Re: Transformer insulation class
65C is not a common temperature rise for dry type transformers. Typical temperature rises are:
55 or 65C for liquid filled and 80, 115, or 150C for dry type. Temperature rise by itself is not an indication of how much heat is generated by a transformer (that depends on losses) but rather it is how hot the insulation gets (heat causes insulation to age faster). To get a low temperature rise all you need is better heat transfer (either more surface area or a better cooling medium).
Dry type transformer temperature rise is a specification for how hot the windings of a transformer will get when at full load. This is the rise above a 40C ambient and with an additional allowance for an internal hot spot of 30C.
The insulation class is the actual temperature rating of the winding insulation. The combination of the ambient + hot spot + rise should not exceed the insulation class.
Using the standard dry type temperature rises and maximum temperature per insulation class:
Class B insulation = 150C-(40C+30C)= 80C rise
Class F insulation = 185C-(40C+30C)= 115C rise
Class H insulation = 220C-(40C+30C)= 150C rise
It is very common to find a transformer built with a high insulation class and still be designed for a low temperature rise. For example an H-80 transformer would have class H insulation but still be designed with an 80C rise, which is commonly achieved by designing the transformer to have fewer losses and therefore less heat generation.
edit: corrected formulas
[ December 04, 2005, 01:10 PM: Message edited by: jim dungar ]
