NASA CP Basic Principles Book
Indice ROCarneval


Operating and Maintenance Instructions

adaptaed from:

Hard spot inspection trolley • PLAMAT-M • 18201
Operating and Maintenance Instructions • V2.0
Fundamental principles

2.3 Eddy current

The EC testing method is based on the approach of generating electrical currents in conductive materials, here the pipe wall. The figure below summarizes the principle. An alternating electric current with defined amplitude and frequency is applied to a coil system. The driving current generates an alternating primary magnetic field which causes eddy currents to flow in the surface of the nearby pipe wall by mutual inductance. The currents in the pipe wall produce a secondary magnetic field which is opposed to the primary field inducing it. Defect damage such as corrosion leads to a change of the EC’s flow direction, which influences the mutual inductance. This can be described by a variation in the electrical impedance of the coil, i.e., its Ohmic resistance and inductive reactance. The impedance is usually measured across a bridge circuit in which any imbalance can be measured accurately. On the basis of this imbalance, material inhomogeneities can be detected and their properties determined by the evaluation of the amplitude and the phase shift between the input and output signals.

EC principle
Figure 4: The EC principle

In addition to material properties such as the electric conductivity and magnetic permeability, the frequency of the input signal also determines the so-called skin depth of eddy currents.
The skin depth is a measurement of the distance to which an alternating current can effectively penetrate beneath the surface of a conductive material, in this case the pipe wall.

For suitable frequencies and standard steel grades the skin depth for carbon steel is well below 1 mm. Thus, the EC approach can be considered to be a surface-sensitive method.

The magnetic permeability generally depends on the operating point of magnetization, which usually comprises:
  • demagnetized operation point
  • residual magnetized operating point

Histereses loop
Figure 5: Magnetic hysteresis loop and operating points

Since the EC approach considered here is surface- bonded, surface influences may have and disturbing character, e.g. tinder, roughness, etc.

The following micro-magnetic parameters are considered for the EC analysis:
Zmax [Ohm] maximum magnitude of the impedance
Zmin [Ohm] minimum magnitude of the impedance
Zmean [Ohm] averaged magnitude of the impedance
Phizmax [rad] maximum phase of the impedance
Phizmin [rad] minimum phase of the impedance
Phizmean [rad] averaged phase of the impedance
W3Z [Ohm] 3 % widening of the eddy current loop
W10Z [Ohm] 10 % widening of the eddy current loop

EC impedance plane
Figure 6: EC impedance plane

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