What Are Eddy Currents?

Eddy currents are closed loops of electric current induced within a conductive material when it is exposed to a changing magnetic field, in accordance with Faraday's law of electromagnetic induction and Lenz's law. The term "eddy" refers to the circular or swirling path these currents follow within the bulk of the conductor, perpendicular to the changing magnetic flux. Unlike current deliberately conducted along a wire, eddy currents circulate inside the material itself and dissipate energy as heat through Joule heating (I²R losses), a phenomenon classified as a form of magnetic core loss alongside hysteresis loss. They arise in any electrically conductive material subjected to an alternating or time-varying magnetic field, including transformer cores, motor stators and rotors, induction coils, and the conducting shells of radio frequency devices.

Eddy currents were first systematically studied by Léon Foucault in the mid-nineteenth century, which is why they are also called Foucault currents in French-language technical literature.

Physical Mechanism and the Skin Effect

The magnitude of eddy currents in a material is governed by three factors: the rate of change of the magnetic flux (which depends on excitation frequency), the electrical conductivity of the material, and the geometry of the region in which the currents circulate. A key consequence of the underlying electromagnetic equations is the skin effect: at higher frequencies, induced currents concentrate increasingly near the surface of the conductor and decay exponentially with depth. The standard depth of penetration, known as the skin depth, is proportional to the inverse square root of the product of frequency, conductivity, and magnetic permeability. At power-line frequency (50 or 60 Hz), the skin depth in silicon steel is on the order of a fraction of a millimeter, which directly motivates the laminated construction used in transformers and motors.

The Texas Instruments technical literature on eddy current losses in transformer windings provides detailed analysis of how eddy currents in copper conductors compound proximity-effect losses in high-frequency switching power supplies, a problem distinct from core losses and requiring conductor design solutions such as Litz wire.

Magnetic Losses and Mitigation

Eddy current losses in magnetic cores are proportional to the square of the excitation frequency and the square of the lamination or grain thickness, a relationship described by the Steinmetz empirical formula and by more rigorous electromagnetic analysis. This frequency-squared dependence is why power transformers operating at 50 or 60 Hz use laminated silicon steel cores (with individual laminations typically 0.23 to 0.65 mm thick, each insulated by a surface oxide or varnish layer), while transformers and inductors operating at kilohertz frequencies use ferrite cores, whose much higher electrical resistivity drastically reduces eddy current circulation.

Grain-oriented silicon steel, developed in the 1930s by Goss, aligns crystallographic grain orientation to reduce both hysteresis and eddy current losses in the rolling direction, and it remains the standard core material for large power transformers. IEEE standards for power transformers specify core loss limits and testing methods that characterize both eddy current and hysteresis components. At radio and microwave frequencies, powdered iron and ferrite composites with particle diameters smaller than the skin depth are used to interrupt eddy current paths while maintaining useful magnetic permeability.

Active use of eddy currents in electromagnetic braking and induction heating applications exploits the same I²R heating mechanism that makes them unwanted in transformers, channeling the induced power into controlled retardation or heating of conductive workpieces.

Applications

Eddy currents have applications in a wide range of electrical and mechanical engineering contexts, including:

• Power transformer and electric motor core design, where loss minimization is critical to efficiency
• Induction heating: melting, welding, and surface hardening of metals
• Electromagnetic braking in rail vehicles, amusement rides, and laboratory dynamometers
• Nondestructive testing of conductive materials for cracks, corrosion, and thickness
• Electromagnetic shielding design for radio frequency enclosures