VOLTAGE REGULATION OF THE ALTERNATOR. 12 1 



curve EF y the average value (during half-a-cycle) and the effec- 

 tive value are easily found. 



In Fig. 107 it is assumed that there is no fringe of flux beyond the pole tips, and 

 the distance apart of each pair of slots a and b is less than the distance between 

 pole tips. Under these conditions there is never an instant when the induced 

 electromotive forces in the different parts of the given armature winding oppose each 

 other, and, such being the case, the average value (during half-a-cycle) of the elec- 

 tromotive force of the alternator is equal to Z times the average value (during half- 

 a-cycle) of the electromotive force in a single conductor ; and when this relation 

 holds, the effective electromotive force of the alternator can be expressed as p$Zn 

 times the form factor of the electromotive force curve. In general, however, the 

 form factor can be used in this way only in case of a concentrated armature winding. 



Harmonic distribution of flux. Figure 108 shows an arma- 

 ture between two nearly flat pole-pieces, so that the flux density 

 in the gap space is greatest at the points aa. If the pole faces 



Fig. 108. 



were so shaped that the flux density at any point b in the gap 

 space would be proportional to the cosine of the angle a, we 

 would have what is called a harmonic distribution of flux, and 

 the electromotive force induced in each armature conductor 

 would be a harmonic electromotive force capable of being repre- 

 sented by a line in a clock diagram. 



(a) Concentrated armature winding. In the case of a con- 

 centrated winding, the average value (during half-a-cycle) of the 

 electromotive force of the alternator is equal to pQZn abvolts as 

 stated above, and if the electromotive force is harmonic (harmonic 

 distribution of flux) the ratio, effective value divided by average 



