TRANSFORMERS 381 



in the core and coils. Sometimes external tubes or radiators, 

 through which the oil circulates, are used for the same purpose. 

 Water-cooled oil immersed transformers depend upon the circu- 

 lation of water through a coil placed in the top of the tank to carry 

 away the heat from the oil, about J gallon being required per 

 minute per Kw. loss, the temperature of the incoming water being 

 15 C. 



For conditions where long and definite periods of light and 

 heavy load occur, as in small winter and large summer service, 

 a combination self-cooling and water-cooling design has been 

 provided. Such transformers are placed in the regular sheet- 

 steel tanks of the self-cooled design, excepting that they have 

 smaller surfaces and are in addition provided with water-cooling 

 coils to take care of the super-load. They can readily be designed 

 to carry 50 per cent of the maximum load without water circula- 

 tion and not exceed the rated temperature rise. The increase in 

 the cost over the water-cooled design is slight and will often be 

 found a good investment when cooling water has any appreciable 

 value. 



Special precautions must naturally be taken to protect trans- 

 formers of the outdoor type both from the extreme heat and from 

 the cold in the winter. The former can readily be obtained 

 by providing sunshades, and in certain instances very good results 

 have been obtained by simply painting the tanks white. It is 

 more difficult, however, to provide for the cold winter temperatures, 

 especially with water-cooled transformers. With the trans- 

 formers in service there seems to be no danger of freezing, and if 

 such should be the case some sort of heating grids could readily 

 be provided in the bottom of the tanks. The main difficulty lies 

 in the formation of moisture which takes place when the temper- 

 ature of the transformer is allowed to fall below that of the sur- 

 rounding air; this applies also to indoor transformers. Pre- 

 cautions must, therefore, be taken that this does not happen, and 

 may be accomplished by either reducing the water rate at times of 

 cold weather, or by using the cooling water over and over again. 

 An oil with special low freezing-point may be used in transformers 

 in rare locations experiencing extreme low temperatures. 



Single and Polyphase Transformers. Transformers are made 

 either as single or polyphase units, the latter being generally of the 

 three-phase type. The single-phase design is by far the most 



