186 THE DESIGN OF STATIC TRANSFORMERS 



coin-mi and eagerness that the permissible temperature rise 

 should be increased above that at present generally accepted. 



But in very small transformers (i.e., transformers from, say, 

 2 k\v rated capacity downwards), the question of heating does 

 not control the design, for if the transformer is designed for 

 good regulation and low core loss, it wilt also be a correct 

 design as regards heating. Consequently the course of events 

 as regards oil-immersed transformers will, in virtue of inevitable 

 evolutionary processes, in all probability be in the direction of 

 the general use of the core type for large sizes and of various 

 types for very small sizes. In very small sizes, particularly 

 transformers for 1 kw output and less, the labour cost and the 

 establishment charges constitute a very large component, and 

 it is less imperative to select a type requiring a minimum of 

 material. 



Quantity of oil used in transformers. In the early days 

 of oil-immersed transformers, the idea was to immerse the 

 transformer in oil, in a case of just sufficient size to contain 

 the transformer. A transformer designed on this plan to 

 comply with customary heating specifications will run at 

 needlessly-low current densities, and, except as regards pres- 

 sure regulation, the rating may be considerably increased 

 if by any means the temperature rise may be maintained 

 within specified limits. By placing the transformer in a case 

 with sufficient radiating surface, the temperature rise may be 

 kept within the desired limits without increasing the size of 

 the transformer itself, i.e., without increasing the amount of 

 material in the windings and core. 



It is still the practice of some manufacturing firms to provide 

 cases of only just sufficient size to contain the transformers and 

 to fill in the intervening space with oil. For small tranformers 

 one may, by this plan, be able to obtain sufficient radiating 

 surface, but for large ratings it is insufficient. Such practice 

 leads either to a great weight of active material per kw of 

 output, if the temperature rise is not to be excessive, or to an 



