TRANSFORMERS 



415 



voltage. The respective current capacities are 0.421, 1, and 1 

 times the line current. 



One advantage of this system is the fact that voltages and 

 currents do not exceed those which would occur in single-phase 

 operation, giving an internal power-factor of the system of 100 

 per cent, whereas in the T connections the average power-factor 

 is only 96.4 per cent. The three-phase side may be connected 

 either delta or Y. This connection, requiring less copper and 



FIG. 258. 



being slightly more efficient than the T connection, is recom- 

 mended in place of the T connection for three-phase units, pro- 

 vided no taps are required on the two-phase side. If single-phase 

 units are desired, the use of this connection becomes doubtful 

 owing to the multiplicity of leads and coils on the two-phase side. 

 The connection is very seldom used, principally on account of 

 the electrical interconnections of the phases on the two-phase 

 side. This prevents it from being used on a three-wire system, 

 while, on the other hand, a cross between the two phases results in 

 a short-circuit. 



Three-phase to Three-phase Two-phase. It is possible by means 

 of transformer connection to derive from a three-phase primary 

 circuit a four-wire secondary circuit, three wires of which rep- 

 resent a three-phase system and the four wires making a two-phase 

 system. From such a system independent three-phase or two- 

 phase loads may be taken simultaneously. This may be accom- 

 plished by three single-phase transformers provided with special 



