Exam 13: Transformers

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The core-and-coil assembly is placed in a pressed steel tank and is completely covered with an insulating oil. This insulating oil removes heat from the core and the coil windings and insulates the windings from the core and the transformer case. The core-and-coil assembly for a typical transformer contains channels or ducts. These ducts permit the oil to circulate and remove the heat. As the oil gains heat, its density decreases and it rises. As it rises and circulates in the transformer, it contacts the tank walls and the heat is transferred from the oil to the tank walls. The walls are cooled by air circulation. The specific gravity of the oil increases as it loses heat. As a result, the oil flows down to the bottom of the tank and again circulates up through the coil ducts to repeat the cooling process.