A core-type transformer is constructed from a stack of U and I shaped laminations, which are interchanged from one layer to the next for better mechanical integrity. The laminations are clamped together for a solid structure.
In a core-type transformer, the primary and secondary windings are wound on two different legs of the core. This results in a large leakage flux. The effect of leakage flux is additional reactances in series with the primary and secondary windings, contributing to additional voltage drop under load and resulting in poor voltage regulation.