THE HEATING OF TRANSFORMERS 



123 



ratings of from 1 to 50 kva. The temperatures plotted in 

 the curves are those of the windings as determined by 

 mea>uring the resistances before and after the tests. In the 

 instances of the 20-, 30-, and 50-kva transformers, when run 

 in jiir, the figures for the ultimate temperature rise (curve A) 

 are not based on actual observations, but are inferred from 

 thr results of the eight-hour tests (curve B), since the ultimate 

 temperatures would have been so high as to injure the 

 insulation. The curves show that the difference in tempera- 

 tun- for air and oil operation is greater for the large than for 



KM;. 81. Carve showing time f.r r.u-kva t nuisformcr to n-:i'-h I" C. at 

 various loads. 



mall transformers. The curve in Fig. si, which n 1 

 to a 50-kva oil-immersed tninstnnner, shows the number of 

 hours during which the transformer can carry various 

 loads without sustaining a temperature rise of more than 

 40 above the temperature of the room. Tin curve >hows 

 the transformer can carry double its rated load for 



[y l\so hours without -ii-tainiii- a temperature rise of 

 moiv than 40 if, at the commencement of that time, its tnu- 



mvis that of the room in \\hieli it is installed. The 

 standard practice of m..st transformer manufacture! 

 guarantee that at rated load the temperature riM at the nd of 



