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| Figure 6-1 An autotransformer. |

| In this transformer, the same ratio of transformation holds as in the two-winding transformer: |

| Where the primary current IP flows in the direction shown by the (closed head) arrow in the figure, the secondary current IS will flow in the opposite direction. Hence, in the portion of the winding between b and c, the current is only the difference between IP and IS1. When the ratio of transformation is small, the difference between EP and ES is small, as is the difference between IP and IS1, so that the portion of the winding between b and c, which carries difference of these currents, can be made of smaller conductor since it will carry only a small current. The portion of the winding between a and b, however carries the sum of the currents IP (the exciting current) and IS2 which constitutes the relatively large untransformed current, and must be made of a conductor sufficiently large to carry this sum. |

| The percent of the volt amperes transformed is the same as the percent of the voltage transformed (using the high voltage as a base). For example, if the autotransformer boost the voltage 10 percent, it actually transforms only 10 percent of the volt-amperes (or kVA) supplied to the load. And since the size of the unit depends on the volt-amperes (or kVA) transformed, the size of the autotransformer need be only 10 percent of the loadif the load to be supplied was 50 kVA, the size of the autotransformer would be only 5 kVA. |

| Under these circumstances, it is obvious that the autotransformer is more economical than the two-winding transformer. However, as the ratio of transformation increases, the advantages decrease and generally disappear at a ratio of approximately 5 to 1. The need for a large part of the coil to be made of heavy conductor necessarily having the same heavy high-voltage insulation since both portions are connected electrically adds considerably to the cost. |

| One disadvantage of the autotransformer is that, because of the metallic connection between the high and low-voltage circuits, each circuit is affected by electric disturbances originating in the other. For example, a ground on either circuit is a ground on both circuits, and a ground on the high-voltage side may subject the low-voltage side (and the connected loads) to the high |

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