What causes phase shift in an eddy current test coil?

Phase shift in an eddy current test coil is primarily caused by a change in the ratio of inductive reactance to resistance. In eddy current testing, an alternating current is passed through a test coil, generating a magnetic field that induces eddy currents in the conductive material being tested.

Inductive reactance is a function of frequency and the inductance of the coil, while resistance is associated with the material's conductive properties. When the properties of the tested material change—such as its conductivity, permeability, or proximity to a flaw—the balance between inductive reactance and resistance alters. This shift causes the current phasor to rotate, leading to a phase difference between the applied voltage and the resulting eddy currents. The change in phase can be interpreted to assess the material's properties and detect any flaws.

Shifting the sensitivity setting of the instrument or utilizing modulation analysis are more about enhancing or adapting the measurement process rather than directly causing a phase shift. The foundational mechanism that leads to phase shift is rooted in the relationship between the induction and resistance in the coil's circuit.