208 ARMATURE CONSTRUCTION 



Slots 1 to 12 lie within a pole pitch ; and coil I covers slots 1, 

 2, and 3 on one side, and slots 10, 11, and 12 on the other six 

 slots in all. 



12 

 The spread of the winding is -^ = 50 %. 



The designation of this winding is a six-pole, two-phase, whole- 

 coiled, triple-coil, six-slot spiral winding. 



12 



Three-phase. Fora three-phase winding there will be -~-=4 



o 



coils per phase. Phase A will, consist of coils I, IV, VII, and X; 

 phase B of coils II, V, VIII, and XI ; phase C of coils III, VI, IX, 

 and XII. 



Now consider, for purposes of connecting up, phase A alone. 

 Neighbouring coils, as coils I and IV, do not lie close together 

 (as was the case in the two-phase winding), but they are distant, 



FIG. 242. Nine-turn Triple Coil. 



as between conductors 12 and 19, an amount about equal to 

 the width of one coil. If we place one pole opposite coil I and 

 another opposite coil IV, the space between these coils can be 

 occupied by another pole; and carrying this out around the 

 armature, we obtain eight poles in all. Thus there are eight 

 poles and four coils per phase, which gives one coil per phase 

 per pole pair ; and herein this winding differs from the same 

 connected as a two-phase, where there are six poles and one coil 

 per phase per pole. 



A reference back to Fig. 201 will be useful at this point. 

 Here we started out with six poles, and evolved a half-coiled 

 winding which had three coils per phase or one coil per phase per 

 pole pair. In this diagram it will be seen that the neighbouring 

 coils of one phase are displaced from one another by an amount 

 equal to the pole-pitch, the same as occurs in Fig. 244. In 

 Fig. 244 we have coloured the three phases A, B, and C, black, 

 red, and blue (still retaining the dotted coils to distinguish 



