Autotransformers

One winding, tapped

A conventional transformer has separate primary and secondary windings. An autotransformer has a single winding with a tap, and the input and output share part of it. Only the difference in voltage is transformed, so for a modest ratio the unit is far smaller and cheaper than a two winding equivalent of the same kVA.

The limit

There is no galvanic isolation. A fault or a high voltage on the input side appears on the output side. That is why autotransformers are used freely for modest ratios between systems that share a ground reference, and are generally not used to derive a low voltage system from a medium voltage one, where the consequences of that shared path are severe.

Where you meet them

Buck boost units, motor starting autotransformers that reduce voltage during start, and utility voltage regulators. In distribution work the pad mounted transformer feeding a building is nearly always a two winding machine, precisely because the isolation matters.

What is the difference between an autotransformer and a regular transformer

An autotransformer uses one winding with a tap, so input and output share part of it. A conventional transformer has two separate windings with no electrical connection. The autotransformer is smaller and cheaper for a given rating but provides no isolation.

Why is an autotransformer smaller than a two winding transformer

Because only the voltage difference is actually transformed. The rest of the power passes through the shared winding conductively, so less iron and copper is needed for the same throughput.

Is a buck boost transformer an autotransformer

Yes. A buck boost unit is a small transformer connected in an autotransformer configuration, which is exactly why it is so compact for the load it supports.