There is a changing magnetic field in the core. In addition to generating current in the winding, the induced current is also generated in the core itself, which is called eddy current. Since the core itself is also a conductor, it can also be regarded as a coil. In order to reduce such loss, the core should use silicon steel material to increase its resistivity to reduce this current, form a thin sheet and use paint or oxidized insulating layer between the laminations to prevent the formation of eddy currents. Conductive silicon steel sheets are made of ferrite materials with higher resistivity, which are used to reduce the loss of induced current in the core. Of course, in addition to the transformer eddy current loss, the core loss also causes hysteresis loss, which also causes the core to heat up.