What Are Magnetic Losses?

Magnetic losses are the energy dissipated within a magnetic material when it is subjected to a time-varying magnetic field. Any transformer core, inductor winding, or rotating machine stator converts a portion of the electromagnetic energy passing through it into heat, reducing efficiency and requiring thermal management. These losses are a central concern in power electronics, electric machine design, and high-frequency magnetics, where operating frequency, flux density, and material microstructure all interact to determine how much energy is wasted in each cycle.

The total energy lost in a magnetic core is often called core loss and is commonly separated into two principal components: hysteresis loss and eddy-current loss. A third component, excess or anomalous loss, accounts for discrepancies between predictions from classical theory and measured values, particularly in soft ferromagnetic alloys.

Eddy Currents

Eddy currents are circulating electrical currents induced inside a conductive magnetic material by Faraday induction when the flux density changes with time. Because the material has finite resistivity, these currents dissipate energy as resistive heat. Eddy-current loss scales with the square of both the frequency and the peak flux density, making it the dominant loss mechanism at elevated operating frequencies. The standard mitigation strategy is lamination: dividing the core into thin sheets separated by insulating layers constrains eddy currents to smaller cross-sectional areas, reducing the induced current magnitude and thus the loss. Ferrite cores, which have much higher resistivity than silicon steel, are preferred for frequencies above roughly 100 kHz because their near-insulating ceramic structure suppresses eddy-current paths almost entirely. A review of power losses models for magnetic cores published in PMC surveys the classical models for eddy-current loss and their limitations under non-sinusoidal flux waveforms common in switched-mode power supplies.

Hysteresis Loss

Hysteresis loss arises from the energy required to cyclically reverse the magnetic domain alignment in a ferromagnetic material. Each time the applied field reverses, domain walls must move against pinning sites in the material's crystal structure, dissipating energy that is proportional to the area enclosed by the B-H hysteresis loop. Hysteresis loss is linear in frequency and depends on the choice of core material: grain-oriented silicon steel, amorphous ribbons, and nanocrystalline alloys are all engineered to minimize coercivity and loop area while maintaining high saturation flux density. Charles Proteus Steinmetz formulated an empirical power-law relationship in 1892 describing hysteresis loss as a function of peak flux density, and the Steinmetz equation and its modern variants remain standard tools for core loss estimation. A core loss estimation study using an improved Steinmetz equation published in AIP Advances demonstrates how asymmetric triangular waveforms, as produced by inverters, deviate from the classical model and require modified coefficients for accurate prediction.

Loss Measurement

Accurate separation and measurement of magnetic loss components is essential for material characterization and for validating simulation models. The two-winding method measures total core loss by applying a known voltage to a primary winding and measuring the in-phase current component; subtracting winding copper losses yields the core loss. The separation of core losses in distribution transformers using harmonic analysis provides a method to distinguish hysteresis and eddy-current contributions from frequency-sweep measurements without requiring cryogenic separation techniques.

Applications

Magnetic losses are a primary design constraint in:

• Power transformers and distribution equipment, where core loss determines no-load efficiency ratings
• Switched-mode power supply inductors and transformers operating at frequencies from tens of kilohertz to several megahertz
• Electric motor stators and rotor laminations in traction drives and generators
• Wireless power transfer coils, where high-frequency excitation demands low-loss ferrite or nanocrystalline cores
• Electromagnetic shielding materials, where controlled loss is used to absorb rather than reflect incident fields