ALTERNATING-CURRENT WINDING 179 



Figs. 218, 219, and 220 we have only one conductor situated under 

 each pole, in Figs. 221, 222, and 223 we have a number of con- 

 ductors in a concentrated group under each pole in place of each 

 single conductor in the former case. 



Hence in Fig. 221 we have a set of three coils, each consisting 

 of a number of turns, connected together and represented by a 

 thick line, as in Fig. 211. 



Similarly, in Fig. 222 we have two sets of coils the red and 

 the black each set of three connected together as in Fig. 221, and 

 each constituting one of the two phases. Also in Fig. 223 we 

 have three sets black, red, and blue each set constituting one 

 of the three phases A, B, and C. 



We shall, as in the single-phase windings, mark the two 

 terminals of each phase S and T ; but to distinguish between the 

 phases, the letter of each phase is suffixed thus S A and T A indi- 

 cate the terminals of phase A, and so on. 



In Figs. 219-223 we have shown one separate pair of collector 

 rings connected to the terminals of each phase. In practice it is 

 very rare to provide two slip-rings for each phase, or in the case 

 of stationary armatures, two lines leaving the machine terminals 

 for each phase, as polyphase windings lend themselves, by virtue of 

 the properties of polyphase currents, to convenient interconnection 

 of the phases, so that some of the rings or terminals may be dis- 

 pensed with. 



Three-phase armatures are almost invariably interconnected, so 

 that instead of six terminals and six lines constituting three 

 independent phases, there are only three. The two methods of 

 connecting three-phase windings are the star grouping and the mesh 

 grouping. These names are suggested by the appearance of the 

 diagrams illustrating them shown in Figs. 224, A and B. 



These two arrangements are also commonly known as Y 

 connected and A (Delta) connected, which designations are more 

 suggestive. 



In Figs. A and B, the coloured limbs represent each the 

 windings of a phase with its terminals marked S and T, as in the 

 case of the winding diagrams. 



For the Y grouping, the three similar terminals of the phases, 

 for instance T A , T fi , T C , are connected together, and the other three 

 similar terminals, S A , S fi , S c , constitute the machine terminals 

 which are connected to the line. The junction of T A , T B , T C , is 

 known as the mid-point or common junction of the system. 



