ALTERNATING CURRENTS 



are assembled around them, these bundles being arranged in radial 



planes. For the sake of clearness only three of these are shown in 



the figure. 



In Fig. 83 is shown another type of construction. The coils are 



of oblong shape, the core-sheets being built up around them. The 



thick lines in the lower figure indi- 

 cate insulating barriers between the 

 sections of the high-voltage and those 

 of the low-voltage windings. The core- 

 plates are pressed together between 

 two heavy end-plates connected by 

 bolts passing outside the core-plates. 



For three-phase work, three single- 

 phase transformers may be used. A 

 cheaper form of construction, however, 

 results by combining the cores of the 

 transformers so as to form a single 

 three-phase core. One method of doing 

 this is shown in Fig. 84.* The coils 

 (not indicated in figure) are placed 

 around three upright laminated cores, 

 connected at the top and bottom by 

 laminated yoke-rings. The cores are 

 pressed against the yoke-rings by means 

 of end- plates fitted with conical rims, 

 which bear against the chamfered ends 

 of the cores. In this, as in all other 

 cases where butt joints are used, thin 

 sheets of insulating material are inter- 

 posed at the joints, in order to prevent 



FIG. 83. Type of Transformer Con 

 struction. 



the formation there of conducting grids 

 (by the contacts between the two sets 

 of plates), which would cause dissipation of energy by eddy currents. 

 The algebraical sum of the magnetic fluxes in the three cores 

 being zero, any two of the cores will, at a given instant, form a 

 return magnetic circuit for the flux in the third core, so that the 

 principle of a closed magnetic circuit will be realized. 



In Fig. 85 is shown another method of combining the cores of 

 the three transformers. The coils are shown in position in the lower 

 part of the figure. This arrangement is not quite symmetrical, the 

 flux passing through the middle core encountering a somewhat lower 

 reluctance on account of the shorter average length of path than 

 that passing through either end-core. 



* This form of construction was a favourite one in the early days of three-phase 

 currents, but is now being abandoned, mainly on account of its expense. 



