176 ARMATURE CONSTRUCTION 



Thus if the winding of Fig. 214 were constructed with three 

 coils instead of six, i.e. with one coil per pair of poles (as in 

 Fig. 204), each coil would cover eight slots on each side, which is 

 two-thirds of the pole pitch, and the spread would still he 66 

 per cent. In both cases the percentage of the total periphery 

 occupied by windings, which is the definition of the spread, is 

 66 per cent., which is correct, as the windings are electrically 

 identical. The number of slots per pole is in both cases eight, 

 and were the whole of the periphery occupied by slots, there 

 would be twelve slots per pole. The spread is then the ratio of 

 the number of slots occupied by windings per pole to the number 

 of slots per pole. In polyphase windings the spread of each phase 

 is the number of slots per pole per phase divided by the number 

 of slots per pole. 



The above will suffice to give a broad idea of the subject of 

 single-phase windings in general. We have now established two 

 sets of fundamental terms: (1) " Whole-coiled" and "half-coiled"; 

 (2) "single-coil" and ''multi-coil." 



However, thus far we have only considered windings having 

 spiral coils, similar to the element shown in Fig. 208. 



There is another way of arranging the coil and still obtaining 

 an open-circuit winding equivalent to any of the above. 



In this type of winding we designate the element a lap coil, 

 and its nature and difference from the spiral coil will be apparent 

 from Fig. 215, p. 171, which should be compared with Fig. 208. 



The lap coil lends itself better to coil form winding than the 

 spiral coil, as in the former the pitch of the conductors is a con- 

 stant; for instance, in Fig. 215 the distance between any conductor 

 in group A and the one to which it is connected in group B is the 

 same as for any other pair, and thus at both back and front of the 

 armature the end portions of the coils are identical in shape and 

 size, and can be made all on one former. 



In the spiral coil (Fig. 208), however, this is not practicable, as 

 the pitch is different for each conductor in the group. The spiral 

 coil is used in connection with semi-closed or entirely closed slots, 

 and the winding is generally done by hand. 



Fig. 216 shows the 6-pole winding of Fig. 214 carried out with 

 lap coils. 



Fig. 217 is a photo of a similar winding, but for a much greater 

 number of poles than six. The spread of this winding is 100 per 

 cent., and each coil is distributed over twelve slots, which cover a 

 whole pole pitch. This winding has twelve slots per pole and 



