REGULATION OF ALTERNATORS 303 



value OP is therefore the difference of terminal voltage E Ei 

 divided by the load voltage E t , or, expressed as a percentage of 

 the lower voltage, 



Thus the error in predetermining the inherent regulation de- 

 pends upon the degree of accuracy within which curves such as 

 those shown in Fig. 124 can be drawn before the machine has 

 been built and tested. 



105. Factors Influencing the Inherent Regulation of Alter- 

 nators. By enumerating all the factors which influence the 

 terminal voltage of a generator driven at constant speed with 

 constant field excitation, it will be possible to judge how nearly 

 the methods about to be considered approximate to the ideal 

 solution of the problem. These factors are: 



(a) The total or resultant flux actually cut by the armature 

 windings (this involves the flux linkages producing armature 

 reactance) . 



(6) The ohmic resistance of the armature windings. 



(c) The alteration in wave shape of the generated electro- 

 motive force, due to changes in air-gap flux distribution. This 

 means that the measured terminal voltage depends not only 

 upon the amount of flux cut by the conductors but also upon the 

 distribution of flux over the pole pitch, because the amount of 

 flux cut determines the average value of the developed voltage, 

 while the form of the e.m.f . wave determines the relation between 

 the mean value and the virtual or r.m.s. value. 



By far the most important items are included under (a), and 

 it will be well to consider exactly how the resultant flux cut by 

 the armature windings varies when load is put on the machine. 



Considering first the flux cut by the active belt of conductors 

 under the pole face, this is not usually the same under load con- 

 ditions as on open circuit (the field excitation remaining con- 

 stant), for the following reasons. The current in the armature 

 windings produces a magnetizing effect which, together with the 

 field-pole magnetomotive force, determines the resultant mag- 

 netomotive force and the actual distribution of the flux in the air 

 gap. When the power factor of the load is approximately unity, 

 the armature current produces cross-magnetization and dis- 

 tortion of the resultant field, accompanied usually by a re- 

 duction of the total flux owing to increased flux density in the 



