The eddy current testing machine is only suitable for conductive materials and can only detect defects on the surface or near the surface layer, which is inconvenient to use for components with complex shapes.
In thermal power plants, eddy current testing equipment is mainly used to detect condenser tubes, steam turbine blades, steam turbine rotor center holes and welds, etc.
Principle of eddy current testing machine: When alternating current is passed into the coil, if the voltage and frequency used are unchanged, the current passing through the coil will also remain unchanged. If a metal tube is placed in the coil, the surface of the tube will experience circumferential current, that is, eddy current.
The direction of the eddy current magnetic field is opposite to the magnetization direction of the applied current, so a part of the applied current will be canceled, so that the impedance of the coil, the magnitude and phase of the current through the coil will change. Changes in the diameter, thickness, electrical conductivity and magnetic permeability of the tube, as well as the existence of defects, will affect the impedance of the coil.
If other factors remain unchanged, only the signal of the impedance caused by the defect is taken out, amplified by the eddy current testing equipment and tested, and the purpose of flaw detection can be achieved.
The eddy current signal can not only give the size of the defect, but also because the eddy current under the surface lags behind the surface eddy current by a certain phase during eddy current testing, the position t (depth) of the defect can be judged by phase analysis.
The loss resistance of the vortex channel and the reverse magnetic flux generated by the vortex are reflected to the detection coil, thereby changing the current size and phase of the coil. Therefore, when the probe moves on the metal surface and encounters defects or changes in material, size, etc., the eddy current magnetic field will have a different reaction to the coil, causing the impedance of the coil to change. This change can be measured by the eddy current testing equipment to identify whether the metal surface has defects or other changes in physical properties.