core loss

Core loss is the energy dissipated as heat inside the magnetic core of an electrical device, regardless of whether the device is loaded or running at no load. It occurs in transformers, electric motors, generators, inductors, and any device with an iron or ferrite core carrying alternating magnetic flux. This loss happens even when the core carries no electrical current through a winding, which makes it distinct from copper loss or I squared R loss in the conductors themselves.

Core loss has two main components. Hysteresis loss arises because the magnetic material does not return to zero magnetization instantly when the field reverses; energy is absorbed in pushing the material around its magnetization curve, and this energy becomes heat. Eddy current loss occurs because the changing magnetic flux induces small circular currents within the core material itself, and these currents dissipate energy in the resistance of the iron or ferrite. At higher frequencies, eddy currents dominate; at lower frequencies, hysteresis typically contributes more.

Measuring and managing core loss

Core loss is measured in watts per kilogram or watts per cubic meter of core material, and it increases roughly with the square of the magnetic flux density and the frequency of operation. A 50 Hz transformer core loses less than a 400 Hz core made of the same material at the same flux density. Manufacturers reduce core loss by using materials with high permeability and low coercivity, laminating the core (stacking thin sheets separated by insulation to block eddy currents), annealing the material to improve its magnetic properties, and controlling flux density through core geometry and winding design.

Core loss is not load-dependent, so it represents a fixed penalty on efficiency. In a large power transformer rated for 10 MVA, core loss might consume 15 to 20 kW continuously, adding to operating cost over the life of the device. In rotating machinery, core loss generates heat that must be removed by cooling ducts or fan circulation. Poor core lamination, overfluxing due to overvoltage, or damage to the insulation between laminations can raise core loss significantly above nameplate values.

The term reflects the location of the loss rather than its cause. The core is the magnetic path, typically iron or silicon steel in power equipment or nickel-iron in precision inductors. Understanding core loss is essential when comparing transformer efficiencies, designing noise and thermal specifications for motors, or selecting materials for high-frequency power conversion circuits where eddy current loss can become dominant.