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Page 27

| Figure 2-7 Concentric windings. |

| In the core-type transformer, the low-voltage coils are usually placed next to the core and the high-voltage coils are external and concentric with them. This reduces the insulation requirements of both coils; if the high-voltage winding was placed next to the core, two layers of high-voltage insulation would be required, one next to the core, and the other between the two windings (Fig. 2-7). (Sometimes, where large and heavy connections are involved, this arrangement may be reversed.) The high-voltage windings may be separated from the low-voltage windings by insulating cylinders; the high voltage may be composed of several disc-shaped coils, each disc insulated from others by insulating strips. If the windings were placed on separate legs of the core, a relatively large amount of the flux produced by the primary windings would fail to link the secondary winding, resulting in a large loss of effectiveness of the flux. |

| For very small sizes of transformer, a cruciform-type core (cross-shaped) is often used for economy (Fig. 2-8 on page 28). In both this type and the core-type transformers, the coils may be made cylindrical in shape, which enables the insulation to have a factor of safety since there are no sharp bends in the insulating material and a better opportunity for the radiation of heat. Both these factors permit the use of less material for a given output. |

| In the shell-type transformer, pancake construction is often employed (Fig. 2-9 on page 28). Here, the high-voltage and low-voltage coils are alternately placed around the core, with the required insulation between them, each coil having the rough appearance of a pancake. Often, space is left between coils for cooling purposes. Such an arrangement of coils reduces the reactance between the coils and improves the operation of the transformer, |

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