232 ARMATURE CONSTRUCTION 



however, done this to show how Fig. 271 can be readily convert r<l 

 into a two-phase winding. We have six slots per pole (two per 

 pole per phase), and in Fig. 271, under the first pole, phase A 

 takes in conductors 1 and 2, phase B 3 and 4, and phase C 5 and 6. 

 Fig. 272 shows the same winding as a two-phase, phase A taking 

 in conductors 1, 2, and 3, and phase B conductors 4, 5, and 6. 



The number of poles remains the same, and there are still six 

 slots per pole ; but since there are now only two phases, the number 

 of slots per pole per phase is three, and the number of turns per 

 phase is 3/2 times greater than in Fig. 271. We thus have a wave 

 winding which can be used for either two or three phase. It will 

 be remembered that earlier in this chapter (p. 205) we obtained 

 coil windings (either spiral or lap) which had this property. The 

 only condition is, as before, that the number of slots per pole shall 

 be a multiple of six, for the reason that this is the smallest number 

 divisible by both three and two. 



The smallest possible number of poles is two, and hence the 

 minimum number of slots is twelve, and the permissible numbers 

 of slots for a wave winding common to two and three phases must 

 be multiples of twelve, and are the same as in the case of coil 

 windings (p. 210). 



RETROGRESSIVE WAVE WINDINGS. 



The wave windings heretofore described are generally more appli- 

 cable to armatures with one conductor per slot carried out as bar 

 windings. If there are two or any even number of conductors per 

 slot, the wave winding most suitable is the retrogressive type, which 

 is carried out in exactly the same way as a continuous-current barrel 

 wave winding. The nature of this winding is shown by Fig. 273, 

 which has eight poles and one slot per pole per phase, each slot 

 containing two conductors. The two conductors are shown close 

 together, the top one being represented by a full line and the 

 bottom one by a dotted line. If the end connectors are all to lie 

 up together, it is necessary to connect from the top conductor in one 

 slot to the bottom conductor in the corresponding slot under the 

 next pole. Thus, conductor 1 is connected to 4, 4 to 5, and so on, 

 arriving at 16. We have now progressed once round the armature 

 in going from the point marked A to the point marked B, but we 

 have only taken in one-half of the conductors. To connect up this 

 half independently, we may start at, Say, conductor 14, which is 

 a bottom conductor. It must be borne in mind that for the end 



