PHASE TRANSFORMATION 113 



of phase transformation have been devised, but the best known is 

 that due to C. F. Scott. The principle of Scott's method is illustrated 

 in Fig. 90. Let OA, OB, and OC in Fig. 90 (a) represent the 

 primaries of three single-phase transformers which are star-connected 

 to the neutral point 0. Let A, B, and C be connected to a system 

 of three-phase supply mains, and let the instantaneous values of the 



FIG. 90. To illustrate Principle of Phase Transformation. 



star p.d.'s be reckoned positive when acting towards 0, as shown by 

 the arrows in the diagram. In Fig. 90 (b) is given a vector diagram 

 of the star p.d.'s. Now since, at a given instant, the p.d. across BC 

 in diagram (a) is the algebraical difference of the p.d.'s across the BO 

 and CO, we obtain the vector of this p.d. by compounding, in diagram 

 (b), the vector OB with OE (which is OC reversed). We thus obtain 



TWO -PHASE MAINS" 



THREf -PHASE 

 MAINS 



FIG. 91. Scott's Method of Phase Transformation. 



the vector OD, which is at right angles to OA. Hence the p.d. across 

 OA, Fig. 90 (a), differs by 90 from that across BC. In order that 

 these p.d.'s may form a two-phase system, OD, in Fig. 90 (b), must 



This is easily accomplished by 



OA 



be reduced in the ratio .-^r = 

 OD 



adopting the arrangement shown in Fig. 91. 



The primary windings 



I 



