SYNCHRONOUS GENERATORS 



283 



the different slots are somewhat out of phase with one another, 

 and for this reason the slot factor k s , for which the values are 

 given in Table XLI, must be introduced in the formula. With 

 two-layer windings the value of k s should correspond to the num- 

 ber of slots per layer per pole per phase and not to the total num- 

 ber of slots per pole per phase. 



TABLE XLI 

 VALUES OF SLOT FACTOR k s 



1.00 



30.80 



a 



0.70 



The windings may be arranged for full or fractional pitch. In 

 the former case the coil spans a distance exactly equal to pole 

 pitch while in the latter case it 

 spans a lesser distance. Frac- 

 tional pitch windings are very 

 generally used, the advan- 

 tages being a better wave and 

 shorter end connections of the 

 windings, resulting in a sav- 

 ing of armature copper be- 

 sides making the machine 

 shorter. This is especially 

 the case for machines with a 

 small number of poles. It is 

 evident that the e.m.f's. in- 

 duced on both sides of the 

 same coil are not exactly in 



phase with each other in a fractional pitch winding, so that a 

 larger flux will be required than with a full-pitch winding. This 

 is allowed for in the voltage formula by introducing a winding 



50 



60 70 80 90 



Per cent Winding Pitch 



100 



FIG. 153. Values of Winding-pitch 

 Factor. 



