394 



ELECTRICAL EQUIPMENT 



single-phase circuits, the two e.m.f.'s and currents being 90 elec- 

 trical degrees or one-fourth of a cycle out of phase with each other 

 (Fig. 234). 



Two single-phase transformers are mostly used for two-phase 

 systems, and the most common connection is that shown in 

 Fig. 235. The high-voltage windings of the two transformers are 

 connected respectively to the two phases of the supply mains. 



6 

 FIG. 234. 



FIG. 235. 



It is sometimes also desirable to operate a three-wire two-phase 

 distribution, as shown in Fig. 236. In this case the voltage across 

 the outside wires is \/2 or 1.41 times the voltage of each individual 

 transformer. This is clearly understood by a reference to the 



FIG. 236. 



FIG. 237. 



vector diagram in Fig. 237, and is due to the 90 phase difference 

 between the two e.m.f.'s, so that instead of adding them numer- 

 ically they must be added vectorially. The current in the neutral 

 wire is also 1.41 times the current in either of the outside wires, 

 provided the load is balanced. 



Transformers in two-phase work are sometimes intercon- 

 nected, as shown in Fig. 238, where a common return is used on 



