REGULATION OF ALTERNATORS 305 



This quantity was calculated in Art. 87, Chap. XII, and ex- 

 pressed in formula (99), the calculation being based upon an 

 amount of end flux per pole (<f> e ) given by the empirical formula 

 (98). Although the writer likes to think of the cutting of the 

 end flux by the conductors projecting beyond the ends of the 

 slots, the idea of flux-linkages and a coefficient of self-induction, 

 L e , expressed in henrys, may be preferred by others. If it is de- 

 sired to substitute the terms of the formulas (98) and (99) in the 

 expression 2irfL e I c , the value of the coefficient of self-induction, 

 in henrys, will.be 



" 



X 10* 



106. Regulation on Zero Power Factor. In practice, any 

 power factor below 20 per cent, is usually considered to be equiva- 

 lent to zero, so that the calculations can be checked when the 

 machine is built, by providing as a load for the generator a suit- 

 able number of induction motors running light. On these low 

 power factors with lagging current the phase displacement of the 

 armature current causes the armature magnetomotive force to 

 be almost wholly demagnetizing, that is to say, it directly opposes 

 the magnetomotive force due to the field windings, the distor- 

 tional or cross-magnetizing effect being negligible. Its maximum 

 value per pole is given by formulas (100) and (101) of Art. 94, 

 Chap. XIII, and its effect in reducing the flux in the air gap is 

 readily compensated (on zero power factor) by increasing the 

 field excitation so that the resultant ampere-turns remain un- 

 changed. This statement is not strictly correct because the in- 

 creased ampere-turns on the field poles give rise to a greater leak- 

 age flux, and this alteration should not be overlooked, especially 

 when working with high flux densities in the iron of the magnetic 

 circuit. If the estimated leakage flux for a given developed 

 voltage on open circuit is </ maxwells, then, for the same voltage 

 with full-load current on zero power factor, the leakage flux would 



be approximately 3>'j = $j \~~M ~) w ^ere M is the number of 



field ampere-turns on open circuit, and (M + Jlf a) is the number 

 of field ampere-turns with full-load current in the armature, the 

 power factor being zero. The quantity M a is the demagnetizing 

 ampere-turns per pole due to the armature current. 

 Let curve A of Fig. 125 be the open-circuit saturation curve 

 20 



