CHARACTERISTIC CURVES. 



403 



is of course E a , so that the distance QP is equal to E a E x or to l? a /. Now 

 7 a is as yet unknown, but since PQ = R a I a and QP // =I a djN it is evident that 

 the direction of PP f/ is independent of I a . In fact the direction of PP f/ may be 

 once for all determined by laying off PM equal to j? a / a and MN equal to 

 2 a dlN for any arbitrary value of /. The line PN then fixes the direction of 

 PP", and the point P" is found as the intersection of PN and OA, as shown in 

 the figure. 



Having thus determined the point P ff y the point P f on the total characteristic 

 which corresponds to the chosen point P, lies in a horizontal line drawn through P", 

 and the abscissa of P* is determined by the condition that the corresponding ordi- 



Mr -;N 



Amperes 1$ O 



Fig. 36. 



nate of SC shall be equal to PQ(= -/?/<.). The point P* thus determined is 

 most easily located by drawing a line through P parallel to SC. Such a line will 

 intersect the horizontal line through P" at the desired point P'. 



The value of d is equal to 20/360 -Z//', where a is the angle of forward lead 

 of the brushes in degrees, Z is the number of armature inductors and p' is the num- 

 ber of paths in parallel between the brushes. 



33. Examples showing applications of the external characteris- 

 tics of a shunt generator. The point of a characteristic curve at 

 which a shunt generator operates is the point at which the gen- 

 erator characteristic is intersected by the receiving circuit charac- 

 teristic, exactly as in the case of the series generator. 



The curve OA, Fig. 37, is the external characteristic of a 

 given shunt generator, OB is the characteristic of a given receiv- 



