> 



FIG. 21. 



the variations of voltage at the terminals of the converter, as a function 

 of its current-output. It is only necessary to reverse the problem 

 and to find the output corresponding to each voltage, within the limits 

 of variation prescribed or allowed for the voltage. 



Let e, e', e", . . . represent a series of values of the voltage e. Let 

 these values be represented, in Fig. 21, by the corresponding distances 

 BO, BO', BO" ---- From the points O, O', O", ... let circular arcs 

 be described, with a radius equal to E, if the supply E.M.F. is constant, 

 or equal to E, E', E". . . respectively, if the supply E.M.F. is variable. 

 Let the points of intersection C, C', C", ... of these circles with the 

 lag-characteristic line QP, Q'P', Q"P". . . correspond, respectively, 

 tc the E.M.F. values e, e', e", .... These points will define the cor- 



