A transformer that has the secondary winding connected in series with the primary at a junction terminal is called a(n) _____.

An autotransformer is defined by its configuration, where the primary and secondary windings share a common winding or portion of the winding. In this design, the secondary winding is effectively part of the same coil as the primary winding but functions at a different voltage. The key characteristic of an autotransformer is that it provides voltage transformation through a single winding that is tapped at a specific point, allowing for a more compact and efficient design compared to traditional transformers that have separate primary and secondary windings.

Autotransformers are often used in applications where a relatively small change in voltage is required, such as in voltage regulation, motor starting, and other scenarios where efficiency is important. This setup allows for direct electrical coupling of the primary and secondary sides, which can lead to a reduction in size and weight compared to dual-winding transformers.

In contrast, a step-up transformer increases voltage while having distinct primary and secondary windings, an isolation transformer serves to decouple two circuits without changing voltage levels, and a delta transformer represents a specific three-phase connection configuration. These variations each serve different functions and applications, reinforcing the unique role of an autotransformer in electrical systems.