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Page 89

| 6 Other Transformer Types.. |

| Autotransformers |

| Autotransformers may be viewed as a particular type of transformer, whose characteristics make it more advantageous to use than two-winding transformers under certain conditions. |

| When the ratio of transformation desired is low, usually not greater than about 5 to 1, and the electrical isolation feature of the two-winding transformer is not essential, use is made of the autotransformer. |

| The autotransformer has only one winding, a portion of which serves both as primary and secondary. In this type transformer, only a portion of the electrical energy is transformed and the remainder flows conductively through its windings. (In a two-winding transformer, all the energy is transformed.) The autotransformer, therefore, having to provide only for that portion of the energy to be transformed can be of smaller size than the equivalent two-winding transformer; this results in lower cost, not only of the unit, but also in its installation. Also, since it handles only a portion of the transformed energy, the losses caused by currents flowing through it are lessened resulting in greater efficiency and better voltage regulation. |

| A schematic diagram of an autotransformer is shown in Fig. 6-1 (on page 90). |

| If a source of voltage EP is applied across all the turns between a and c, this part of the coil will serve as a primary winding. Some of the turns, between b and c, will also serve as a secondary winding giving the voltage ES. This arrangement uses the transformer for step-down purposes, reversing ES and ES will allow the transformer to be used equally well as a step-up transformer. |

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