These are compared to standard-efficiency aluminum-wound units, that are designed for a 150C rise. The examples of 1,500 kVA and 75 kVA transformers in the table below are of high-efficiency, copper-wound transformers designed to achieve an 80C rise and high efficiency. Be careful that a unit carrying a low temperature rise figure is not also inefficient, using fans to remove the excess heat. For example, a transformer with an 80C temperature rise uses 13-21% less operating energy than a 150C rise unit.Ī more efficient transformer generates less waste heat in the first place, but transformer temperature rise results from not only how much heat is generated but also how much heat is removed. In that case, temperature rise is a rough indicator of transformer efficiency. It is best to obtain the actual load and no-load losses in watts from the transformer manufacturer, but sometimes those data are not available. Transformer Efficiency and Temperature Rise (So-called hot spots within the transformer may be at a higher temperature than average.) Since most dry transformers use the same insulation on their windings (typically rated at 220C), irrespective of the design temperature rise, the 80C rise unit has more room for an occasional overload than a 150C rise unit, without damaging the insulation or affecting transformer life. That means, for example, that an 80C rise dry transformer will operate at an average winding temperature of 120C when at full-rated load, in a 40C ambient environment. These values are based on a maximum ambient temperature of 40C. Liquid-filled transformers come in standard rises of 55C and 65C. Standard Ratings and Overload Capacityĭry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Transformer temperature rise is defined as the average temperature rise of the windings above the ambient (surrounding) temperature, when the transformer is loaded at its nameplate rating. In general, more efficient transformers tend to have lower temperature rise, while less efficient units tend to have higher temperature rise. The heat generated in transformer operation causes temperature rise in the internal structures of the transformer. What Is Temperature Rise of a Transformer?Īll devices that use electricity give off waste heat as a byproduct of their operation. This article is excerpted from "Premium-Efficiency Motors and Transformers", a CD-ROM available from CDA by going to the Publications List. Temperature Rise and Transformer Efficiency
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