174 ARMATURE CONSTRUCTION 



winding diagrams, and the number of turns per coil can be 

 specified in any particular case. 



Fig. 212 l shows a 6-pole single-coil winding with the coils 

 drawn in this way. Let us suppose that each coil consists of four 

 complete turns, then we should have eight conductors located in 

 each slot. If now these eight conductors were spread out over 

 eight slots instead of being concentrated in a single slot, we 

 should obtain the winding shown in Fig. 213. This is an example 

 of a thoroughly distributed single-phase winding, the conductors 

 being located in slots distributed over the whole of the armature 

 periphery at equal intervals. This is now a " quadruple-coil " 

 winding, but it is also an eight-slot winding, as there are eight 

 slots per pole. 



This arrangement of the conductors gives the minimum value 

 for the total electromotive force, whereas the single-coil winding 

 of Fig. 2 1 2 would give the maximum value. 



It is possible to arrange the winding in other ways between 

 these two extreme cases, and Fig. 214 shows one such intermediate 

 arrangement. This winding is still an eight-slot winding, but the 

 slots are concentrated together in groups, so that they occupy 

 two-thirds of the complete periphery of the armature. 



The comparison of the windings in Figs. 212, 213, and 214 

 leads up to the matter of spread of the winding, on which the 

 total electromotive force depends. 



The spread of the winding is defined as the percentage of the 

 periphery which is occupied by windings. Thus the spread of the 

 winding in Fig. 212 would be only about 10 per cent., whereas 

 in Fig. 213 it is 100 per cent. i.e. in the latter case the slots con- 

 taining the winding are uniformly spread over the whole of the 

 periphery. In Fig. 214 it will be seen that the slots are only 

 distributed over about two-thirds of the periphery, and the spread 

 of the winding is here 66 per cent. 



In the case of the whole-coiled windings (having one coil per 

 pole) the spread is the percentage of the pole pitch which a single 

 coil occupies on both sides. Thus in Fig. 214 each side of the coil 

 occupies one- third of the pole pitch, and the two sides together 

 66 per cent., which is the percentage spread. In the case of 

 the "half-coiled" windings (having one coil per pole pair) the 

 spread is the percentage of the pole pitch occupied by a single coil 

 on one side. 



1 The connections between the coils in Fig. 212 and in Figs. 213 and 214 will 

 be clear from what has been said above in connection with Figs. 203 and 204. 



