How Eddy Current Works

To properly conduct an eddy current inspection, it is important to understand the principles of how eddy current works.

When an oscillator is used to generate an AC signal in a coil, an alternating magnetic field is generated around the coil. When that coil is placed near an electrically conductive material (metal) eddy currents are induced in that material. These eddy currents will generate a secondary magnetic field which will cause an impedance change in the coil. The eddy currents that flow in the conductive material are affected by surface geometry, hardness, temperature and metallurgical makeup of the material (especially near the surface). Therefore, the impedance of the coil is affected by these same factors.



The oscillator driven coil is placed in a specific location on an acceptable part (master) near a test feature (machined detail, threaded hole, bushing, nut, etc.). An impedance reading is taken by the system and is used as a reference zero for the system. The same coil is then placed in the same relative position near a test part. If the test part is the same material, temperature and hardness, geometrical differences can be detected. Because temperature, material, hardness and geometry are never exactly the same even on seemingly identical parts, a window is set up to allow for slight variations in the parts. When a part with acceptable readings is detected the solid state relay is energized indicating the part has passed.