Which type of transformer has its windings connected in series?

• A. Voltage transformer
• B. Step-up transformer
• C. Autotransformer
• D. Current transformer

Answer: (D)

Windings in series describe an autotransformer. In an autotransformer, the two windings share a common portion and their connections place the windings in series, with taps that provide the desired voltage ratio. This means the same winding acts as both primary and secondary for part of the circuit, yielding a compact, often more efficient design for moderate voltage changes. The trade-off is reduced isolation between primary and secondary compared to a two-winding transformer.

A voltage transformer (potential transformer) is built to provide a scaled-down voltage while keeping isolation, with the secondary typically connected to a measuring device. A step-up transformer is a type of two-winding transformer that increases voltage but remains isolated between windings. A current transformer is placed in series with the circuit to measure current, with the secondary used for sensing, but the windings themselves are not connected in series with each other in the way an autotransformer’s windings are.