SYNCHRONOUS GENERATORS 297 



star connection is preferred, the main advantages of this con- 

 nection being: 



1. It is possible to bring out a lead from the neutral point, 

 which is useful for various purposes. 



2. The cost is less than with delta connection, requiring 

 approximately only 58 per cent of the turns. 



3. It is not possible for circulating currents of triple frequency 

 to flow in the windings. 



If E represents the effective e.m.f. of each group and I the lim- 

 iting current which can be carried by the same, the corresponding 

 three-phase capacities of the various arrangements will be 



Fig. 164: 



Fig. 165: 



Fig. 166: 3X27X#=6.E7; 



Fig. 167: 



Fig. 168: 



For two-phase connections the capacities are the same for the 

 different combinations shown in Figs. 169 to 172. If E\ repre- 

 sents the e.m.f. of each group and 7 the permissible current it 

 equals 4EiI. 



The armature winding of single-phase generators can be 

 arranged either for purely single-phase duty or on the basis of the 

 same winding being used both for polyphase and single-phase 

 service, the latter method being the one mostly used. When 

 intended for three- and single-phase service any one of the con- 

 nections shown in Figs. 173 to 175 can be used, although the star 

 connection in Fig. 175 is by far the most common.. 



The single-phase e.m.f 's. will be the same as three-phase with 

 the exception of the arrangement shown in Fig. 174, where the 

 single-phase connection is obtained from diametrically opposite 

 points on the closed-coil winding. 



The comparative capacities of the machines when used for 

 single-phase and three-phase service should obviously be based 

 on the losses and heating in the individual armature coils or group 

 of coils and not on the total armature losses. The reason for this 

 is that the armature loss is not equally divided among the differ- 

 ent groups of coils and the heating therein will consequently be 



