Transformer Temperature Rise Ratings

When a temperature is at no load, its temperature is slightly greater than the ambient temperature.  When the transformer is loaded the temperature rises.  The temperature rise rating of a transformer gives the maximum value to which the temperature of the transformer would rise. 

Dry type transformers are usually available in three standard temperature rises, 80C, 115C or 150C.  Liquid filled transformers have ratings of 55C and 65C.  These values are based on a reference value of 40C.

For instance, a transformer with a temperature rise rating of 80C will reach a maximum temperature of 120 C (40+80) during operation. 

The lower the temperature rise rating, the better is the ability of the transformer to withstand momentary overloads.  Thus a transformer with a temperature rise rating of 80C will have a better overloading capacity than a transformer with a temperature rating of 120C.

The temperature rise rating gives and idea of amount of heat produced and the amount of heat removed.  Transformers with lower temperature rise ratings use windings with lower resistivity. 

Transformers with low temperature rise are used in special applications such as in underground installations, air conditioned buildings