TRANSFORMERS 



311 



Core 



full load with a temperature rise not exceeding 40 C. above 

 standard room temperature of 25 C. 



Since the output and losses in a transformer increase in pro- 

 portion to its volume or as the cube of its linear dimensions 

 while the radiating surface 

 increases only as the square 

 of the linear dimensions, as 

 the output is increased special 

 methods of cooling must be 

 adopted. 



Transformers up to 300 kw. 

 are usually immersed in tanks 

 containing oil of good insulat- 

 ing qualities. This oil serves 

 the double purpose of increas- 

 ing the insulation of the trans- 

 former and conducting away 

 the heat developed by the 

 losses. Such transformers are 

 called oil-insulated self-cooled 

 transformers. The cases are 

 made with deep corrugations 

 to give a larger radiating sur- 

 face exposed to the air. 



A second method of cooling 

 is to pump air through the 

 transformer to carry off the 

 heat developed. Transformers 

 cooled in this way are termed " air blast " and are used on elec- 

 tric locomotives where their light weight is an advantage and in 

 places where oil cannot be used on account of the danger of fire. 

 They cannot be operated satisfactorily above 30,000 volts as the 

 insulation rapidly deteriorates due to ozone set free in air at high 

 voltages. Oil is then a necessary protection. 



For air-blast transformers about 150 cu. ft. of air per minute are 

 required for each kilowatt lost, to keep the temperature within 

 the 40 C. rise allowed. 



Large transformers, above 300 kw., are cooled by placing in 

 the upper part of the tank cooling coils which carry a continuous 

 flow of cold water, which conveys the heat away from the oil. 



FIG. 286. 



Three-phase shell-type trans- 

 former. 



