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Page 41

| Review |

| 1. The basic components of a transformer are the primary coil, the secondary coil, and the iron or steel core (Fig. 2-1). Other major components include the tank, oil, and bushings. Transformer coils are designed to get the required number of turns into a minimum space. At the same time, the cross section of the conductor must be large enough to carry the current without overheating, and sufficient space must be provided for the insulation and for cooling paths, if any. These coils may be made of copper or aluminum, the choice depending on the cost to achieve the low resistance and small space requirements. |

| 2. Special care is taken to hold losses down in the core. Heat created by hysteresis, friction from the reversing magnetic molecular particles (Fig. 2-2), is minimized by using a special grade of heat-treated, grain-oriented silicon steel. Heat created by eddy currents, created when the alternating magnetic field cuts through the metal core (Fig. 2-3), is minimized by laminating the core, and insulating the adjacent laminations with insulating varnish. Cores may be the core type, where the windings are outside the core (Figs. 2-4 and 2-5), or of the shell type, where the core surrounds the coils (Figs. 2-4 and 2-6). |

| 3. The magnetic fields in both coils repel each other; during short circuits, these forces may become large enough to wreck the transformer. Hence, the coils must be suitably wound (Figs. 2-7 and 2-9), mounted on the core, and firmed to prevent movement, bending or distortion; this may be done by suitable bracing and blocking. (This incidentally also reduces noise from the vibration of the several parts.) Extra insulation and barriers are often used as a precaution to keep a failure on the high-voltage winding from imposing a high voltage on the low-voltage coil. |

| 4. The coils and core are usually enclosed in a steel tank to protect them from the elements, vandalism and, for safety purposes, from possible contact by people. Oil is usually placed in the tank to cover the coils and core to provide a means of cooling (Fig. 2-11) and filling small air voids which may eventually cause failure. Moisture reduces the dielectric strength of oil, while oxygen helps form sludge. Moisture is condensed in the tank from changes in volumes of air and oil due to changes of temperature of the transformer itself or that of the surrounding air. Sludging is principally due to decomposition of oil resulting from exposure to the oxygen of the air while hot. Accessories to keep air from affecting the oil use gas seals (Fig. 2-12) or conservators (Fig. 2-13). |

| 5. Increased cooling, and hence increased transformer capacity, may be obtained by the use of radiators which increase the area of cooling surfaces, by fans to increase the movement of air over the cooling surfaces, and by pumps to increase the circulation of oil (Figs. 2-14, 2-15, |

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