402 



ELEMENTS OF ELECTRICAL ENGINEERING. 



and PQ is the value of RJ a . Therefore RJ a is known, and 

 the abscissa of P' is determined by the condition that the cor- 

 responding ordinate of SC shall be equal to R a l a y as indicated 

 in the figure. The position of P' thus determined may be most 



Fig. 35. 



easily found by drawing a line through Q parallel to SC\ this 

 line will intersect the horizontal line through P at the desired 

 point P 1 '. Additional points on the total characteristic curve 

 may be located in a similar manner. 



(b] When the demagnetizing action of the armature current is considered and 

 assumed to be proportional to the armature current. The curve OA, Fig. 36, is 

 the magnetization curve of the machine for the given speed, plotted to the left of the 

 origin O, as in Fig. 35. The ordinates of the straight line OB, Fig. 36, represent 

 values of E X (=R S I S ] as in Fig. 35. 



Consider any given point P on OB, Fig. 36. The abscissa OS is the shunt 

 field current I 8 and the ordinate SP is E x . It is required to find the point P / 

 on the total characteristic corresponding to P. 



Let N be the number of turns of wire in the shunt field winding, and let */be the 

 number of demagnetizing turns on the armature through each of which, the entire 

 armature current may be considered to flow. Then the net field excitation is 

 (NI S dl a } ampere-turns, which may be written N(I S I a djN^. Therefore 

 the actual net field excitation and the actual value of E a is that which would be pro- 

 duced by a field current equal to (I 8 ladjN] acting alone; or, in other words, 

 the distance Q_P, f/ Fig. 36, is equal to I a dj N, P" being the unknown point 

 on the magnetization curve corresponding to P. Furthermore, the ordinate of P^ 



