The primary and secondary windings of an Auto-transformer are

• A. None of these
• B. Both magnetically and electrically coupled
• C. Electrically coupled
• D. Magnetically coupled

Answer: (B)

In the conventional two-winding transformer the primary winding is electrically insulated from the secondary winding. The two windings are coupled together magnetically by a common core. Thus, it is a magnetic induction that is responsible for the energy transfer from the primary to the secondary winding.
When the two windings of a transformer are also interconnected electrically, it is called an autotransformer. An autotransformer may have a single continuous winding serving as both primary and secondary winding, or it can consist of two or more distinct coils wound on the same magnetic core. In either case, the principle of operation is the same. The direct electrical connection between the windings ensures that a part of the energy is transferred from the primary to the secondary winding by conduction. The magnetic coupling between the windings guarantees that some of the energy is also delivered by induction.