Eddy Current

7 Key excerpts on "Eddy Current"

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  eBook - ePub

  Electromagnetic Fields

  Theory and Applications

  • Ahmad Shahid Khan, Saurabh Kumar Mukerji(Authors)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  21  Eddy Currents

  21.1 Introduction

  The terms current, current density, and their types were introduced in Chapter 8 . The displacement current and displacement current density were described at length in Chapter 15 . Yet another type of current, referred to as an Eddy Current, is the subject of the present chapter. This chapter describes the genesis, meaning, importance, and the applications of Eddy Currents. The Eddy Current equation and its solution in one, two, and three dimensions also is discussed. Finally, Eddy Currents in anisotropic medium are described.

  21.2 Basic Aspects

  Eddy Currents are circular electric currents induced within conductors by a changing magnetic field in the conductor, in accordance with Faraday’s law of induction . These are also referred to as Foucault currents . These flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an electromagnet or transformer, for example, by 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.

  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 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

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  Eddy Currents

  Theory, Modeling, and Applications

  • Sheppard J. Salon, M. V. K. Chari, Lale T. Ergene, David Burow, Mark DeBortoli, Sheppard Salon, Lale Ergene(Authors)
  • 2023(Publication Date)
  • Wiley-IEEE Press

    (Publisher)

  1 Part I Theory 3 1 Basic Principles of Eddy Currents 1.1 Introduction The discovery of Eddy Currents is usually attributed to French physicist Leon Foucault. In 1855, Foucault measured the force on a rotating conducting disk. He found that it took a greater force to rotate the disk when the disk was in a magnetic field produced by an electromagnet. He also noticed that the disk was heated when spun through a magnetic field. These observations, that eddy cur- rents can produce force and torque and also heating, are still major applications of Eddy Currents. The production of force or torque by Eddy Currents is key to the operation of induction motors and induction generators, Eddy Current brakes, Eddy Current magnetic bearings, liquid metal stirring, and electromagnetic metal forming, to name a few applications. The forces produced by Eddy Currents may be a hindrance. In such cases, methods of Eddy Current mitigation will be necessary. Eddy Current heating also has many applications such as induction heating for metal treatment and induction cooktops. There are also many applications in which Eddy Currents result in unwanted losses, limiting the efficiency of devices and requiring more thermal management. We will use a very broad definition of Eddy Currents in this treatment. We include all electric currents induced by time-varying magnetic fields and/or relative motion between conductors and magnetic fields. This also includes the redistribution of currents due to the self-field of conductors excited with external sources. All currents induced by a time-varying magnetic field can be thought of as Eddy Currents, but in this book, we are mainly dealing with applications in which the current density is nonuniform in the conductor. Whether we speak of Eddy Currents, skin effect, or proximity effect, we are speaking of the same physical phenomenon and this is described by the same set of equations.

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  Inductors and Transformers for Power Electronics

  • Vencislav Cekov Valchev, Alex Van den Bossche(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  163 5 Eddy Currents in Conductors 5.1 Introduction Eddy Current effects in conductors were recognized at an early time. At the end of the nineteenth century, it was known that a conductor of a (telegraphic) coaxial cable has an increased AC resistance for higher frequencies and that the inductance reduces with frequency [1]. The method that was applied used mathematical series and the derivative of the current. This approach is differ-ent from the methods we use now, but the result was correct. Eddy Currents have long been recognized in electrical machines as well. For example, the starting torque in squirrel cage induction motors is improved by a rotor resistance increase and inductance decrease caused by the Eddy Currents. In large machines and transformers, Eddy Currents have a big impact on the process of manufacturing coils because of efforts to avoid them, for example, paralleling wires and Röbel bars. Although some physical properties were known before, the real break-through of ferrites came after 1945 in the Netherlands [2]. Using ferrites, the magnetic components could be made much smaller since the main flux path was not going through the conductors and, thus, most of the Eddy Currents could be avoided. In classical electronics, more attention was given to the total core losses and the Q-factor than to a detailed analysis of Eddy Current losses [3]. Current Power Electronics Needs Because of improvements in semiconductors and soft switching topologies, much higher switching frequencies are now possible, compared to 20 years ago. As a result, most of the actual designs of magnetic components in power electronics are highly influenced by Eddy Currents. Skin Effect At high frequencies, the major part of the current flows in a thin outer layer (skin) of the conductor, so the phenomenon is called skin-effect . 164 Inductors and Transformers for Power Electronics Proximity Effect The effect by a transverse field is called proximity effect .

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  Non-Destructive Testing

  And Testability of Materials and Structures

  • Gilles Corneloup, Cécile Gueudré, Marie-Aude Ploix(Authors)
  • 2021(Publication Date)
  • PPUR

    (Publisher)

  CHAPTER 8 Eddy Current TESTING Eddy Currents (EC) have an important place in the NDT field and can be applied to all highly conductive metallic materials. They can also be applied to various industrial fields such as aeronautics, automotive, nuclear, and steel industries, etc. A conductive part is subjected to a variable, sinusoidal, or ordinary magnetic field to make induced currents appear by electromagnetic induction. These eddy cur- rents parallel to the surface are limited to the inducing magnetic field area and mainly flow close to the surface. A standard penetration depth is defined and is equal to the depth for which the amplitude of the surface is divided by Napier’s constant e. For non-magnetic materials (μ r = 1), this depth value is around a millimeter, but for magnetic materials (μ r = 100 or more), the depth of the inspection is small (0.1 mm approx.). Eddy Currents are influenced by many parameters, which is both an advantage and a restriction for the method. Their distribution depends on the frequency, con- ductivity, and magnetic permeability of the material, as well as the relative part-coil geometry. The presence of a defect, emerging or close to the surface, changes these characteristics and modifies the distribution of the induced currents. Direct measurement of the distribution is impossible, so a search is carried out indirectly. As the induced currents have the same properties as the inducing currents, they also create a magnetic field opposed to the inducing field. The value of the result- ing field represents the EC distribution, that is, the presence of defects, and can be detected at the terminals of the induction coil or by another magnetic receiver (Hall Effect probe). (a) (b) (c) Fig. 8.1 Coil in the air (a), creation of EC (b), changed by a defect (c). The measurement is indirect via the evolution of the induction coil impedance.

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  High-Frequency Magnetic Components

  • Marian K. Kazimierczuk(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  According to Lenz’s law, the induced magnetic field opposes the applied magnetic field. The magnetic field induces an EMF v( t ) = d φ/ dt = A ( dB / dt ) = A μ( dH / dt ) in a conducting material of conductivity σ , which in turn produces Eddy Currents i E ( t ) . The flow of Eddy Currents causes power losses in winding conductors and magnetic cores. The applied time-varying magnetic field H a ( t ) induces the electric field E ( t ) , which induces Eddy Currents i E ( t ) , and these currents generate a time-varying magnetic field H ( t ) that opposes the original z y x Conductor H = H m cos ω t J e Figure 1.12 Eddy Current 30 HIGH-FREQUENCY MAGNETIC COMPONENTS applied magnetic field H a ( t ) , according to Lenz’s law. The direction of the induced Eddy Currents i E ( t ) with respect to the induced magnetic field H ( t ) is determined by the RHR, as shown in Fig. 1.12. The opposing magnetic flux can be found using Amp` ere’s law ∇ × H = J a + J e , (1.188) where J a is the applied current density and J e is the Eddy Current density. When the applied current J a is zero and the magnetic field is generated by adjacent conductors, Amp` ere’s law becomes ∇ × H = J e . (1.189) The eddy-current density in a conductor of conductivity σ = 1 /ρ can be described by J e = σ E = E ρ (1.190) For sinusoidal waveforms, the phasor of the eddy-current density is given by J e = − j ωσ A , (1.191) where A is the phasor of the magnetic vector potential. Eddy Currents can be reduced using selecting high-resistivity materials (such as ferrites) or using thin plates, called laminations. These currents dissipate energy as heat in magnetic cores and winding conductors of inductors and transformers. There are two effects associated with Eddy Currents: skin effect and proximity effect. The skin effect current density J s is orthogonal to the proximity effect current density J p . The winding power loss due to Eddy Currents is P = 1 2 σ ( J s J ∗ s + J p J ∗ p ) dV .

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  University Physics Volume 2

  • William Moebs, Samuel J. Ling, Jeff Sanny(Authors)
  • 2016(Publication Date)
  • Openstax

    (Publisher)

  Identify those paths for which ε = ∮ E → · d l → ≠ 0. Check Your Understanding A long solenoid of cross-sectional area 5.0 cm 2 is wound with 25 turns of wire per centimeter. It is placed in the middle of a closely wrapped coil of 10 turns and radius 25 cm, as shown below. (a) What is the emf induced in the coil when the current through the solenoid is decreasing at a rate dI /dt = −0.20 A/s? (b) What is the electric field induced in the coil? 13.5 | Eddy Currents Learning Objectives By the end of this section, you will be able to: • Explain how Eddy Currents are created in metals • Describe situations where Eddy Currents are beneficial and where they are not helpful As discussed two sections earlier, a 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 in the conductor, we refer to that current as an Eddy Current. Magnetic Damping Eddy Currents can produce significant drag, called magnetic damping, on the motion involved. Consider the apparatus shown in Figure 13.20, which swings a pendulum bob between the poles of a strong magnet. (This is another favorite physics demonstration.) If the bob is metal, significant drag acts on the bob as it enters and leaves the field, quickly damping 602 Chapter 13 | Electromagnetic Induction This OpenStax book is available for free at http://cnx.org/content/col12074/1.3 the motion. If, however, the bob is a slotted metal plate, as shown in part (b) of the figure, the magnet produces a much smaller effect. There is no discernible effect on a bob made of an insulator. Why does drag occur in both directions, and are there any uses for magnetic drag? Figure 13.20 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.

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  Numerical Modelling

  • Peep Miidla(Author)
  • 2012(Publication Date)
  • IntechOpen

    (Publisher)

  Part 2 Maxwell's Equations 8 Numerical Modelling and Design of an Eddy Current Sensor Philip May 1 and Erping Zhou 2 1 Elcometer Ltd. and 2 University of Bolton UK 1. Introduction Eddy Current testing involves exciting a coil with a fixed frequency or pulse and bringing it into close proximity with a conductive material. The electrical impedance of the coil changes due to the influence of electrical ‘Eddy Currents’ in the material. Using an Eddy Current technique, the sizing of surface and sub-surface defects, measurements of thickness of metallic plates and of conductive and non-conductive coatings on metal substrates, assessment of corrosion, ductility, heat treatment and measurements of electrical conductivity and magnetic permeability are all possible and quantifiable. The Eddy Current method has become one of the most successful non-destructive techniques for testing conductive coatings on conductive substrates. The data acquired from Eddy Current sensors however is affected by a large number of variables, which include sample conductivity; permeability; geometry and temperature as well as sensor lift off. The multivariable properties of sample coatings add an even greater level of complexity. Many of these problems have been overcome in the laboratory using precision wound air-cored coils, multiple excitation frequencies and theoretical inversion models. High levels of agreement between theoretical models and measurement however are only possible with accurately constructed coils, which are difficult to manufacture in practice. Coils are also prone to poor sensitivity, poor resolution, and a poor dynamic range as well as self-resonance at high frequencies, which make them unsuitable for online process control. Many of the problems associated with air-cored coils however can be overcome when the coils use ferrite cores or cup cores.

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