ISO THE DESIGN OF STATIC TRANSFORMERS 

 RELATION BETWEEN TEMPERATURE RISE AND OUTPUT 



Although it is practicable to increase the rating of a trans- 

 former, provided a higher temperature rise is allowable, and 

 although this is often done in practice, it is not to be recom- 

 mended. Not only is the regulation rendered worse, but the 

 insulation is liable to deteriorate if subjected to high tempera- 

 ture. The rating of a transformer designed for a temperature 

 rise of 40 C. may usually be increased some 15 per cent, for a 

 temperature rise of 50 C., and some 40 per cent, for a 60 C. 

 rise. The price of a transformer designed for a higher rating 

 and temperature rise, but with the same amount of material, 

 would be substantially the same as for the lower rating and 

 temperature rise. A transformer of a given rating and with 

 high temperature rise may, however, be built cheaper than one 

 of equal rating, but with normal temperature rise. A com- 

 parison of two transformers built by a certain large manufactur- 

 ing firm is given in Table 13, and bears on this point. 



TABLE 13. COMPARISONS OF COST OF HIGH AND Low TEMPERATURE- 

 RISE TRANSFORMERS. 



the case. The temperature guarantee of standard distributing transformers is 

 50 degrees Centigrade rise of the windings after continuous operation at normiil 

 load. If this guarantee were based on oil temperatures, rather than on that of 

 the windings, a guarantee of approximately 40 degrees Centigrade could be 

 made instead of 50 degrees Centigrade. In determining the permissible load at 

 which commercial transformers can be operated for a given time, and the 

 permissible time for a given load, this maximum operating temperature must. 

 not be exceeded." 



