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| Figure 2-8 Cruciform-type core. |

| particularly in large-size transformers where heavy currents are experienced. The shell-type transformers also make arrangements for air cooling paths simpler and easier to provide. |

| In transformers, all coils which carry current in the same direction attract one another and coils which carry currents in opposite directions repel one another. Hence, all the coils of the primary attract one another as do all the coils of the secondary; however, primary coils repel the secondary coils and vice versa. Under normal operating conditions, these forces are relatively small. In case of short circuit or the carrying of very large currents, these forces may become great enough to damage the transformer if the coils are not adequately supported. |

| Coils and cores must be mechanically capable of withstanding these short circuit stresses. Where the coils are concentrically placed, the forces produced are radial which may tend to distort the shapes of the coils. If |

| Figure 2.9 Shell-type unit (pancake coils). |

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