262 



PRINCIPLES OF ELECTRICAL DESIGN 



OP as would be the case if the IR drop only had to be considered. 

 The vector PE represents the e.m.f. component necessary to 

 counteract the reactance drop in the armature windings. Al- 

 though the external power-factor angle is zero, there is an angle 

 \l/ between the current vector and the vector of the developed 

 e.m.f., which may be termed the internal power-factor angle. 



(IX Drop) 



^-Internal Power Factor 



Angle 



FIG. 99. Vector diagram for calculating developed e.m.f. non-inductive 



load. 



In Fig. 100, the external power-factor angle is 8 (power factor 

 of load = cos 8). The construction shows how the reactance 

 voltage (IX) becomes a factor of greater importance on the lower 

 power factors. The e.m.f. that must be developed to obtain 

 a constant terminal pressure must, therefore, be greater on low 

 power factor. This, however, is not the chief cause of poor 

 regulation on low power factors; it is the demagnetizing effect of 



(IX) 



Fia. 100. Vector diagram for calculating developed ejn.f. load partly 



inductive. 



the armature ampere-turns which is chiefly accountable for 

 poor regulation on all but unity power factor. From an in- 

 spection of Fig. 100 it will be seen that the greatest difference 

 between developed and terminal voltage occurs when the ex- 

 ternal power-factor angle is the same as the angle E EtP, be- 

 cause the developed voltage is then simply the arithmetical sum 

 of the terminal voltage OE t and the impedance drop E g Et. 



