Physics of Eddy Current-An Introduction

8/10/2019 Physics of Eddy Current-An Introduction

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Physics of Eddy Current- ET

- An Understanding2014-OctoberMy ASNT Level III Pre-Exam Preparatory Self Study Notes

Charlie Chong/ Fion Zhang


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Expert at Works


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Expert at Works

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


4/49Charlie Chong/ Fion Zhang

Expert at Works

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



Expert at Works




Overview:

Eddy-current test uses electromagnetic induction to detect flaws in conductivematerials. 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 whichinteracts 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.



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 iseliminated 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 treatmentcondition, 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 andexperienced in eddy current test.



Contents

1. Introduction2. The Principle of Eddy Current Testing

3. Eddy Current

4. Faradays Law

5. Lenzs Law6. Inductance

7. Impedance



1.0 Introduction:

Eddy Current Inspection (EC)Eddy Current Inspection (EC) is a technique used to detect surface breakingdiscontinuities 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 TheoryAn 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 eddycurrents. 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 iswidely 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 variationsImpedance - 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 beinglargest 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 currentflow 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 magneticpermeability 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 ( or 1)and used as criteria of ideal measurement. At three standard depth of

penetration (3), 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 toolow 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 EddyCurrents 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.

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


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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:

Faradays 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 adrag 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 ofthe 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 minimizethem. 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:

Lenzs law


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The Laws: As the probe is energized with AC current (the strength of primary magnetic field isalternating and changing), and so an eddy current is set up (Faradays law) in the

counterclockwise direction (Lenzs law)

Faradays law of Induction

Lenzs law Opposing

field & direction


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Michael Faraday, FRS (22

September 1791 25 August

1867) was an English scientistwho contributed to the fields of

electromagnetism and

electrochemistry. His main

discoveries include those ofelectromagnetic induction,

diamagnetism and electrolysis.


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Heinrich Friedrich Emil Lenz

(Russian:

)(12 February 1804 10 February 1865)

was a Russian physicist of Baltic

German ethnicity. He is most noted for

formulating Lenz's law inelectrodynamics 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


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Induction DampingAs 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-conductingbob, 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.

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


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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 oninsulators 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.

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


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Induction Heating

http://inductionbending.co.uk/gallery


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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 nocurrent and no magnetic drag when the plate is completely inside the uniform

field.

Opposing Cfrom entering

Opposing C

from leaving


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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 directionto 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.

Fl i Ri ht H d R l (D R l )


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Fleming Right Hand Rule - (Dynamo Rule)

www.youtube.com/embed/d_aTC0iKO68

www.youtube.com/embed/bBwM3Q6zGag

Ch G Th O i


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Che Guevara The Opposinghttp://www.cyclopaedia.info/wiki/Che-Guevara

http://www.fanpop.com/clubs/che-guevara/images/21468922/title/che-photo

Edd t i d t f i ti it t h t ll


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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 togive 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 iscaused 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.

Levitation


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Levitation

4 0 Faraday's Law


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4.0 Faraday's LawAny 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, thevoltage 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 ina 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


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5.0 Lenz s LawWhen 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 acurrent 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, theinduced field acts in the direction of the applied field to try to keep it constant.

Faradays & Lenzs Laws


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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

YouTube


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YouTube

,

6 0 Inductance


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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 inductanceL may be defined in terms of the emf generated to oppose a given change in

current:

Inductance


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www.youtube.com/embed/NgwXkUt3XxQ

www.youtube.com/embed/X2e9x104AnE

www.youtube.com/embed/4PvOFovZQpQ

7.0 Impedance


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7.0 ImpedanceWhile 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 theform:

where I and V are the rms or "effective" values. The quantity Z is calledimpedance. 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. Moregeneral is the complex impedance method.

Impedance Combinations


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

Impedance


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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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Physics of Eddy Current-An Introduction

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