82 



ALTERNATING CURRENTS 



the vacant spaces on our armature with the conductors forming the 

 second phase of the winding, thereby utilizing the entire available 

 winding-space. 



In a three-phaser, the pole-pitch is divided into three equal parts, 

 and the width of each of these represents the width of a group of 

 conductors belonging to the same phase. Here, again, the entire 

 available winding-surface is utilized. 



Since in a three-phase alternator the width of a group of conductors 

 belonging to one phase does not exceed ^rd of the pole-pitch, it 

 follows that the pole-arc may be made equal to f rds of the pole-pitch 

 without introducing any appreciable amount of differential action. 

 This, accordingly, is the generally adopted value for the ratio 



T)olG~lI*C 



-, -r ^ in three-phase alternators, and it is about the same as that 

 pole-pitch 



commonly employed in continuous-current machines. 



41, Standard Type of Alternator 



The standard modern type of alternator is one in which the 

 armature is stationary, and the field revolves. The general construc- 



VENTILATING APERTURES 



FIELD POLES 



RMATURE LAMINATIONS 

 YOKE 



FIG. 61. General Arrangement of Kevolving Field Alternator. 



tion of this type will be understood from Fig. 61. The armature 

 laminations are supported by a casting which forms the " yoke- ring," 



