198 ALTERNATING CURRENTS 



The secondaries, or low-side coils, are placed adjacent to the iron in 



order to minimize the amount of high-voltage insulation required. 



87. Type H Transformer. In designing a transformer it is 



desirable that the mean length of turn be as short as possible. 



This reduces both the weight of copper and the resistance and 



reactance of the winding. This is accomplished in the Type H 



transformer of the General Electric Co. by making a shell-type 



. transformer in which the core is cru- 



ilica Sbieldi HDP Secondary > 



ciform in shape, as shown in Fig. 185. 

 The central core around which the 

 coils are wound is operated at much 

 higher flux density than the four 

 wings. Although the reluctance and 

 channels i osses j n this CO re are high, they are not 

 FIG. 185. Core and windings excess i ve w hen the entire magnetic 



of type H transformer. ... _,, 



circuit is considered. These trans- 

 formers are used mostly as distribution transformers for stepping 

 down from 2,200 and 1,100 volts to 220 and 110 volts, so that the 

 primary is the high side. It will be observed that the low side, 

 the secondary, is next the iron. That is, one of the two low-side 

 coils is next the central core and the other is next the iron of the 

 four wings. The two high-side coils lie between the two low-side 

 coils, and are not adjacent to the iron. The two high-side coils 

 are insulated from the low-side coils by the mica shields repre- 

 sented by heavy lines. The advantage of this design is that 

 only moderate insulation is required between the low-side coils 

 and the core. As high-voltage insulation need be used only 

 between the high- and the low- voltage coils, a minimum amount 

 of high-voltage insulation is required. 



In designing a transformer, provision should be made for keep- 

 ing it cool. Spaces or ducts should be left between coils and 

 between coils and core. Such ducts, or channels, are shown in 

 Fig. 185. The oil in these ducts becomes heated, its specific 

 gravity decreases, and the oil rises. When it comes in contact 

 with the transformer case it cools, which increases its specific 

 gravity, and it therefore flows downwards outside the trans- 

 former coils, and is subjected to further cooling. There is a 

 continuous circulation of oil up through the coils and the core 



which carries away the heat. 



-^ ^ **'" - 



PROPERTY CF ELECTRIC. AlURY, \ 



FACULTY OF APPLIED SCK 4 



