FIELD-EXCITATION OF ROTARY CONVERTERS 199 



is Constant, the Generator E.M.F. being always Constant. Let us 

 suppose, as in the case of synchronous motors, that the converter is 

 supplied by a constant E.M.F. E, and excited by an exciting current 

 which can be regulated at will. 



The fundamental diagram (Fig. 2), combined with the preceding 

 curve (Fig. 16), will enable us to find readily, for each value of the 

 active current, I w , the values of reactive current corresponding to 

 each value of the excitation, expressed in ampere-turns. 



Let, for example, I w be the given active current, and BD the 

 line of reference, which is drawn after the value of y has been obtained. 

 The locus of the point C, characteristic of the load-conditions, for that 

 constant value of I w , is the 

 right line DQ, drawn per- 

 pendicular to BD, at the 

 distance BD=ZI W , whereas 

 the locus of the point O the 

 second end of the vector E, 

 is the line OBX. 



All the load -conditions 

 corresponding to the active 

 current /, will therefore be 

 obtained by drawing, from 

 the first points C, C", C'", . . ., 

 on the first right line, circular 

 arcs of radius E, intersecting 

 the second line at the points 



O, O', O", ____ The segments BO, BO', BO", . . . thus formed 

 measure the values of the corresponding internal E.M.F.'s, e, e', 

 f", . . ., and the segments DC, DC', DC", . . . divided by Z, measure 

 the values of the reactive current Id- 



For each pair of values of e and Id, thus determined, the correspond- 

 ing ampere-turns of excitation, A e , are obtained, from the excitation- 

 curve (Fig. 16), by subtracting the counter ampere-turns of the arma- 

 ture from the resultant ampere turns (A) corresponding to the ordinate 

 (s) of this curve; whence we have 



It should be remembered that the reactive current 7j is demagnetiz- 

 ing and positive if it is counted to the right of the axis BD, and that 



