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AUTO TRANSFORMERS

An autotransformer is a transformer which has part of its winding common to the primary and secondary circuits. Fig. 1(a) shows the circuit for a double-wound transformer and Fig. 1(b) that for an auto transformer. The latter shows that the secondary is actually part of the primary, the current in the secondary being (I2-I1). Since the current is less in this section, the cross-sectional area of the winding can be reduced, which reduces the amount of material necessary.
Figure 1: (a) double-wound transformer (b) auto transformer

ADVANTAGES OF AUTO TRANSFORMERS

The advantages of autotransformers over double wound transformers include:
1) a saving in cost since less copper is needed
2) less volume, hence less weight
3) a higher efficiency, resulting from lower I2R losses
4) a continuously variable output voltage is achievable if a sliding contact is used
5) a smaller percentage voltage regulation.

DISADVANTAGES OF AUTO TRANSFORMERS

The primary and secondary windings are not electrically separate, hence if an open-circuit occurs in the secondary winding the full primary voltage appears across the secondary.

USES OF AUTO TRANSFORMERS

Auto transformers are used for reducing the voltage when starting induction motors and for interconnecting systems that are operating at approximately the same voltage.
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