CALCULATION OF E.M.F. 



93 



per pole, the r.m.s. value of the e.m.f. is increased by reducing the ratio 



i )i i ] j > J1T*P 



From this one might at first sight conclude that there 

 pole-pitch 



is a great advantage in making the pole-pieces as narrow as possible. 

 It must be remembered, however, that for a given flux a reduction in 



<--iT- 



fT + 



T 



*E 



I 



(a) (*) 



FIG. 78. To illustrate Effect of Pole-width on E.M.F. 



the polar area involves an increase of field intensity in the gap, and 

 this means an increase in the field copper. Beyond a certain point, 

 therefore, the increase of e.m.f. per conductor and consequent decrease 

 of armature copper would be more than counter-balanced by the 

 increase of field copper. The gap induction in modern alternators 

 ranges from 6000 to 9000 C.G.S. lines per sq. cm. 



45. Effect of Varying Number of Slots 



We have next to consider the effect of varying the number of 

 slots in which the coil is embedded. If each side of a coil occupies 

 only a single slot, the e.m.f.'s of the various conductors may be 

 practically taken to be in phase with each other, so that the e.m.f. 

 per coil is equal to the e.m.f. per turn, multiplied by the number of 

 turns in the coil. But by distributing the coil over a larger number 

 of slots, we introduce phase differences between the e.m.f.'s of con- 

 ductors lying in different slots, and the addition of the e.m.f.'s of the 

 various groups of conductors must be carried out vectorially instead 

 of arithmetically. The vectorial sum being, in general, numerically 



