246 



ELECTRICAL ENGINEERING 



With three slots per phase per pole the factor is 

 5 = 0.96, Fig. 223. 



The following table gives the distribution factors for single-, 

 two- and three-phase windings. 



If only two thirds of the slots are used for the single-phase 

 windings their distribution factors must be reduced by multiply- 

 ing them by the constant 0.866. 



145. Short-pitch Windings. The pitch of a winding is the 

 distance between the two sides of one of the coils forming the 



E.M.F. between Terminals 



E.M.F. in O or h 



FIG. 224. Full-pitch coil. 



winding. When the coil pitch is equal to the pole pitch or the dis- 

 tance between the centres of adjacent poles, the winding is full 

 pitch. When the coil pitch is less than the pole pitch the winding 

 is fractional pitch or short pitch. 



In Fig. 224 abcdf shows the distribution of flux under two 

 adjacent poles of an alternator. The area under the section of 

 the curve abc or cdf multiplied by the length of the pole parallel 

 to the shaft gives the flux $ crossing the air gap under each pole. 



As the side g of the coil cuts across the flux in the gap an e.m.f. 

 is generated in it of the same wave shape as the flux distribution. 



