138 THE DESIGN 01- STATIC TRANSFORMERS 



of several makers, curves in which the quantity of oil employed 

 is plotted as a function of the rated output. Since these curves 

 relate to the product of different makers, they should not be 

 taken as representing progress in transformer design during 

 the past few years, but rather as representative of widely- 

 employed proportions. The values given by the curve apply- 

 ing to my own designs appear very high as compared with the 

 values corresponding to the other curves. This is accounted 

 for by several reasons. The transformers corresponding to 

 this curve are designed with modern alloyed steel laminations 

 which render the transformers comparatively small as com- 

 pared with the older makes to which the other curves in 

 Fig. 82 refer. The cases and quantities of oil for these 

 transformers are so proportioned as to ensure an ultimate 

 temperature rise of not more than 40 C. above the surrounding 

 air. 



With transformers having as small quantities of oil as corre- 

 spond to some of the curves in Fig. 82, it is doubtful whether 

 the temperature rise would not be excessive if the transformers 

 were run at full load for many hours or until the ultimate 

 temperature corresponding to continuous running is attained. 

 Under practical conditions, a transformer is, after shipment, 

 rarely operated at full load for other than short periods, and 

 thus there is rarely occasion to observe whether the temperature 

 rise conforms to the specified limit, and there is thus a tempta- 

 tion to take unscrupulous advantage of this circumstance. 



The transformer cases proportioned to conform to my curve 

 in Fig. 82 are designed on the principle that the temperature 

 rise may be kept down to practically any value which could 

 reasonably be specified by providing a case of sufficient surface 

 to dissipate the heat generated by the losses. In other words 

 the temperature rise is, for a given type, fairly proportional to 

 the watts per square decimeter of external surface of the case. 

 Hence, a transformer may be designed to be of small dimen- 

 sions, weight and cost (for copper and core), provided that it 



