240 ALTERNATORS. 



or calculation. On referring to the magnetisation curve, 

 Fig. 92, p. 198, the point on the curve corresponding to the 

 excitation under the assumed conditions, must be found. 

 A line is then drawn vertically upwards through this point, 

 its^length being equal to the loss of voltage on the volt scale. 

 A horizontal line drawn through the upper end of this vertical 

 line to meet the magnetisation curve will represent by its 

 length the increase in excitation required. 



Strictly, the load magnetisation curve, Fig. 94, should be 

 employed, but as this is not often taken experimentally and 

 is practically parallel to the no-load curve, the latter is 

 generally employed. 



Short Circuit Magnetisation Curve. If an alternator is 

 only partially excited and its terminals are short-circuited 

 through an ammeter, the whole of the electromotive force 

 generated in the armature is spent in overcoming the arma- 

 ture impedance. By making measurements of the current 

 under these conditions for various values of the excitation, 

 useful information may be obtained. There are four factors 

 which tend to diminish the current which the armature is 

 able to produce : 



(a) The weakening of the main field produced by the 



armature magnetic reaction. 



(b) The counter electromotive force produced by the 



armature self-induction. 



(c) The armature resistance. 



(d) Production of eddy currents. 



The weakening of the main field is due to the setting up 

 of lines of force by the armature current, and entails the 

 loss of a certain electromotive force exactly in the same way 

 as the self-induction of the circuit requires the expenditure 

 of a certain amount of voltage in setting up a magnetic 

 field. 



The first two factors may consequently be treated 

 together as being similar in their effects to a back electro- 

 motive force or idle voltage 90 out of phase with the 

 current. 



The last two sources of loss may similarly be taken as 

 giving rise to a single energy electromotive force in phase 

 with the current. We may, consequently, represent the 

 conditions in the armature by a triangle of electromotive 

 force, as in Fig. 116, p. 245, the values of the voltages being 

 determined in the experiment to be described. 



