308 ELECTRICAL MACHINERY 



are used to a considerable extent. If three single-phase, 

 core type transformers were wound each on one leg only 

 and the three empty legs were combined to make the return 

 path for the flux of each transformer we would have essen- 

 tially a three-phase transformer. The flux carried by the 

 common return path, however, would be zero (because 

 three equal harmonic fluxes 120 apart in time have a zero 

 resultant). Hence as the common return leg carries no 

 flux, it is useless and so may be dispensed with. 



FIG. 199. A Three-phase Transformer Removed from its Tank. 

 General Electric Co. 



Three-phase Transformer. By this modification the three 

 leg, three-phase transformer is reached, having the primary 

 and secondary windings of one phase on each leg. The design 

 mentioned above, altered slightly to make the core easier to 

 construct, yields a transformer of the form shown in Fig. 199. 

 It may be seen that no common leg is used in such a design. 



The three-phase transformer is slightly more efficient than 

 three single-phase transformers as there is less iron per kw. 

 output, but the cost of upkeep and of spare units is greater. 

 In general three single-phase transformers are used because 

 of the greater reliability and flexibility of the installation. 



