Overloading within limits
Overloading within limits
Dry-type transformers are available in standard temperature rises of 80°C, 115°C, or 150°C. These values are based on a maximum ambient temperature of 40°C for spot temperatures and 30°C average ambient temperature over a 24 hour period. That means, for example, that an 80°C rise dry transformer will operate at an average winding temperature of 120°C when at full-rated load, in a 40°C ambient environment without damaging any one spot on the transformer winding, however, if a long time period of loading is expected the 30°C ambient average is a better target to use since hot spots within the transformer may be at a higher temperature than average. Most dry transformers use the same insulation on their windings (typically rated at 220°C), irrespective of the design temperature rise, the 80°C rise unit has more room for an occasional overload than a 150°C rise unit, without damaging the insulation or affecting transformer life. However, the 150°C unit is better able to cool itself from a temporary overload than an 80°C unit. Thus the 150°C rating even though it will be less efficient than the 80°C unit. Overall the insulation rating is given as the temperature rise + maximum ambient + Hot Spot Allowance = Maximum wind temperature. Thus 150° + 40° + 30° = 220°C.
You do have to be careful when determining temperature rise also sine the ANSI definition of temperature rise is the average rise as determined by change in D.C.resistance of the windings. You may have about a 15% margin to play with but an alternative may be to add forced cooling in the form of fans to draw air through the unit at a higher than normal CFM cooling flow with natural convection. I would suggest you check with the manufacturer for recommendations.
Hope this helps,
NewtonLaw
