264 



PRINCIPLES OF ELECTRICAL DESIGN 



self-induction in the windings. The slot flux may be consider- 

 able, especially on a heavy load of low power factor, and it will 

 have an appreciable effect on the inherent regulation of the 

 machine. The m.m.f. of the conductors in the slot accounts for 

 the fact that a certain percentage of the flux in the air gap does 

 not enter the armature core below the teeth; but the point here 

 made is that the slot leakage flux does not induce a counter e.m.f. 

 in the armature windings. 



The core loss will depend upon the flux necessary to develop 

 the e.m.f. OE of Figs. 99 and 100, where the component E P is the 

 reactance voltage drop due to the flux linkages of the end con- 

 nections only; or, in other words, where E P is the e.m.f. induced 

 in the conductors outside the slots by the cutting of the flux lines 

 created by the currents in all the phase windings. 



87. Calculation of Armature Inductance. The flux cut by 

 the conductors which project beyond the ends of the armature 

 slots is very difficult to calculate, and empirical formulas based 



FIG. 101. Illustrating flux cut by end projections of armature coils. 

 on experimental data are generally used for predicting the prob- 

 able value of the inductance of the armature end connections. 

 In Fig. 101 let r represent the pole pitch of a three-phase generator 

 and V the average axial extension of the coils beyond the ends 

 of the armature core. 1 If the total flux produced by the arma- 

 ture currents, in the space r cm. wide by I' cm. deep, as shown 

 cross-hatched in Fig. 101, can be estimated, the voltage developed 

 by the cutting of this flux can readily be calculated. Without 

 attempting to go into the niceties of mathematical analysis 

 which would apply only to one of the many different arrange- 

 ments of coils it can be shown that the flux produced through 



1 The equivalent projection of a coil that is bent up to clear the coils of 

 other phases might be considered equal to the projection of the same coil if 

 flattened out. With r expressed in inches, an approximate value for l f t 

 based on the assumptions made in Art. 83, is 



V = 1.27 (k.v. +3 



cm. 



