A magnetic core driven by a time-varying field is not perfectly lossless. Part of the electrical or mechanical energy supplied to the device is converted into internal thermal energy in the core.

Two important mechanisms are hysteresis loss and eddy-current loss.

Hysteresis loss

A ferromagnetic material driven around a hysteresis loop does not return all of the magnetic energy supplied during one cycle. The energy dissipated per unit volume per cycle is related to the area enclosed by the $B$-$H$ hysteresis loop.

Repeating the cycle more frequently repeats that loss more often, increasing the associated average power loss. Magnetic cores intended for alternating excitation therefore often use soft magnetic materials with relatively narrow hysteresis loops.

Eddy-current loss in a core

Changing magnetic flux also induces circulating eddy currents inside a conducting core. Those currents dissipate electrical energy through the core's finite resistivity.

The general eddy-current mechanism is not specific to magnetic cores. In this application, the design goal is usually to reduce the unwanted current paths while preserving useful magnetic flux.

A common method is to build the core from thin insulated laminations rather than one solid conducting block. The insulation interrupts large current loops and increases their effective resistance. High-resistivity magnetic materials such as ferrites can suppress eddy-current loss further, especially at higher frequencies.

Core loss versus winding loss

Core losses occur in the magnetic material itself. They are distinct from winding loss, where current in the electrical windings produces resistive heating.

A transformer or electrical machine can therefore heat through several simultaneous mechanisms even when all of them ultimately draw energy from the same input source.

Design consequences

Core loss depends on quantities such as

• magnetic material and hysteresis behavior;
• flux-density excursion;
• excitation frequency;
• electrical resistivity;
• lamination thickness and geometry.

Increasing frequency can be especially costly because it repeats hysteresis cycles more rapidly and can strengthen induced-current effects.

Magnetic-core design therefore requires both magnetic and electrical reasoning: the core must carry useful magnetic flux while minimizing irreversible energy conversion into heat.