Upload
charlie-chong
View
222
Download
0
Embed Size (px)
Citation preview
8/10/2019 Physics of Eddy Current-An Introduction
1/49
Physics of Eddy Current- ET
- An Understanding2014-OctoberMy ASNT Level III Pre-Exam Preparatory Self Study Notes
Charlie Chong/ Fion Zhang
8/10/2019 Physics of Eddy Current-An Introduction
2/49
Charlie Chong/ Fion Zhang
Expert at Works
8/10/2019 Physics of Eddy Current-An Introduction
3/49
Charlie Chong/ Fion Zhang
Expert at Works
http://eddycurrent.net/gallery/index.php/Palm-Cooling-Discovery-Gardens/IMG_2100
8/10/2019 Physics of Eddy Current-An Introduction
4/49Charlie Chong/ Fion Zhang
Expert at Works
http://ropax.co.uk/eddy-current-inspection.html
8/10/2019 Physics of Eddy Current-An Introduction
5/49Charlie Chong/ Fion Zhang
Expert at Works
http://ropax.co.uk/eddy-current-inspection.html
8/10/2019 Physics of Eddy Current-An Introduction
6/49Charlie Chong/ Fion Zhang
Expert at Works
http://ropax.co.uk/eddy-current-inspection.html
8/10/2019 Physics of Eddy Current-An Introduction
7/49Charlie Chong/ Fion Zhang
8/10/2019 Physics of Eddy Current-An Introduction
8/49Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
9/49Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
10/49Charlie Chong/ Fion Zhang
Contents
1. Introduction2. The Principle of Eddy Current Testing
3. Eddy Current
4. Faradays Law
5. Lenzs Law6. Inductance
7. Impedance
8/10/2019 Physics of Eddy Current-An Introduction
11/49Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
12/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
13/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
14/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
15/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
16/49
Charlie Chong/ Fion Zhang
Depth of Detection Vs Frequency
8/10/2019 Physics of Eddy Current-An Introduction
17/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
18/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
19/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
20/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
21/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
22/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
23/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
24/49
Charlie Chong/ Fion Zhang
Introduction to Physics of Eddy Current:
http://www.youtube.com/embed/djFvnFy3rJc
http://www.youtube.com/watch?v=V-IW6cFIt9E
8/10/2019 Physics of Eddy Current-An Introduction
25/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
26/49
Charlie Chong/ Fion Zhang
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/
8/10/2019 Physics of Eddy Current-An Introduction
27/49
Charlie Chong/ Fion Zhang
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/
8/10/2019 Physics of Eddy Current-An Introduction
28/49
Charlie Chong/ Fion Zhang
Induction Heating
http://inductionbending.co.uk/gallery
8/10/2019 Physics of Eddy Current-An Introduction
29/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
30/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
31/49
Charlie Chong/ Fion Zhang
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 )
8/10/2019 Physics of Eddy Current-An Introduction
32/49
Charlie Chong/ Fion Zhang
Fleming Right Hand Rule - (Dynamo Rule)
www.youtube.com/embed/d_aTC0iKO68
www.youtube.com/embed/bBwM3Q6zGag
Ch G Th O i
8/10/2019 Physics of Eddy Current-An Introduction
33/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
34/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
35/49
Charlie Chong/ Fion Zhang
Levitation
4 0 Faraday's Law
8/10/2019 Physics of Eddy Current-An Introduction
36/49
Charlie Chong/ Fion Zhang
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.
8/10/2019 Physics of Eddy Current-An Introduction
37/49
Charlie Chong/ Fion Zhang
8/10/2019 Physics of Eddy Current-An Introduction
38/49
Charlie Chong/ Fion Zhang
5 0 Lenz's Law
8/10/2019 Physics of Eddy Current-An Introduction
39/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
40/49
Charlie Chong/ Fion Zhang
Faraday s & Lenz s Laws
www.youtube.com/embed/1-aoGz5X_j0
www.youtube.com/embed/ZMAd9DrnNGY www.youtube.com/embed/Vs3afgStVy4
YouTube
8/10/2019 Physics of Eddy Current-An Introduction
41/49
Charlie Chong/ Fion Zhang
YouTube
,
6 0 Inductance
8/10/2019 Physics of Eddy Current-An Introduction
42/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
43/49
Charlie Chong/ Fion Zhang
www.youtube.com/embed/NgwXkUt3XxQ
www.youtube.com/embed/X2e9x104AnE
www.youtube.com/embed/4PvOFovZQpQ
7.0 Impedance
8/10/2019 Physics of Eddy Current-An Introduction
44/49
Charlie Chong/ Fion Zhang
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
8/10/2019 Physics of Eddy Current-An Introduction
45/49
Charlie Chong/ Fion Zhang http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imped.html#c4
Impedance
8/10/2019 Physics of Eddy Current-An Introduction
46/49
Charlie Chong/ Fion Zhang
www.youtube.com/embed/Pj4Rq1ZIeDI
www.youtube.com/embed/FEERuJlwBxE
www.youtube.com/watch?v=xyMH8wKK-Ag
www.youtube.com/embed/y1ES6WrALzI
8/10/2019 Physics of Eddy Current-An Introduction
47/49
Charlie Chong/ Fion Zhang
Good Luck!
8/10/2019 Physics of Eddy Current-An Introduction
48/49
Charlie Chong/ Fion Zhang
Good Luck!
8/10/2019 Physics of Eddy Current-An Introduction
49/49
Charlie Chong/ Fion Zhang