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Page 29

| interleaved coils are not exactly balanced, axial forces develop, also tending to distort the coils; these are usually so interleaved that forces between coils are balanced, except at the ends. Hence, when assembled, coils must be carefully centered on cores and rigidly blocked to prevent any movement, bending or distortion from normal positions under the stresses caused by heavy currents. Such bracing and blocking also tends to reduce the noise emanating from the vibration of the several elements, brought about by the effects of the alternating magnetic fields. |

| Precaution should also be taken so that a failure of the high voltage insulation to the low voltage side will not impose the high voltage on the windings of the low voltage coil. Additional insulation and barriers are often placed between the primary and secondary coils to lessen the chances of such occurrences. |

| Enclosure and Cooling Systems |

| The complete core and the coil assembly are placed in an enclosure, generally a steel tank, to protect them from the elements, vandalism, and, for safety purposes, from possible contact by people. In commercial transformers, the complete assembly is usually immersed in a special mineral oil to provide means of insulation and cooling. The oil covers the assembly completely and fills small voids, especially in the insulation, or pockets where air or contaminants may collect which may eventually cause failure of the transformer. |

| Method of Transformer Cooling.. |

| Transformers are usually cooled by oil or air (Fig. 2-10 on page 30). The wasted energy in the form of heat generated in transformers due to unpreventable iron and copper losses must be carried away to prevent excessive rise of temperature and injury to the insulation about the conductors. The cooling method used must be capable of maintaining a sufficiently low average temperature. It must also be capable of preventing an excessive temperature rise in any portion of the transformer, and formation of hot spots. This is accomplished, for example, by submerging the core and coils of the transformer in oil, and allowing free circulation for the oil. As the coil and core assembly heats up from both load and losses, it gives up its heat to the surrounding oil. The heated oil will tend to rise to the top causing the cooler oil to drop toward the bottom of the tank, thus setting up a natural circulating current of oil within the tank or enclosure (Fig. 2-11 on page 30). The oil in contact with the tank, in turn, gives up its heat to the tank walls and to the surrounding air. In this manner, the temperature of the coil and core assembly is held to safe limits. |

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