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Page 26

| Figure 2-6 Typical construction of shell-type transformer. |

| Coils or Windings |

| Transformer coils are designed to get the required number of turns into a minimum of 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. |

| For small units, the coil wire may be round, insulated with cotton, enamel, shellac, varnish, paper, or a combination of these. For larger units, the wire may be square or rectangular ribbon, usually insulated with oil impregnated paper. Where transformer operation at high temperatures is desired, special glass or plastic insulation may be used. The insulation should provide not only for normal operating voltages, but also for surges of high voltage resulting from lightning or switching. |

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