124 



ELEMENTS OF ELECTRICAL ENGINEERING. 



group. This ratio chord/arc, as here defined, is sometimes 

 called the phase constant of a distributed winding. The follow- 

 ing table gives the values of this phase constant based not upon 

 the length of circular arc but upon the actual length of the 

 proper portion of the periphery of the electromotive force poly- 

 gon, see Fig. 1 10. 



VALUES OF PHASE CONSTANTS FOR DISTRIBUTED 

 ARMATURE WINDINGS. 



The slots for a given winding are always arranged in p similar 

 groups, p being the number of field magnet poles. The slots in 

 a group are always equidistant, and the width of a group is un- 

 derstood to mean ns, where n is the number of slots in a group 

 and s is the distance from center to center of adjacent slots. It 

 is evident that if the width of the groups of slots is ^ of the dis- 

 tance from N to S, center to center, then three such windings 

 can be placed on the armature giving a three-phase alternator, 

 and if the width of the groups of slots is y 2 of N to S, center to 

 center, then two such windings can be placed on the armature 

 giving a two-phase alternator. Figure 1 1 1 shows the outline of 

 an 8-pole alternator with its armature slotted for a winding dis- 

 tributed in 3/ slots, the width ns of each group of slots being j 

 of the distance N to 5, center to center, so that there is room 

 on this armature for two additional windings of the same type. 



It is undesirable to spread the winding of a single-phase alter- 

 nator over the whole armature surface. Thus the phase con- 

 stant of a winding spread over the whole surface is 0.637 and the 

 phase constant of a winding spread over ^ of the armature sur- 

 face is 0.784. The number of armature conductors in the first 



