Glossary term
Eddy Current
A circulating electric current induced inside a conductor by a changing magnetic field or by motion through a magnetic field.
Definition
phenomenonA circulating electric current induced inside a conductor by a changing magnetic field or by motion through a magnetic field.
Eddy currents are induced by electromagnetic induction and flow in closed loops within conductive material. They can be useful for braking, heating, sensing, and non-destructive testing, but they can also create unwanted heat, loss, drag, shielding, and measurement error.
An eddy current is a loop of current induced within a conductive body when magnetic flux through that body changes. The current circulates locally, rather than travelling through a defined external circuit. Its direction opposes the change in magnetic flux that produced it, following Lenz’s law.
Engineering role
Eddy currents are important in transformers, electric machines, induction heating, magnetic braking, metal detectors, electromagnetic shielding, non-destructive testing, and high-frequency conductors. Sometimes they are deliberately created, as in induction cooktops and eddy-current brakes. In other cases they are unwanted losses, as in transformer cores, motor laminations, busbars, and conductive structures near alternating magnetic fields.
Loss and heating
Because eddy currents flow through finite resistance, they generate heat by Joule heating. The loss tends to increase with magnetic field amplitude, frequency, conductor thickness, and electrical conductivity. Laminated cores, ferrites, powdered metals, slots, and high-resistivity materials are used to reduce circulating current paths in magnetic components.
Skin and proximity effects
At higher frequencies, induced currents concentrate near conductor surfaces and are influenced by nearby conductors. This changes effective resistance, current distribution, heating, and electromagnetic compatibility. Engineers may use stranded conductors, litz wire, busbar geometry, shielding, or finite-element electromagnetic analysis when eddy-current effects are significant.
Testing and sensing
Eddy-current non-destructive testing uses probe coils to induce currents in a conductive part and then measures impedance changes caused by cracks, thickness variation, conductivity, coating thickness, or heat-treatment condition. The method is sensitive to surface and near-surface defects, but penetration depth depends strongly on frequency, conductivity, and magnetic permeability.
Common mistakes
Common mistakes include ignoring eddy-current heating near high-frequency magnetic fields, assuming a solid magnetic core behaves like an ideal flux path, and using conductive mounting hardware where changing magnetic fields are strong. In testing, a frequent error is interpreting eddy-current signals without accounting for lift-off, surface condition, material conductivity, and geometry.