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What is an auto transformer ? What are its advantages and its limitations?

In auto transformer, a single winding is used for primary and secondary circuits.
The volt amperes  of primary winding is equal to the volt amperes  of secondary winding.

Advantages:

Since the same winding is used for primary and secondary, it requires less material for the same output than that required by two winding transformer.
Used in 400KV class  power transformers ( capacity 315 MVA/500MVA) and some time also for 220KV class power transformers.
Used as a Variac in different electrical testing.

Limitations:

Low impedance much lesser than that of two winding transformer. Hence, high short circuit currents for short circuits on secondary side.
In auto transformer, if section of winding common to primary and secondary is opened, full primary voltage will come across secondary, resulting in higher voltage on secondary causing chances of accidents.
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