ALTERNATING-CURRENT WINDING 209 



the front and back ranges irrespective of their colour), and have 

 also added the eight poles. Since the four coils of one phase 

 are situated under poles all of which are of the same polarity (as 

 coils I, IV, VII, and X), the direction of current circulation will 

 be the same in all of them. Hence we must connect up the four 

 coils in the same direction : 



The end of coil I to the beginning of coil IV 

 IV ., VII 



VII X, 



leaving the beginning of I and the end of X for terminals 

 S A and T A . Similarly, connecting up the other two phases, B and 



C, gives the completed diagram of Fig. 244. This winding is now 

 an 8-pole, three-phase, half-coiled, triple-coil, three-slot spiral 



72 

 winding in two ranges. The number of slots per pole is -^- = 9, or 



3 per pole per phase, which will also be seen from Tables 

 XIV. and XV. This winding having one coil per phase per 

 pole pair, a single coil should cover three slots on one side 

 (see p. 174). 



This will be seen from the diagram in Fig. 244. Slots 1 to 9 lie 

 within a pole pitch, and of these slots 1, 2, and 3 are covered by 

 one side of coil No. I of phase A. 



The spread of the winding is thus 9/3 = 33 per cent. 



One interesting point is brought out by retaining the dotted- 

 line coils to represent the back range in Fig. 244. It will be seen 

 that each phase consists of two full-line coils and two dotted-line 

 coils two in the front range and two in the back. The three 

 phases are exactly similar, and should the length of the mean 

 turn of the front range coil differ from that of the back range 

 coil, the phases will still all have the same total length of conductor 

 in them, and be consequently of the same resistance and quite 

 balanced. 



The following are a few points which may be noted in con- 

 nection with this winding : - 



1. Any . three-phase two-range winding having an even number 

 of coils per phase will have half of that number situated in one 

 range and half in the other range, and the three phases will 

 be exactly similar. 



The following table shows a comparison between the two- 

 phase and three-phase arrangements of the two-range twelve-coil 

 winding shown in Figs. 243 and 244. 



14 



