THE HEATING OF TRANSFORMERS 115 



cooling surface. Some tests are reported to have been made 

 on a 5,5-kw transformer of this type in comparison with an 

 ordinary transformer of the same size. The transformer- 

 operated for a sufficient length of time to obtain the ultimate 

 temperature rise. In the transformer without the ribs thi> 

 occurred after eight or nine hours, and iu a somewhat shorter 

 time in the ribbed Iran-former. It is stated that it was found 

 that the construction permitted of rating the transformer at 

 aa great a load as could, for the >ame temperature rise, be 

 obtained by the ordinary construction, and that for a given total 

 loss the temperature rise of both copper and iron is halved by 

 the use of the ribbed construction. It is further claimed that a 

 more uniform temperature was secured throughout all parts of the 

 transformer. It is stated that no appreciable eddy loss occurred 

 in the ribs, and that the efficiency of the 5,5-kw transformer 

 about 94,4 per cent. 



OIL-IMMERSED TRANSFORM i . 



The natural circulation of air affords insufficient means of 

 dissipating the heat from other than very small transformers, 

 and the simplest alternative consists in immersing the tran*- 

 former in a tank of oil. If the case is filled with oil and if 

 suitable attention is given to providing for a free natural 

 circulation of oil through all portions of tin transformer, then 

 tli.- ultimate temperature rise of an oil-immersed tran>f.>rm. -r 

 will be proportional to the watts total loss in the active 

 material of the tnii^f<>nncr pi-r square decimeter of external 

 radiating >urface of the case. Oil-immersed tran>fon 

 were employed in England at an early dftte, but soon came 

 into disrepute owing, in my opinion, to a failure to understand 

 the requirements of a design of this kind and to the on 

 poor grades of oil. Suitable iran-l'Mi-mer oil is expensive, but 

 the use of the best qualities should be rigorously insisted nj 

 and, furthermore, the greatest of care should be exercised in 



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