402 ELECTRICAL EQUIPMENT 



If this is not done, the direct current will produce a uni-directional 

 magnetism in the transformer, which, superimposed on the mag- 

 netic cycle, would tend to raise the magnetic induction beyond 

 saturation, and thus cause excessive exciting current and heating 

 except where the unbalanced current is comparatively small. 

 Such a connection is shown in Fig. 246 which represents a delta- Y- 

 connected step-down transformer with the neutral brought out. 

 It is evident that in this case each transformer low- voltage winding 

 receives one-third of the neutral current, and if this current is not 

 small, as compared with the exciting current of the transformer, 

 it will cause an increase in the magnetic density. 



A system with a distributed Y or " zig-zag " connected low- 

 voltage winding, as shown in Fig. 247, has, however, been devised, 

 and will eliminate the flux distortion due to the unbalanced 

 direct current in the neutral. Two separate interconnected wind- 

 ings are used for each leg of the Y. The unbalanced neutral cur- 

 rent flowing in this system may be compared in action to the 

 effect of a magnetizing current in a transformer. The effect of 

 the main transformer currents in the high- and low-voltage wind- 

 ings is balanced with regard to the flux in the transformer core, 

 which depends upon the magnetizing current. When a direct- 

 current is passed through the transformer, unless the fluxes 

 produced by the same neutralize one another, its effect on the 

 transformer iron varies as the magnetizing current. For example, 

 assume a transformer having a normal ampere capacity of 100 and, 

 approximately, six amperes magnetizing current, and assume 

 that three such transformers are used with Y-connected low- 

 voltage windings for operating a synchronous converter connected 

 to a^three-wire Edison system. Allowing 25 per cent unbalancing, 

 the current will divide equally among the three legs giving 8.33 

 amperes per leg, which is more than the normal magnetizing cur- 

 rent. The loss due to this current is, however, inappreciable, 

 but the increased core losses may be considerable. If a dis- 

 tributed winding is used the direct current flows in the opposite 

 direction around the two halves of each core, thus entirely neu- 

 tralizing the flux distortion. 



Whether the straight Y or the interconnected Y connection is 

 to be used is merely a question of balancing the increased core loss 

 of the straight Y connection against the increased copper loss and 

 the greater cost of the interconnected Y system. The straight Y 



