244 ARMATURE CONSTRUCTION 



that by omitting the commutator they are at once alternating- 

 current windings only. 



The carrying out of these windings, and the laws which 

 govern them, are the same as for continuous-current windings. 

 These have been dealt with in Chapter VIII. It will therefore 

 suffice if we give a few examples of typical polyphase rotary 

 converter windings. For three-phase multiple-circuit simplex 

 windings there are three sections of winding per pair of poles. 

 In two-circuit windings there are three sections per winding 

 independently of the number of poles. 



Thus a six-pole machine with a six-circuit simplex winding 

 n 



has H X 3 = 9 sections, i.e. three per pair of poles. 



To tap such a winding for three-phase, leads are taken out 

 to the collector rings at equal ninths through the armature from 

 beginning to end. 



Fig. 281 shows such a winding having 108 conductors. The 

 portion of the winding lying within one pair of poles is divided 

 into three sections coloured black, red, and blue, and taps are 

 taken out to the three collector rings at the junctions of these 

 sections, giving nine taps in all. 



Had the winding been a two-circuit single winding there 

 would have been only three sections, and taps would have been 

 required at equal thirds around the armature. 



Fig. 282 illustrates an eight-pole two-circuit winding with 90 

 conductors and a winding pitch of 11. Tappings are brought to 

 the collector rings from the leads to the three conductors numbered 

 1, 31, and 61. The phases when distinguished by colouring, stand 

 as in Fig. 282. 



The distinctive feature which will be noticed from Figs. 281 

 and 282 is the overlapping of the phases in three-phase closed 

 circuit windings. This is clearly brought out by the colours. 

 Thus any portion of the armature periphery carries conductors 

 belonging t to two different phases, such conductors occurring in 

 the same slots. For instance, in Fig. 282 the portion of the 

 armature from conductor 1 to 6 carries red and blue conductors, 

 from 7 to 13 black and blue, and so on round the whole armature, 

 the conductors of one phase being distributed over two-thirds of 

 the periphery. This does not occur with six-phase windings, and 

 as this feature results in a less output for the three-phase machine, 

 the six-phase rotary converter is coming to be more commonly used. 



The positions of tappings for six-phase windings are shown in 



