184 ARMATURE CONSTRUCTION 



The general distinction between the two cases is summarised 

 by the following : 



If the three conductors from which similar terminals are taken 

 (as in S A , S fi , S c ) lie evenly distributed within one pole pitch, one 



phase must be reversed. 



If these three conductors lie evenly in the region of a pair of 

 poles, i.e. within a double pole pitch, the phases are connected up in 

 order, as in Fig. 224, B. 



This distinction is brought out also by Figs. J and K, which are 

 respectively really three- and six-phase, both Y-connected. These 

 windings correspond to the wave windings of Figs. 269 and 271. 



The Y connection is most commonly used for generators. This 

 is because the pressure per phase in the Y grouping is only 



Q'577 ( = -Q) of the terminal pressure; for instance, if the 

 V is/*>/ 



generator is required to give 10,000 volts at. the terminals e.g. 

 between terminals I. and II., Fig. A then each phase will only 

 have to generate 5770 volts; and this will be the pressure between 

 each terminal and the mid-point, as, for instance, between S A and 



T A . The advantage in this is that the neutral point may be 



grounded and the slot insulation need only be of such propor- 

 tions as are allowed for 5770 volts, which is much less than for 

 10,000 volts. 



In the A connection, if we had 10,000 volts at the terminals as 

 between I. and II. , the volts per phase will also be 10,000 as 

 between S A and T A , and the machine must be insulated accordingly. 



As most alternators are for very high voltages, this point 

 becomes of importance ; but with low-voltage machines, as the 

 ordinary induction motor, it is not serious. For other reasons 

 also, relative to the size of the wires and their arrangement in 

 the slots, which do not generally apply to generators, the A 

 connection is sometimes preferable for induction motors. 



Two-phase windings are not so often interconnected, and the 

 ordinary case of a two-phase winding with independent phases 

 and four lines is shown, diagrammatically, in Fig. 226, A. 



The other common grouping for two-phase is a three-line 

 grouping shown in Fig. 226, B. Here the ends of the two phases 

 are connected together and to a common line, and the beginnings 

 of the phases to the other two lines. This connection is sometimes 

 known as two-phase with common return. 



We shall not deal with the connections of four-phase and six- 



