What is an Eddy Current?

An eddy current is a whirlpool of resistance generated when two electromagnetic fields cross. It circulates in a direction that opposes the original current. The resistance resulting from the collision of the two fields effectively converts some of the electrical energy present into heat, an undesirable byproduct when the intention is simple conveyance of electricity, as in a transformer. Other applications, however, harness the opposing magnetism of eddy currents to achieve other results, including the identification of metals, testing the properties of materials and technical assemblies and the braking of railroad cars.

In electromagnetic applications such as transformers, where the point is to conduct electricity with minimal interference, special construction is needed to ensure that an eddy current does not impede the primary electrical force. Layers of conducting material are separated by layers of insulating material. The result is that the natural magnetic attraction of an opposing force to the conducting material is fragmented and doesn't get a chance to form a counterproductive eddy current.

Sometimes, generating heat by means of eddy currents is the point, most notably in industrial furnaces used to melt metals. Residential induction cooktops rely on the same principle, whereby the electromagnetic field of a burner reacts with the magnetic field of special iron cookware. Heat occurs only where the two surfaces meet, so the rest of the cooktop does not get hot.

Two low-technology uses for eddy currents are found in vending machines and recycling. In a vending machine, a stationary magnet will cause an invalid item such as a steel slug to be rejected. On a much larger scale, types of cans and other recyclable metals can be sorted, because each metal responds to the opposing magnetic force in its own way.

In an eddy current brake, the magnetic resistance is great enough to stop a railroad car. In a system comparable to friction, the applied magnetic force resists the movement of the steel wheels. As the wheels slow, the resistance decreases, allowing for a gradual slowdown and smooth stop. The shutoff mechanisms for power tools such as circular saws work in a similar manner.

Eddy current inspection allows for the nondestructive analysis of conductive metals and assemblies that contain them. With this technique, the inspector induces an eddy current in the test material and then looks for irregularities in the flow of the current. For example, a discontinuity in the interaction of the two magnetic fields could indicate the presence of a crack. This kind of testing is sensitive enough to check for changes in the thickness of a material, corrosion or other undesirable, hidden conditions.

One notable user of eddy current inspection is the United States National Aeronautics and Space Administration (NASA). The agency often must troubleshoot problems with materials and systems that are already in place, so the nondestructive aspect of an eddy current probe is crucial. In the spring of 2009, NASA discovered a flaw in a flow control valve, a critical part that governs fuel flow during the launch of the space shuttle and other rockets. Eddy current testing enabled the agency to monitor the condition of the valves, and eventually it was determined that all of them should be replaced.