170 ARMATURE CONSTRUCTION 



Windings having one coil per pair of poles we designate half- 

 coiled 1 windings (only half of the total number of poles being 

 subtended by coils). 



The above relates to single-phase windings, but for polyphase 

 windings the definitions will be : " whole-coiled " windings have 

 one coil per phase per pole ; "half-coiled" windings have one coil per 

 phase per pair of poles. 



The direction of the current circulation in the coils in each 

 case is of interest as bearing on the connecting up of the coils 

 to one another. 



If we study the direction of the arrow-heads on the conductors, 

 we see that in Fig. 203 the current circulates in a clockwise direc- 

 tion in three of the coils, and counter-clockwise in the alternate 

 three coils. 



The difference is, of course, due to the situation of the coils 

 relative to the poles, as in Fig. 203 any two adjacent coils are 

 situated under poles of opposite polarity, whereas in Fig. 204 they 

 are under poles of the same polarity. The direction of the 

 current circulation has been indicated by a dotted circular arrow 

 at the centre of each coil. 



If we wish to connect up the coils so that the current traverses 

 the whole winding in the same direction, in the case of Fig. 203 we 

 must connect the end of one coil to the end of its neighbour, or the 

 beginning of one to the beginning of the next. 



If we mark the left-hand end of all coils S ( = start) and the 

 right-hand end T ( = terminate), then T of the first coil must be 

 connected to T of the second one, and S of the second to S of the 

 third, and so on. This is equivalent to reversing every alternate 

 coil on its predecessor. 



In Fig. 204, however, all the coils are connected up in the 

 same way for the current to traverse the whole winding in the 

 same direction. 



Hence T of the first coil is connected to S of the second, T of 

 the second to S of the third, and so on. This gives a less compli- 

 cated arrangement of the connections between the coils than is the 

 case in Fig. 203. 



This is not a serious matter so far as single-phase windings are 

 concerned ; but when we come to polyphase windings, it is more 

 important. As a matter of fact, the type of winding shown in 



1 The style of winding shown in Fig. 204 has been designated by Dr S. P. 

 Thompson as " Hemitropic," literally signifying " half-turned " or "half-coiled."' 

 See Polyphase Electric Currents, S. P. Thompson, 2nd ed., 1900, p. 85. 



