Large power transformers have their core and windings submerged in an oil bath to transfer heat and muffle noise, and also to displace moisture which would otherwise compromise the integrity of the winding insulation. Heat-dissipating “radiator” tubes on the outside of the transformer case provide a convective oil flow path to transfer heat from the transformer’s core to ambient air: (Figure below)
Oil-less, or “dry,” transformers are often rated in terms of maximum operating temperature “rise” (temperature increase beyond ambient) according to a letter-class system: A, B, F, or H. These letter codes are arranged in order of lowest heat tolerance to highest:
- Class A: No more than 55o Celsius winding temperature rise, at 40o Celsius (maximum) ambient air temperature.
- Class B: No more than 80o Celsius winding temperature rise, at 40o Celsius (maximum)ambient air temperature.
- Class F: No more than 115o Celsius winding temperature rise, at 40o Celsius (maximum)ambient air temperature.
- Class H: No more than 150o Celsius winding temperature rise, at 40o Celsius (maximum)ambient air temperature.
Article extracted from Lesson in Electric Circuits AC Volume Tony R Kuphaldt under Design Science License