What Are Eddy Currents?
A transformer designer tries to limit them. An induction heater relies on them. A displacement sensor measures their effects. Eddy currents are circulating electrical currents induced within a conductive material, and their usefulness depends on what the equipment is intended to do.
Unlike the current following a deliberately wired circuit, these currents can form closed paths inside a solid metal component. They are still ordinary electric currents; the name describes their circulating pattern. Foucault currents is another term used for the same phenomenon.
How Can Current Flow Without a Wired Supply?
The explanation comes from electromagnetic induction and the work of Michael Faraday. Changing magnetic flux through a closed path produces an electromotive force around that path. Like voltage, electromotive force is measured in volts.
Faraday’s law: ε = −dΦ/dt
Here, ε is the induced electromotive force and Φ is magnetic flux. The negative sign expresses the direction described by Lenz’s law: the induced response opposes the change in flux that caused it.
The flux may change because the field varies with time or because the conductor moves through a suitable field arrangement. A stationary conductor in a steady magnetic field does not sustain eddy currents after transients have died away. Motion alone is also insufficient if it leaves the relevant flux unchanged.
The metal does not have to be ferromagnetic. Copper and aluminium support eddy currents even though an ordinary permanent magnet does not attract them as it attracts iron.
Three Ways Industry Uses the Effect
Heating a workpiece: An induction heating coil establishes an alternating magnetic field. Currents induced in the workpiece produce heat within its conductive material. Ferromagnetic workpieces may also receive heat from hysteresis. Coil geometry, operating frequency and the required heating depth all matter; simply increasing frequency does not guarantee a more efficient process.
Slowing a moving component: In an eddy current brake, interaction between the induced currents and the magnetic field produces drag. The moving system supplies the energy, which becomes heat. A passive brake operating in a static field loses its braking action at standstill and cannot provide a holding function by itself. A drive’s braking resistor works differently, dissipating energy delivered through the DC link.
Detecting metal: An inductive proximity sensor can detect the effect of target eddy currents on its oscillator. Target size, material and sensor construction influence the usable sensing distance.
From a Metal Detector to a Precision Measuring System
An eddy current displacement probe does more than report whether metal is present. With a suitable controller, it can track the gap to a conductive target and measure changes associated with position or vibration. A switching proximity sensor and a continuous displacement system serve different purposes.
The target is part of the measurement setup. Material, thickness, geometry and temperature can affect the response, so calibration must suit the intended application. Replacing a steel target with an aluminium one may require a different calibration rather than a simple mechanical adjustment.
Depending on the controller, the measurement may be available as an analogue voltage, current signal or digital value. When connecting it to a PLC, check the actual interface specification. Familiar 0–10 V and 4–20 mA signals are options only where both devices support them.
Eddy current testing also uses changes in the probe response to inspect conductive parts. A crack can interrupt current paths, but changes in conductivity, permeability or probe lift-off can alter the signal too. Reliable inspection requires suitable reference standards and interpretation; a changed reading is not automatically evidence of a crack.
Why Eddy Currents Become an Unwanted Loss
Conductive materials have finite electrical resistance. Circulating currents therefore generate heat. In equipment intended to transfer energy efficiently, that heating increases the cooling requirement without contributing to the useful output.
In a transformer, eddy current loss in the core is distinct from resistance loss in the windings. It also differs from hysteresis loss, which is associated with repeated magnetisation of the core material. A hot transformer may have several contributing loss mechanisms.
There is no free energy involved. The power dissipated by induced currents ultimately comes from the electrical excitation or from the mechanical work that maintains motion.
Why Magnetic Cores Use Insulated Laminations
A laminated core replaces a thick conducting section with thin sheets separated by electrical insulation. The sheets are arranged so that the main working flux largely follows their planes. This restricts large circulating current paths while retaining a useful magnetic path through the core.
The insulation between sheets matters as much as the layered appearance. Conductive bridges caused by burrs or damage can undermine the intended separation. Laminations reduce eddy current loss; they do not remove every source of heating.
Material choice provides another approach. Electrical steels offer useful combinations of magnetic properties and resistivity. Ferrites have much higher electrical resistivity and can limit eddy currents at frequencies where solid metallic cores would be unsuitable. Their saturation behaviour, temperature limits and other losses still need consideration.
Frequency, Thickness and the Limits of a Simple Formula
For thin laminations with approximately uniform sinusoidal flux and limited skin effect, the classical eddy current loss density follows this proportional relationship:
pe ∝ Bpeak2 × f2 × t2 / ρ
Bpeak: peak flux density · f: frequency · t: lamination thickness · ρ: electrical resistivity
At otherwise unchanged conditions, halving the sheet thickness reduces this modelled loss component to one quarter. Doubling frequency increases it fourfold only if peak flux density remains unchanged. Neither comparison predicts the total loss of a complete machine.
Changing a real supply frequency can also change flux density and waveform. The relationship is therefore a design model, not a universal instruction to reduce operating frequency.
What About Skin Effect?
Skin effect describes how alternating current becomes concentrated nearer the surface of a conductor. It is related to electromagnetic induction, but it is not simply another name for eddy currents.
With resistivity and permeability held constant, characteristic penetration depth varies inversely with the square root of frequency. A fourfold frequency increase therefore halves that depth. Penetration depth is a measure of decay, not a sharp boundary beyond which current disappears.
A Motor Drive Has More Than One Relevant Frequency
With a variable frequency drive, distinguish the motor’s fundamental operating frequency from the switching used to create its output. Pulse-width modulation introduces a switching pattern whose carrier frequency is separate from the motor’s rated frequency.
The resulting harmonic content can affect motor losses. Changing the carrier frequency also affects the drive’s operating conditions, so its overall thermal effect cannot be inferred from the simple lamination formula alone. Use the motor and drive manufacturers’ limits when selecting settings.
Electromagnetic interference is a further, separate consideration. Solving an interference problem and reducing core heating may require different measures, even when both are associated with the same switching equipment.
Technical references: OpenStax — Faraday’s Law · Separation of Magnetic Core Losses · TDK — Ferrite Definitions and Losses · Ambrell — Induction Heating · Micro-Epsilon — Eddy Current Sensors · Evident — Eddy Current Testing