Which transformer type has windings that share a common part?

The autotransformer is characterized by having a single winding that serves as both the primary and secondary winding. In this configuration, a part of the winding is common to both sides of the transformer. This design allows for a more compact and efficient way to step voltage levels up or down, compared to traditional transformers that have separate primary and secondary windings.

In autotransformers, the voltage can be varied by tapping into different points along the winding, which contributes to both the size and the cost-effectiveness of the autotransformer. This unique attribute makes it particularly useful for applications where the voltage change is small, as less copper is used compared to standard transformers.

Other transformer types, such as isolation transformers, step-up transformers, and three-phase transformers, do not operate on the principle of sharing a common winding between the primary and secondary sides in this way. Their designs require distinct windings to achieve their intended functions, which distinguishes them from the autotransformer. Thus, the autotransformer stands out due to its shared winding configuration, leading to its designation as the correct answer in this context.