CHARACTERISTIC CURVES. 



387 



Volts 



R, of the armature and field windings, as explained in Art. 46, 

 Chapter III., so that : 



E*=E. + & (0 



Let the values of RT be plotted for various values of /. These 

 plotted points will lie on the straight line OB, Fig. 21, since R 



is a constant. Plot the curve OC, 



of which the ordinate at each point 

 exceeds the ordinate of the given 

 curve OA, by the amount RI. 

 The curve C is the required in- 

 ternal characteristic. 



The effect of residual magnet- 

 ism upon the characteristic curve 

 of a series generator is to cause 

 the curve to intersect the axis of 

 volts above the origin, as shown 

 in Fig. 21. 



Relation between the magneti- 

 zation curve and the internal or 

 total characteristic of a series gen- 

 erator. If it were not for the de- 

 magnetizing action on the field, of Fig> 2I * 

 the current in the armature, the internal or total characteristic of a 

 series dynamo would be identical* with the magnetization curve. 

 However, the effect of the demagnetizing action of the armature 

 current is either to reduce the armature flux and therefore the total 

 induced electromotive force which corresponds to a given field cur- 

 rent, or to necessitate an increased field current to give the pre- 

 scribed total induced electromotive force. Let <^be the number of 

 demagnetizing turnsf of wire on the armature (per magnetic circuit) 



* Except for a slight loss of electromotive force due to the inductance of the ele- 

 ments of the armature winding. 



fit is evident from equation (35) that 



