412 



ELECTRICAL EQUIPMENT 



a way that they are displaced 180 from each other, thus producing 

 the six-phase relation. 



The voltages are the same as for the T-connected three-phase 



wwwww 



d 



wwwwwv 



MAM 



3-Wire 

 20 



_I 



lc 



4-Wire 

 20 



12345 

 Collector Rings 



FIG. 255. 



system, and each transformer must be 15 per cent greater than 

 half of the power required for the rotary. 



The neutral can also be brought out on the six-phase side, 

 although this furthermore increases the complication of the con- 

 nection. 



Three-phase to Two-phase. A number of schemes for three- 

 phase to two-phase transformation, and vice versa, have been 

 devised, but the most commonly used method is the T con- 

 nection for either balanced or unbalanced service. 



Balanced T or Scott Connection. This connection is shown in 

 Fig. 256 and requires two transformers which on the three-phase 

 side are connected in T, the number of effective turns in the teaser 

 winding being 86.6 per cent of the number of turns in the main 

 winding. On the two-phase side both mains and teaser windings 

 are identical and, as shown in the figure, are electrically inde- 

 pendent, when supplying a two-phase, four-wire system. Gen- 

 erally, the main and teaser transformers are made identical for 

 the sake of interchangeability, in which case the three-phase 

 winding is provided with both a 50 per cent and an 86.6 per cent 

 tap, as shown by the dotted lines in Fig. 256, so that when used as a 

 main the 50 per cent tap is used and when used as a teaser the 

 86.6 per cent tap is used, the 13.4 per cent winding being left idle. 



