[ad_1]
Eddy currents are generated when two electromagnetic fields cross, creating resistance and converting electrical energy into heat. They can be used for identifying metals, braking railroad cars, and non-destructive analysis of conductive metals. In transformers, special construction is required to prevent eddy currents from impeding the primary electric force. NASA uses eddy current inspection for troubleshooting problems with materials and systems.
An eddy current is a vortex of resistance generated when two electromagnetic fields cross each other. 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 simply to transport electricity, such as in a transformer. Other applications, however, take advantage of the opposing magnetism of eddy currents to accomplish other things, including identifying metals, verifying material properties and engineering assemblies, and braking railroad cars.
In electromagnetic applications such as transformers, where the purpose is to conduct electricity with minimal interference, special construction is required to ensure that an eddy current does not impede the primary electric force. Layers of conductive 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 has no chance to form a counterproductive eddy current.
Sometimes, eddy current heat generation is the point, particularly in industrial furnaces used to melt metals. Residential induction hobs are based on the same principle, whereby the electromagnetic field of a burner reacts with the magnetic field of special iron pans. Heat occurs only where the two surfaces meet, so the rest of the cooktop doesn’t get hot.
Two low-tech 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 bullet, to be rejected. On a much larger scale, you can select types of cans and other recyclable metals, 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 down, the resistance decreases, allowing for a smooth slowdown and smooth stop. Stop mechanisms for power tools such as circular saws work in a similar way.
Eddy current inspection allows for non-destructive analysis of conductive metals and the assemblies containing them. With this technique, the inspector induces an eddy current in the test material and thus 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 type of test is sensitive enough to check for changes in a material’s thickness, corrosion, or other hidden, undesirable conditions.
One notable user of eddy current inspection is the United States National Aeronautics and Space Administration (NASA). The agency often has to troubleshoot problems with existing materials and systems, so the nondestructive aspect of an eddy-current probe is crucial. In the spring of 2009, NASA discovered a defect in a flow control valve, a critical part that regulates fuel flow during space shuttle and other rocket launches. Testing for eddy currents allowed the agency to monitor the condition of the valves and it was ultimately determined that they all needed to be replaced.
