SYNCHRONOUS GENERATORS 299 



higher in groups carrying the highest current. When a poly- 

 phase machine, therefore, is loaded single-phase its capacity is lim- 

 ited by the current which any individual coil can carry, and this 

 current is obviously the same whether polyphase or single-phase. 

 With the connection, as shown in Fig. 173, the three-phase 

 rating is the same as in Fig. 164; viz., 3xV%EXl, or equal to 

 5.196#7. The corresponding single-phase rating is \/3#Xl.57 

 or equal to 2.598 E; the two groups of the winding carrying the 

 limiting current / while the other four groups carry a current equal 



to -, the total current thus being 1.57. The single-phase capacity 



with this connection is, therefore, equal to 50 per cent of the cor- 

 responding three-phase rating. 



The diametrical connection shown in Fig. 174 gives a much 

 higher rating than the previous one. The relative capacities, 

 however, depend on whether the six terminals are utilized in 

 connection with transformers for obtaining three-phase power. 

 As each half of the winding can carry the limiting current 7, the 

 total current is equal to 27, and the single phase rating 4EI, the 

 single-phase e.m.f. being 2E. The corresponding six-phase rating 

 is equal to 6EI, and the single-phase rating is, therefore, 66.7 

 per cent of this rating. For straight three-phase connection, 

 however, the three-phase rating becomes 5.196#7 and in this case 

 the single-phase capacity is 77 per cent of the three-phase. 



With star connection shown in Fig. 175, two of the phases 

 carry all of the current while the third phase is idle and could be 

 omitted, although it is generally added, being a reserve in case 

 of accident to either of the other phases. With the star arrange- 

 ment, as shown, two-thirds of the winding is almost in phase with 

 the single-phase terminal e.m.f., being 86.6 per cent effective, and 

 this arrangement is, therefore, about 15 per cent more effective 

 than the delta connection shown in Fig. 173. 



The three-phase rating is SEXl X\/3 or equal to 5.1967*77, 

 while the single-phase rating is equal to 3EI; thus 57.7 per cent 

 of the three-phase rating. This is by far the most common method 

 of connecting armature windings for single-phase service. 



The general practice in building single-phase generators is to 

 use a Y-wound stator and give it a rating from 65 per cent to 70 

 per cent of the three-phase rating. This is possible, since one- 

 third of the armature slots will either be vacant or filled with 



