180 



ARMATURE CONSTRUCTION 



For the A grouping, the ends of the three phases are connected 

 up into a closed circuit (or mesh) the end of one (T A ) to the 



beginning of the next (S fi ). The three junctions between the 

 phases are connected to the machine terminals. 



If, when any winding is completed, the terminals of the phases 

 are lettered S and T at their beginning and end, then to connect 

 up for Y or A, one has only to remember the following : 



Y Grouping. Connect the beginnings (S) of the three phases 

 to the three machine terminals ; and connect the ends (T) of the 

 three phases together, forming the mid-point i.e. 



Connect phase A between terminal I. and mid-point. 

 >j Et/ )> )> ** )) 



CTTT 

 'JJ 5> JLiAi 55 5 > 



FIG. 224, A. 



M 



Y and A Three-phase Groupings. 



A Grouping. Connect the beginnings (S) of the three phases to 

 the machine terminals and also to the ends (T) of the preceding 

 phases i.e. 



Connect phase A between terminals I. and II. 

 B II. III. 



C III. I. 



The appropriateness of these connections will be seen from the 

 following considerations relating to the diagrams in Fig. 225. 



One complete cycle is passed through by any armature con- 

 ductor while passing from a certain point opposite one pole piece, 

 say the middle of the north pole, to the corresponding point 

 opposite the next pole piece of the same polarity. This angular 

 distance is 360 electrical degrees, independent of the number of 

 poles of the machine. Now, a three-phase armature winding is 

 merely three single-phase windings laid on the same armature, the 



