144 THE DESIGN OF STATIC TRANSFORMERS 



I ' ntilation Ducts. It is desirable that the temperature 

 throughout the transformer should he as uniform as possible, 

 and that there should be no excessive local heating at any 

 part of the transformer. In the interests of achieving this 

 result, the windings and core should be provided with ducts 

 or channels through which the oil can circulate. In ohU-r 

 designs of transformers this provision was not made, and the 

 maximum temperature attained was considerably in excess of 

 the temperature at any accessible part. In a modern trans- 

 former liberally provided with wide ventilating ducts, there is 

 much less liability to local heating, and the ratio of the 

 maximum temperature to the highest accessible temperature 

 does not vary greatly from unity. 



In large transformers it is absolutely essential in the interests 

 of maintaining the insulation in good condition, to provide 

 ventilating ducts both in the windings and the core. In the 

 case of transformers of the core type I usually prefer the 

 arrangement of the ventilating ducts illustrated in Figs. 49, 

 50, 51, 53, 54, and 55. In this construction the coil is 

 separated from the core by means of specially-shaped distance 

 strips of suitably-impregnated wood or other material. In this 

 manner, parts of the inside of the coils, as well as of the out- 

 side, are subject to the cooling eifects of the oil. It is most 

 desirable to avoid undue local heating, as this conduces to 

 ageing of the iron, and to deterioration of the insulation. 



