388 ELEMENTS OF ELECTRICAL ENGINEERING. 



through each of which the whole of the armature current may 

 be considered to flow, and let N be the number of turns of wire 

 in the field winding per magnetic circuit. Then the number of 

 ampere-turns, net, acting on each magnetic circuit is I'(N d) 

 when the current output of the generator is /' amperes. Now, a 

 current / flowing in the field winding alone gives a total induced 

 electromotive force E a , and in order that the same induced elec- 

 tromotive force E a may be obtained when the machine is deliver- 

 ing /' amperes as a series generator, the net ampere-turns act- 

 ing must be equal to NI. That is, we must have 



// = 7v' / (0 



That is, the current must be increased in the ratio of (N d) 

 to N. Therefore the total characteristic curve of a series gener- 

 ator may be derived from the magnetization curve of the machine 

 by multiplying the abscissas of the magnetization curve by 

 Nj(N ' d). This result is deduced on the assumption that the 

 demagnetizing action of the armature is strictly proportional to 

 the armature current. 



Dependence of the characteristic of a series dynamo on speed. 

 The flux & has a definite value for a given value of current 

 output of a series generator independently of speed. Conse- 

 quently the total induced electromotive force <$>Z' n is propor- 

 tional to the speed of the generator for a given value of current 

 output. Therefore the external characteristic corresponding to 

 speed n f may be derived from the given external characteristic 

 corresponding to speed n as follows : 



Add RI to each ordinate of the given characteristic, thus find- 

 ing the internal or total characteristic for the same speed n. Mul- 

 tiply the ordinates of this total characteristic by n' jn y thus find- 

 ng the total characteristic for speed n' '. Subtract RI from each 

 ordinate of this total characteristic for speed n f ', thus finding 

 the required external characteristic for speed n' . 



