Classification and Structural Introduction of Detection Coils Matching Eddy Current Flaw Detector

Eddy current flaw detector is a device that uses a coil with alternating current to establish an alternating magnetic field which induces eddy currents in a conductive material that is close to the coil. The eddy currents produce their own magnetic field, which changes the strength of the original magnetic field and results in changes in coil voltage and impedance when the magnetic field is passing through the conductor. When a defect occurs on or near the surface of the conductor, it affects the intensity and distribution of the eddy currents, causing changes in voltage and impedance of the detection coil. This change indirectly indicates the presence of a defect inside the conductor.

Classification of detection coils of eddy current flaw detector

In eddy current testing, a detection coil is used to establish an alternating magnetic field, transfer energy to the tested conductor, and obtain quality information about the tested conductor through the alternating magnetic field generated by the eddy currents. Therefore, the detection coil is a transducer. The size, shape, and technical parameters of the detection coil are crucial to the testing process and are often selected based on the shape, size, material, and quality requirements of the tested conductor.There are three main types of detection coils.

Through-coil

The through-type coil of the eddy current flaw detector is a coil used to test the test specimen by placing it inside the coil for inspection.Through-coil is suitable for the testing of pipes, rods, and wires.Due to the magnetic field generated by the coil primarily affecting the outer wall of the test specimen, the detection of outer wall defects is more effective, while the detection of inner wall defects is carried out using penetration.

In general, the sensitivity of inner wall defect detection is lower than that of the outer wall. Defects in thick-walled pipes cannot be detected using through-type coils.

Internal-insertion coil

Internal-insertion coil is specially designed to test the thick wall or inner wall of a pipe and the quality of pipes in equipment such as a heat exchanger.

Probe coil

The probe-type coil of the eddy current flaw detector is placed on the surface of the test specimen for inspection.The probe coil is suitable for surface scanning and testing of small defects on flat plates, slabs, square billets, round billets, and bars as well as complex-shaped mechanical parts. The probe coil is smaller in volume and has a smaller field range compared to the through-coil, making it more suitable for detecting small surface defects.

The structure of the detection coil of the eddy current flaw detector

The structure of the detection coils varies depending on the objects and purposes of use. Sometimes, a single coil is used for detection, and sometimes two coils connected in reverse are used for differential detection. Sometimes, multiple coils are connected in series, parallel, or in a certain arrangement to achieve a particular non-destructive testing purpose.

These coils are sometimes wound around a framework, known as the self-comparison method, and sometimes wound around two frameworks. One coil holds the sample, while the other is used for actual testing, known as the mutual comparison (or standard comparison) method.

The electrical connections of the detection coils can also vary, such as using a single winding that functions as both an excitation and detection coil, using separate excitation and detection windings, or using a coil that is part of an electrical circuit.