410 



ELECTRICAL EQUIPMENT 



transformation. All values except for power are given with 

 reference to straight single-phase as unity. The total value of 

 power delivered is the same in all cases. By straight single-phase 

 is meant connecting one transformer between two wires of a three- 

 phase system. The only condition under which there seems to 

 be an advantage is in schemes 1 and 3 where it will be noticed 



A, 1.1551 



B, 2.31 I 



C, 1.1551 



Power Factor = .866. 



FIG. 251. 



Power Factor = . 666 



FIG. 252. 



30, .635 

 = 10, I- 

 Av. .778. 



FIG. 253. 



FIG. 254. 



that a delta-connected generator has a maximum current of 0.577 

 as against 0.667 for the straight single-phase. To offset this, both 

 schemes 1 and 3 require two transformers possessing greater total 

 capacity and also imposed upon the line a greater maximum 

 current. 



Two-phase to Six-phase. The double-T connection, as shown 

 in Fig. 255, is generally used in cases where a six-phase synchronous 

 converter is to be operated from a two-phase supply system, and 

 where the two-phase voltage requires some transformation in 

 order to obtain the correct alternating-current voltage for the 

 converter. The cost of double-T-connected transformers and a 

 standard six-phase rotary converter will occasionally be less than 

 that of two-phase transformers and a special two-phase converter. 



