206 



ALTERNATING CURRENTS 



fluxes must also be zero. No appreciable flux exists in the com- 

 mon leg and this leg may be eliminated, therefore, without disturb- 

 ing existing conditions. A more practical arrangement, from the 

 construction standpoint, is shown in Fig. 194 (6). The reluc- 

 tance of the magnetic circuit for the center coil is less than it is 

 for the two outer coils. This makes the magnetizing current of 

 the middle phase slightly less than that of the two outer phases, 

 but the magnetizing currents are so small that this has no notice- 

 able effect on the operation of the transformer. 



Figure 195 shows a three- 

 phase, shell-type transformer. 

 It does not differ from three 

 single-phase, shell-type trans- 

 formers laid side by side. 

 Owing to the joint use of the 

 magnetic paths between the 

 coils, there is less iron in this 



type of transformer than in 

 three equivalent single-phase 

 units As each phase has a 

 magnetic circuit independent 

 of the others, the three phases 

 are more independent of one 

 another than they are in the 

 a complete three-phase trans- 



FIG. 195. Arrangement of coils and 

 laminations in 3-phase, shell-type trans- 

 former. 



core type. Figure 188 shows 

 former with a tubular tank. 



The lower cost of three-phase transformers and the smaller 

 space occupied by them is often balanced by the fact that if any 

 one phase becomes disabled, the whole transformer must ordi- 

 narily be removed from service. (The shell type may be oper- 

 ated open delta at 58 per cent, of its rating, but this is not always 

 feasible.) If one transformer of a three-phase bank of single- 

 phase transformers becomes disabled, the system may run open 

 delta at reduced capacity or the transformer may be replaced 

 by a single spare which can be readily substituted. 



90. Auto-transformers. Figure 196 shows a drop wire, having 

 a resistance of 10 ohms, connected across a 100-volt supply. 

 The supply may be either direct OF alternating current. A load 

 having a resistance of 5 ohms is tapped to the middle of the drop 



