SYNCHRONOUS MACHINERY 



243 



144. Distribution Factors. The windings in Figs. 207 and 

 208 are all concentrated windings, that is, they are placed in one 

 slot per phase per pole. 



When a winding is made up of a number of coils placed in sep- 

 arate slots the e.m.f.'s generated in the various coils are displaced 

 in phase and the terminal e.m.f . is less than if the winding had been 

 concentrated. The factor by which the e.m.f. of a concentrated 

 winding must be multiplied to give the e.m.f. of a distributed 

 winding of the same number of turns is called the distribution 

 factor for the winding and it is always less than unity. 



When a single-phase winding is distributed in two slots per 

 pole spaced at 90 degrees the e.m.f.'s in the two coils are 90 degrees 

 out of phase. If the effective value of the e.m.f. generated in 

 each coil is e, then the terminal e.m.f. is e t = V2 e, Fig. 217, and 

 the distribution factor is 



When a single-phase winding is distributed in three slots per pole 

 spaced at 60 degrees the terminal e.m.f. is the sum of three e.m.f.'s 



0.666 



FIG. 217. 



FIG. 218. 



e at 60 degrees to one another. It is e t = 2 e, Fig. 218, and the 

 distribution factor is 



= |t = p = 0666 



3e 3e 



When a single-phase winding is distributed in six or more slots 

 per pole the distribution factor may be taken as 



= - = 0.637. 



7T 



