376 ALTERNATING CURRENTS 



symmetrical cyclic magnetization of the iron. This is undesir- 

 able, as it results in an increased magnetizing current whose 

 positive and negative values will be unequal and dissimilar. By 

 splitting each transformer secondary into two sections, a,a', 

 b,b', and c,c r , Fig. 341, and connecting as indicated, it will be 

 observed that the direct current flows in opposite directions in 



B 

 Primaries 



Secondaries 



FIG. 341. Zig-zag connection of three-phase secondaries for eliminating D. C. 

 magnetizing of transformer cores. 



the two-halves of each secondary winding, and consequently, has 

 no appreciable magnetizing effect. 



Figure 338 shows the complete connection for a six-phase, 

 220-volt, three-wire converter, having two series fields and with 

 the direct-current neutral connected to the neutral of the double- 

 Y connected transformer secondaries. Figure 340 shows a 

 direct-current system supplied by a four-phase converter, the 

 neutral being obtained through two star-connected transformers. 



