ARMATURE WINDINGS 89 



Fig. 74. If we consider only a single-phase winding, then it is 

 immediately evident from Fig. 74 that the hemi-tropic winding, by 

 reason of the greater mean length of turn, requires more copper and 

 lends to a higher armature resistance than the ordinary winding. 

 On the other hand, it offers somewhat greater flexibility in the matter 

 of distributing the winding. Thus with the six slots per pole which 

 we have assumed, each half or side of a coil must be embedded in 

 either one or two slots if the ordinary winding is employed ; whereas, 

 with a hemi-tropic winding, each side of a coil may occupy one, two, 

 three, or four slots. 



Considering next a two-phase armature winding, and assuming as 

 before six slots per pole, we have three slots available per pole per 

 phase. The winding must therefore necessarily be hemi-tropic, as 

 shown in Fig. 75, in which the winding of one phase is shown by the 

 full lines, and that of the other by the dotted lines. The positions 



PIG. 75. Two-phase Armature Winding. 



of the field-poles are indicated by the letters N and S. Owing to the 

 overlapping of the coils, the coil-ends of one phase are brought out 

 straight, and lie on a cylindrical surface (as in the typical modern 

 continuous-current armature with a barrel or " straight-out " winding), 

 while those of the other are bent up so as to lie on a conical surface. 

 The bent-up ends of the armature coils are shown in Figs. 63 and 66, 

 and are still more clearly visible in Fig. 77. 



If the winding is to be a three-phase one, we have, assuming as 

 before six slots per pole, two slots available per pole per phase. The 

 winding might therefore be either of the ordinary or the hemi-tropic 

 type. The adoption of the ordinary winding (with one-half of a coil 

 in a single slot) would, however, necessitate a somewhat complicated 

 arrangement of coil-ends, as these would have to be arranged so as to 

 lie on three different surfaces, a different surface corresponding to 

 each phase. By adopting the hemi-tropic type of winding, shown in 

 Fig. 76, this difficulty is overcome, since the coil-ends may now be 



