For economic reasons high-temperature superconductor (HTS) machines, such as HTS motors and generators, are designed to use an iron core to reduce the HTS tape length. In these situations, the HTS tapes are positioned in the iron core and transport the ac current within the ac leakage magnetic field. We investigated the influence of the iron core arrangement on ac loss characteristics. We modified the iron core of a transformer by removing a rectangular segment and then positioned an HTS tape in the rectangular recess created. The iron core was affected by the ac magnetic field generated by the ac coil, and the HTS tape transporting the ac current. We placed the HTS tapes in different rectangular positions and varied their shape. In all conditions, the ac transport current losses were greater than losses experienced when the HTS tapes were situated without the iron core, and the ac leakage magnetic fields produce ac magnetization losses. Therefore, how the HTS tape is arranged in the iron core affects the ac loss characteristics and merits further study.