290 



ELECTRICAL MACHINERY 



Level of oil 



<N r ^ 



5S3/4 



collect in the transformer case and tend to cause short 

 circuits, etc. Air cooling is not used to a great extent. 



Oil-cooled Transformers. The method of cooling most 

 generally used is to place the transformer in a water-tight 

 case, filled with oil. The oil serves both as an insulator 

 and as a cooling medium. Currents circulate in the oil 

 due to convection. These currents are " up " next to the 

 transformer core and coils and " down " on the outside, 

 next to the case. The heat is carried by the oil currents 

 from the transformer to the case, through which it is con- 

 ducted to the atmosphere. The 

 _______ direction of these currents is 



shown in Fig. 181. 



The use of oil in a trans- 

 former increases its capacity in 

 the same way that forced venti- 

 lation increases the capacity of 

 a motor or generator. This 

 method of cooling is the most 

 reliable there is; there is nothing 

 to get out of order as in the 

 case with air or water cooling. 



In the larger sizes difficulty 

 is encountered because of the 

 great amount of heat generated 



in the transformer; the smooth-iron case does not expose 

 enough surface to the air for proper cooling. Because of 

 this the case is made of corrugated metal, as shown in Fig. 

 182, so that the radiating surface of the case is very much 

 increased. But in very large station transformers the heat 

 cannot be radiated fast enough even with a corrugated 

 case, so that resort is had to water cooling. 



Water-cooled Transformer. A water-cooled transformer 

 is one in which the transformer is immersed in oil (as in the 

 oil-cooled transformer) but the oil itself is cooled by cir- 

 culating between coils of pipes, immersed in the oil, through 



-_^. 



FIG. 181. Arrows Show Di- 

 rection of Oil Currents 

 Around a Transformer. 



