An eddy current rotary probe, on an eddy current test system with entry and exit conveyors. An eddy current rotary probe, on an eddy current test system with entry and exit conveyors.

Eddy Current Testing (ECT) Technology in Non-Destructive Testing

MAC’s® Eddy Current Testing (ECT) systems incorporate computer-based test instruments and test coils that use custom coil sensors: encircling and sector coils, and spinning or rotary probe coils.

Overview

Encircling and sector Eddy Current coil tests perform in this manner:

  • The product is passed through or adjacent to an electrical test coil, which has been excited by an alternating current.
  • This induces a flow of eddy currents around the test material or in the case of a sector coil, in the area under the coil.
  • Short, intermittent anomalies or flaws cause a variation in the eddy current pattern, which the instrument detects.

Eddy Current Technology Brochure

A NDT system by MAC, eddy current manufacturers, utilizing a MultiMac EC electronic, and an eddy current encircling coil and eddy current probe.

Rotary probe Eddy Current tests perform as follows:

  • The product moves longitudinally through the rotating test probes resulting in a helical search pattern.
  • As the probe passes over a defect, variations in the induced Eddy Current pattern are detected.
  • The minimum flaw length, which can be consistently detected, is a function of the rotary speed of the probe and the throughput speed of the material.
  • Rotary testing is the method of choice for detecting seam type surface defects in non-magnetic and magnetic grades of wire and bar.

Eddy Current Test Technology in NDT

Typical applications for encircling/sector ECT sensors are:

  • Detect short surface and some subsurface defects, on or off-line, in magnetic and non-magnetic wire, bar and tube.  
  • Inspect welded tube for short ID or OD defects in the weld zone or on the full circumference.
  • Test uniform cross sectional material, including squares, rectangles, hex and round.
  • Inspect small diameter wire or tube for short defects.
  • Check continuity and locate welds in single and multi-conductor insulated wire and cable
  • When testing carbon steel, austentic stainless and alloy steels that have a permeability higher than 1, it is often necessary to saturate the material with a magnetic field. This has the effect of evening out the permeability variations in the material which would otherwise interfere with the ECT. Saturation coils are used to create the saturation field within which the ECT coil is paced.

Eddy Current Test Instruments: 

Typical applications for rotary probe ECT sensors are:

  • Detect seam type surface defects in cold drawn wire or cut length bar stock.
  • Inspect in line with continuous wire operations such as wire drawing, parts forming, re-spooling, or straight and cut.
  • Inspect cut lengths, usually off-line.
  • Test parts, such as small shafts and bearings for longitudinal surface defects.
  • Eddy Current technology can be used on ferrous and non-ferrous materials.

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FAQ about Eddy Current Equipment

Eddy current testing uses specialized eddy current instruments, probes, coils, and inspection systems to detect surface and some subsurface defects in materials. Common equipment includes eddy current flaw detectors, encircling coils, bobbin coils, rotating probes, differential probes, and automated eddy current testing systems. For high-speed tube and bar inspection, systems using eddy current electronics such as MultiMac® and Minimac® can be integrated with coils, rotary systems, and automated material handling for production-line inspection.

Eddy current testing (ECT) is used to inspect tubes for surface and near-surface defects without damaging the material. During tube inspection, an eddy current coil or probe generates an electromagnetic field around the tube. Changes in the material, such as cracks, pits, seams, or corrosion, disrupt the electrical currents and create a detectable signal.

For high-speed production lines, encircling coils can inspect the tube as it passes through the coil, while rotary probes can provide more localized or detailed inspection.

MAC® offers automated eddy current inspection systems, including MultiMac® and Minimac®, that can be integrated with coils, rotary inspection systems, and production-line handling equipment.

There are several types of eddy current testing (ECT), with the appropriate method depending on the material, product geometry, and defects being inspected. Common types include:

  • Conventional eddy current testing: Uses a probe or coil to detect surface and near-surface discontinuities in conductive materials.
  • Encircling coil testing: The product passes through a coil that surrounds it, making it well suited for high-speed inspection of tube, pipe, wire, and bar.
  • Rotary probe testing: Multiple probes rotate around the product to provide detailed inspection coverage and detect localized defects.
  • Bobbin coil testing: A probe passes through the inside of a tube, commonly used for inspecting heat exchanger and other tubing.
  • Array eddy current testing: Uses multiple coils or sensors to inspect a larger area and collect more detailed information in a single pass.
  • Differential eddy current testing: Compares signals from multiple sensing elements to help identify localized changes or defects.

For industrial tube and bar inspection, encircling coils and rotary eddy current systems are commonly used for automated, high-speed production-line testing. MAC® offers eddy current inspection solutions including MultiMac® and Minimac® electronics, which can be configured with different coil and probe arrangements based on the application.

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