Auto Transformers

Introduction to Auto Transformers

An auto transformer also known as an auto step down or auto regulate transformer is an transformer with only one winding. In an auto transformer, parts of the same winding acts as both the primary and secondary sides of the transformer. The windings has at least three taps where the connections are made, but normally we have multiple taps connected to a tap changer, for voltage regulation.


Since an autotransformer only got one winding with "double duty", they have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation between primary and secondary circuits. Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased MVA rating for a given size and mass.

  • Auto transformer can be used as step-down transformer before the furnace transformer.
  • Auto transformer can be used as regulating transformer.
  • Auto transformer can be arranged in the same tank as the furnace transformer.

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Auto-transformer principle
Auto-transformer principle