CHARACTERISTIC CURVES. 



401 



CD at a number which is equal to njn" where n" is the lowest 

 speed (critical speed) at which the given shunt machine can build 

 up as a generator. 



Volts 



O 



Speed 



Fig. 34. 



32. Relation between the magnetization curve of a shunt dynamo 

 and its total characteristic as a generator. (a) On the assumption 

 that the demagnetizing action of the armature current is negligible. 

 In Fig. 35 the magnetization curve OA* of the machine for 

 the given speed is plotted to the left of the origin of coordinates ; 

 ordinates of OA of course represent values of Zj o , and abscis- 

 sas of OA, measured to the left from 0, represent values of 

 shunt field current /. The ordinates of the straight line OB 

 represent values of RJ S . 



Consider any given point P on the magnetization curve. The 

 ordinate SP is the total induced electromotive force a , so 

 that the point P' of the total characteristic which corresponds to 

 P, lies on a horizontal line drawn through P. Draw the straight 

 line SC of which the abscissas measured to the right from ^ rep- 

 resent (/ -f /.), or the total armature current / o , and of which the 

 ordinates represent the values of R a l a . Now, when PS is the 

 total induced electromotive force, QS is the value of x ( 



* The values of f 8 are exaggerated in Fig. 35 for the sake of clearness. 

 26 



