THE ALTERNATING CURRENT GENERATOR 239 



In Fig. 142 are shown two concentric, formed, coils 

 for use in winding an a-c. armature. The smaller coil 

 is shown completely insulated while the larger one is only 

 partially covered with its insulating wrapping. 



Developed Single-phase Winding. Fig. 143 gives a 

 developed winding for a single-phase alternator with full- 

 pitch coils and Fig. 144 shows the same winding when 

 concentric coils are used so that some of them have less 

 width than the distance between the centers of two conse- 

 cutive poles, i.e., they have fractional pitch. 



To slip ring 





Fia. 143. Developed Single-phase Winding, all Coils of Same Pitch. 



Two-phase Winding. Now if the same armature is to be 

 wound two-phase, the full number of coils will be used, 

 i.e., twelve. They will be divided into two groups of six 

 coils each and these two groups will not be connected 

 together at all so that four slip-rings are required. 



Sometimes, however, the beginnings of the two sets of 

 coils are connected together and this junction is connected 

 to one slip-ring, so that only three slip-rings are used, one 

 ring being common to the two phases. The two-phase, 

 four-wire, winding is shown in Fig. 145, concentric coils 

 being used. 



