THE HKATING OF TRANSFORM Kl;s 131 



KMIMATION OK mi: TKMI'KKA i IKI: 1; 



In estimating the temperature rise in oil transformers, a 

 leading factor relates to the capacity of the case for emitting 

 li.-at from its outer surface. The temperature rise may be 

 ->ti mated from the basis of the loss per unit of external radiating 

 surface of the transformer case. Thus we require to know the 

 rate of generation of heat in the transformer per square 

 decimeter of external radiating surface of the case. This is 

 equal to the total of the watts lost in the transformer divided 

 by the total external radiating surface of the case. The 

 quotient bears a well-defined relation to the temperature rise, 

 tli. two quantities being proportional. 



The specific temperature rise is defined as the temperature 

 rise in degrees centigrade per watt per square decimeter of 

 external surface of the case, and since this quantity has a fairly 

 un i form value, the permissible value of the watts per square 

 decimeter as defined above depends on the specified tempera- 

 ture. For example, if a certain transformer has a 

 temperature rise of 40 C., and if the losses amount to 1 watts 

 IM r sj dm of external surface of the case, the specific 



t. ni|M -ratun- ri>- is equal to ^-j =J 10 per watt per sq dm. 



Tin- following data jitl'ord a rough basis for t^tiniatin^ tin- 

 temperature rise of oil-immersed tran-tormcrs : 



A. With iron caves \\itli a smooth external surface, the 

 specific temperature rise may ! taken as 10 per watt J>T 



!IM <>f <-\ten i: ,l surface of the case. Hence, for 

 ultimate rise the internal losses may amount to 4 watts 

 dm of c\termil surface. 



II. With ribbed iron cases, where tin- radiating surface 

 <d,tain-'d by tin- addition of the ribs amounts 

 the surtacc of the case without the i il.s. the i 



ma\ I., taken u increased \>\ ."n p, r rent., and the temperature 

 to 7 i 



K2 



