154 



ALTERNATING CURRENTS 



represent the impedance drop corresponding to the given current. 

 Through D draw DC parallel to OA, and through C draw CA parallel 

 to DO. Then OA = OE gives the required terminal p.d., while EC 

 represents the drop corresponding to the given current (the percentage 



"FC 1 \ 



drop is 100 7^ ) . It is evident from the diagram that the value of 

 OL> / 



EC will, for a given load current, depend very largely on 0, and that 

 when assumes a sufficiently large negative value i.e. when the 

 current becomes a leading one, such as might be obtained with a 

 condenser or over-excited synchronous motor ( 72) in circuit 

 EC may become negative, the drop becoming added to the open- 

 circuit p.d. to give the terminal p.d. This happens when the angle 

 which OD makes with OA becomes sufficiently obtuse to cause CA 

 to intersect OA outside the circle. 



82. Kapp's Diagram 



The variation of the drop with varying power factor for a given 

 constant value of the load current is best studied by the aid of a 

 very elegant construction due to Kapp.* 



FIG. 114. Kapp's Construction for Variation of Drop with Power Factor of Load. 



If the value of the load current remains constant, the vector OD 

 in Fig. 113 will remain of constant length. Let us assume the 



Loc. cit. 



