314 



ELECTRICAL ENGINEERING 



To supply a load of 300 kv.a. from a three-phase system three 

 100 kv.a. transformers are required if connected in " delta " or 

 two 173 kv.a. transformers if connected in " open delta." 



In the vector diagram in Fig. 291 EI, E z and E 3 are the 

 e.m.f.'s generated in the three transformers of a " delta " system; 

 they are also equal to the terminal e.m.f.'s at no load in the " open 

 delta." Iiy 1%, Is' are the load currents with a closed delta. 

 Transformer (3) is disconnected and the two remaining trans- 

 formers (1) and (2) carry currents /i and 7 2 respectively and the 

 voltages between the lines are EBC, EGA and EAB. These voltages 

 are never exactly balanced and they become more and more un- 

 balanced as the load is increased. 



195. Transformation from Two Phase to Three Phase. The 

 " Scott " connection or " T " connection is an arrangement of 

 two transformers by means of which three-phase power can be 

 obtained from a two-phase system or the reverse. (Fig. 292.) 



FIG. 292. Two-phase to three-phase. 



FIG. 293. Non-inductive 

 load. 



Two similar transformers are used with their primaries con- 

 nected to the two-phase system. The secondary A B of one is 

 tapped at its middle point and the secondary FD of the other has 

 a tap at a point 86.6 per cent from the end D. The two taps are 

 connected at C. The part FC of the winding of the second trans- 

 former is not used. 



Three-phase power can be taken off at the terminals A, B and C. 

 If the e.m.fs. generated in the winding FD is 



FD = EQ sin 0, 



then the e.m.fs. generated in the various sections are 

 BCD = 0.866 EQ sin 6, 

 CA = 0.5 E Q sin (B - 90), 

 e CB = 0.5 #o sin (B + 90), 



