Physics of eddy current an introduction

Physics of Eddy Current- ET - An Understanding 2014-October My ASNT Level III Pre-Exam Preparatory Self Study Notes

Charlie Chong/ Fion Zhang

Expert at Works

Charlie Chong/ Fion Zhang

Expert at Works

Charlie Chong/ Fion Zhang

http://eddycurrent.net/gallery/index.php/Palm-Cooling-Discovery-Gardens/IMG_2100

Expert at Works

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http://ropax.co.uk/eddy-current-inspection.html

Expert at Works

Charlie Chong/ Fion Zhang

http://ropax.co.uk/eddy-current-inspection.html

Expert at Works

Charlie Chong/ Fion Zhang

http://ropax.co.uk/eddy-current-inspection.html

Charlie Chong/ Fion Zhang

Overview: Eddy-current test uses electromagnetic induction to detect flaws in conductive materials. The eddy current test set-up consists of a circular coil which is placed on the test surface.

The alternating current in the coil generates changing magnetic field which interacts with the conductive test surface and generates eddy current. The flow of eddy current can be disrupted due to change in resistivity or conductivity, magnetic permeability, any physical discontinuities. The change in eddy current flow and a corresponding change in the phase and amplitude is measured against known values. Eddy current test method can detect very small cracks in or near the surface of the material, the surfaces need minimum preparation. The biggest advantage of the eddy current test method is that is can be employed to determine surface flaws on painted or coated surface. Eddy current flaw detection is commonly used in the aerospace industry, crane industry, concrete pumping industry and other general industries where the protective surface coating cannot be removed.

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The crane industry and crane owners benefits most from the application of eddy current test method to detect surface flaws underneath the protective coating (paint ). The exorbitant cost of paint removal and repainting is eliminated by applying eddy current flaw detection method as compared to magnetic particle test. It is also useful for making electrical conductivity and coating thickness measurements. Eddy current test is commonly employed for rapid thickness testing of coatings – conductive and non-conductive.

The principle of eddy current test which measures the change in resistivity in the conductive material makes it useful in wide range of applications such as conductivity measurement, sorting of material, assessment of heat treatment condition, sorting of materials on the basis of hardness and strength, thickness measurement of thin components. Compared to other surface flaw detection methods, eddy current test requires highly trained, skilled and experienced technicians. LMATS professionals are qualified, certified and experienced in eddy current test.

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Contents 1.

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Introduction The Principle of Eddy Current Testing Eddy Current Faraday’s Law Lenz’s Law Inductance Impedance

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1.0

Introduction:

Eddy Current Inspection (EC) Eddy Current Inspection (EC) is a technique used to detect surface breaking discontinuities in all electrically conducting materials. Uses include material sorting and in-service tube, bar and weld inspection. The main advantage is that testing can be conducted without the need to remove paint or surface coatings. Typical site applications include inspection of crane jibs, pedestals, pad-eyes (pre and post loading), drilling derrick substructures and wind turbine towers.

EC Theory An alternating current is applied to an inspection coil, which creates a magnetic field. When placed next to a suitable test material, it induces an ‘eddy current’ into the test material. The presence of defects and material variations in the test material, affects the characteristics of the induced eddy currents. These changes are detected by an excitation coil and are displayed on a digital screen.

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2.0

Principle of Eddy Current Testing

The eddy current testing method is a nondestructive evaluation method. It is widely used for crack detection as cracks cause very large local conductivity changes. However, there are many other applications in which highly sensitive and spatially resolved conductivity analysis can help to solve various inspection tasks. The basic principle is shown below.

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An applied alternating current to a coil creates a primary magnetic field Provided that there is a conductive sample in this changing field, eddy currents are induced following the law of induction.

The eddy currents generate a second magnetic field. This secondary field is opposed towards the primary field (c.f. Lenz's law).

The properties of sample change the resulting field that can be characterized in various ways, for example, with a pickup coil.

The impedance change provides information about capacitive and resistive properties of the sample.

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Characteristics ■Contactless and nondestructive ■extremely fast (we utilize up to 50,000 samples per second) ■good automation abilities ■high sensitivity Method variations ■Impedance - Spectroscopy (cf. Fraunhofer IZFP Dresden) ■Multi-frequency eddy current testing ■Impulse eddy current ■Frequency sweeping analysis

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Skin effect Eddy currents concentrate near the surface close to an induction coil and their strength decreases with distance from the coil. The EC density is being largest near the surface of the conductor, decreasing exponentially at greater depths. This decay is known as the term, Skin effect. The skin effect occurs when induced Eddy Currents at the surface generate an opposed magnetic field that lowers the entire resulting field, thus causing a decrease in current flow as the depth increases.

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Depth of Detection Vs Frequency

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Standard penetration depth The depth that eddy currents penetrate into a material is affected by the frequency of the alternating current, the electrical conductivity and magnetic permeability of the sample. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability.

The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (δ or 1 δ)and used as criteria of ideal measurement. At three standard depth of penetration (3 δ), the Eddy Current density is down to only 5% of the surface density. So, defects or variation deeper than the three standard depth of penetration cannot be recognized because the EC density in this depth is too low to detect. Thus, achieving the standard penetration depth is the most important factor at Eddy Current testing and this is realized by selecting appropriate frequency suitable for a material property.

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Eddy Current Density Since the sensitivity of Eddy Current inspection depends on the Eddy Current density at the defect location, it is important to know the strength of the Eddy Currents at this location. When detect flaws, a frequency is often selected which places the expected flaw depth within one standard depth of penetration. This assures that the strength of the Eddy Currents would be sufficient to produce a flaw indication.

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http://www.suragus.com/en/company/eddy-current-testing-technology

3.0

Eddy Current

Eddy currents (also called Foucault currents) are circular electric currents induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within (1) nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by (2) relative motion between a magnet and a nearby conductor.

The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.

Keywords: Faraday’s law of induction

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By Lenz's law, an eddy current creates a magnetic field that opposes the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic (changing) field. This effect is employed in eddy current brakes which are used to stop rotating power tools quickly when they are turned off. The current flowing through the resistance of the conductor also dissipates energy as heat in the material. Thus eddy currents are a source of energy loss in alternating current (AC) inductors, transformers, electric motors and generators, and other AC machinery, requiring special construction such as laminated magnetic cores to minimize them. Eddy currents are also used to heat objects in induction heating furnaces and equipment, and to detect cracks and flaws in metal parts using eddy-current testing instruments. Eddy currents can take time to build up and can persist for very short times in conductors due to their inductance.

Keywords: Lenz’s law

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The Laws: As the probe is energized with AC current (the strength of primary magnetic field is alternating and changing), and so an eddy current is set up (Faraday’s law) in the counterclockwise direction (Lenz’s law)

Faraday’s law of Induction

Lenz’s law – Opposing field & direction Charlie Chong/ Fion Zhang

Michael Faraday, FRS (22 September 1791 – 25 August 1867) was an English scientist who contributed to the fields of electromagnetism and electrochemistry. His main discoveries include those of electromagnetic induction, diamagnetism and electrolysis.

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Heinrich Friedrich Emil Lenz (Russian: Эмилий Христианович Ленц) (12 February 1804 – 10 February 1865) was a Russian physicist of Baltic German ethnicity. He is most noted for formulating Lenz's law in electrodynamics in 1833. The symbol L, conventionally representing inductance, is chosen in his honor.

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Introduction to Physics of Eddy Current:

http://www.youtube.com/embed/djFvnFy3rJc http://www.youtube.com/watch?v=V-IW6cFIt9E Charlie Chong/ Fion Zhang

Induction Damping As discussed in motional emf, motional emf is induced when a conductor moves in a magnetic field or when a magnetic field moves relative to a conductor. If motional emf can cause a current loop in the conductor, we refer to that current as an eddy current. Eddy currents can produce significant drag, called magnetic damping, on the motion involved. A common physics demonstration device for exploring eddy currents and magnetic damping. (a) The motion of a metal pendulum bob swinging between the poles of a magnet is quickly damped by the action of eddy currents. (b) There is little effect on the motion of a slotted metal bob, implying that eddy currents are made less effective. (c) There is also no magnetic damping on a non-conducting bob, since the eddy currents are extremely small.

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A more detailed look at the conducting plate passing between the poles of a magnet. As it enters and leaves the field, the change in flux produces an eddy current. Magnetic force on the current loop opposes the motion. There is no current and no magnetic drag when the plate is completely inside the uniform field.

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http://legacy.cnx.org/content/m42404/latest/

When a slotted metal plate enters the field, as shown in Figure below, an emf is induced by the change in flux, but it is less effective because the slots limit the size of the current loops.

Moreover, adjacent loops have currents in opposite directions, and their effects cancel. When an insulating material is used, the eddy current is extremely small, and so magnetic damping on insulators is negligible. If eddy currents are to be avoided in conductors, then they can be slotted or constructed of thin layers of conducting material separated by insulating sheets.

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http://legacy.cnx.org/content/m42404/latest/

Induction Heating

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http://inductionbending.co.uk/gallery

Eddy currents (I, red) induced in a conductive metal plate (C) as it moves to right under a magnet (N). The magnetic field (B, green) is directed down through the plate.

From Lenz's law the increasing field at the leading edge of the magnet (left) (increasing field) induces a counterclockwise current, which creates its own magnetic field (left blue arrow) directed up, which opposes the magnet's field, producing a retarding force. Similarly, at the trailing edge of the magnet (right) (decreasing field), a clockwise current and downward counterfield is created (right blue arrow) also producing a retarding force.

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A more detailed look at the conducting plate passing between the poles of a magnet. As it enters and leaves the field, the change in flux produces an eddy current. Magnetic force on the current loop opposes the motion. There is no current and no magnetic drag when the plate is completely inside the uniform field.

Opposing “C” from entering

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Opposing “C” from leaving

Lenz's law states that the current swirls in such a way as to create an induced magnetic field that opposes the phenomenon that created it. In the case of a varying applied field, the induced field will always be in the opposite direction to that applied. The same will be true when a varying external field is increasing in strength. However, when a varying field is falling in strength, the induced field will be in the same direction as that originally applied, in order to oppose the decline.

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Fleming Right Hand Rule - (Dynamo Rule)

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www.youtube.com/embed/d_aTC0iKO68 www.youtube.com/embed/bBwM3Q6zGag

Che Guevara – The Opposing http://www.cyclopaedia.info/wiki/Che-Guevara

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Eddy currents in conductors of non-zero resistivity generate heat as well as electromagnetic forces emf. The heat can be used for induction heating. The electromagnetic forces can be used for levitation, creating movement, or to give a strong braking effect. Eddy currents can also have undesirable effects, for instance power loss in transformers. In this application, they are minimized with thin plates, by lamination of conductors or other details of conductor shape.

Self-induced eddy currents are responsible for the skin effect in conductors.

The latter can be used for non-destructive testing of materials for geometry features, like micro-cracks. A similar effect is the proximity effect, which is caused by externally induced eddy currents.

When a conductor moves through an inhomogeneous field generated by a source, electromotive forces (EMFs) can be generated around loops within the conductor. These EMFs acting on the resistivity of the material generate a current around the loop, in accordance with Faraday's law of induction. These currents dissipate energy, and create a magnetic field that tends to oppose changes in the current- they have inductance.

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Levitation

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4.0

Faraday's Law

Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.

Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a succinct summary of the ways a voltage (or emf) may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field.

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5.0

Lenz's Law

When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

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Faraday’s & Lenz’s Laws

www.youtube.com/embed/1-aoGz5X_j0 www.youtube.com/embed/ZMAd9DrnNGY www.youtube.com/embed/Vs3afgStVy4 Charlie Chong/ Fion Zhang

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6.0

Inductance

Inductance is typified by the behavior of a coil of wire in resisting any change of electric current through the coil. Arising from Faraday's law, the inductance L may be defined in terms of the emf generated to oppose a given change in current:

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Inductance

www.youtube.com/embed/NgwXkUt3XxQ www.youtube.com/embed/X2e9x104AnE www.youtube.com/embed/4PvOFovZQpQ Charlie Chong/ Fion Zhang

7.0

Impedance

While Ohm's Law applies directly to resistors in DC or in AC circuits, the form of the current-voltage relationship in AC circuits in general is modified to the form:

where I and V are the rms or "effective" values. The quantity Z is called impedance. For a pure resistor, Z = R. Because the phase affects the impedance and because the contributions of capacitors and inductors differ in phase from resistive components by 90 degrees, a process like vector addition (phasors) is used to develop expressions for impedance. More general is the complex impedance method.

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Impedance Combinations

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http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imped.html#c4

Impedance

www.youtube.com/embed/Pj4Rq1ZIeDI www.youtube.com/embed/FEERuJlwBxE www.youtube.com/watch?v=xyMH8wKK-Ag www.youtube.com/embed/y1ES6WrALzI

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Good Luck!

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Good Luck!

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