EDDY CURRENTS
Content
INTRODUCTION
WORKING OF EDDY CURRENT
STRENGTH OF EDDY CURRENT
REVIEW OF LITERATURE
APPLICATION OF EDDY CURRENT.
BIBLIOGRAPHY
Eddy current
introduction
An eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. It is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause a circulating flow of electrons, or a current, within the conductor. These circulating eddies of current create electromagnets with magnetic fields that opposes the change of the magnetic field (see Lenz’s law). The stronger the applied magnetic field, or the greater the electrical conductivity of the conductor, or the faster the field that the conductor is exposed to changes, then the greater the currents that are developed and the greater the opposing field.
The term eddy current comes from analogous currents seen in water when dragging an oar breadthwise: localised areas of turbulence known as eddies give rise to persistent vortices.
Eddy currents, like all electric currents, generate heat as well as electromagnetic forces. The heat can be harnessed for induction heating. The electromagnetic forces can be used for levitation, creating movement, or to give a strong braking effect. Eddy currents can often be minimised with thin plates, by lamination of conductors or other details of conductor shape.
WORKING OF
EDDY CURRENTS
As the circular plate moves down through a small region of constant magnetic field directed into the page, eddy currents are induced in the plate. The direction of those currents is given by Lenz’s law.
When a conductor moves relative to the field generated by a source, then electro-magnetic fields (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 the changes in the field.
Eddy currents are created when a moving conductor experiences changes in the magnetic field generated by a stationary object, as well as when a stationary conductor encounters a varying magnetic field. Both effects are present when a conductor moves through a varying magnetic field, as is the case at the top and bottom edges of the magnetized region shown in the diagram. Eddy currents will be generated wherever a conducting object experiences a change in the intensity or direction of the magnetic field at any point within it, and not just at the boundaries.
The swirling current set up in the conductor is due to electrons experiencing a Lorentz force that is perpendicular to their motion. Hence, they veer to their right, or left, depending on the direction of the applied field and whether the strength of the field is increasing or declining. The resistivity of the conductor acts to damp the amplitude of the eddy currents, as well as straighten their paths. Lenz’s law encapsulates the fact 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.
Eddy currents generate resistive losses that transform useful forms of energy, such as kinetic energy, into heat, which is generally much less useful. In many devices, this Joule heating reduces efficiency of iron-core transformers and electric motors and other devices that use changing magnetic fields. Eddy currents are minimized in these devices by selecting magnetic core materials that have low electrical conductivity (e.g., ferrites) or by using thin sheets of magnetic material, known as laminations. Electrons cannot cross the insulating gap between the laminations and so are unable to circulate on wide arcs. Charges gather at the lamination boundaries, in a process analogous to the Hall effect, producing electric fields that oppose any further accumulation of charge and hence suppressing the eddy currents. The shorter the distance between adjacent laminations (i.e., the greater the number of laminations per unit area, perpendicular to the applied field), the greater the suppression of eddy currents.
The loss of useful energy is not always undesirable, however, as there are some practical applications. One is in the brakes of some trains known as an eddy current brake. During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The eddy currents meet resistance as charges flow through the metal, thus dissipating energy as heat, and this acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion.
Strength of eddy currents
Some things usually increase the size and effects of eddy currents:
- stronger magnetic fields
- faster changing fields (due to faster relative speeds or otherwise)
- thicker materials
- Lower resistivity materials (aluminium, copper, silver etc.)
Some things reduce the effects
- weaker magnets
- slower changing fields (slower relative speeds)
- thinner materials
- slotted materials so that currents cannot circulate
- laminated materials so that currents cannot circulate
- higher resistance materials (silicon rich iron etc.)
Review of Literature
1)Experiments with eddy currents: the eddy current brake
Manuel I González 2004 Eur. J. Phys. 25 463-468
Abstract – A moderate-cost experimental setup is presented to help students to understand some qualitative and quantitative aspects of eddy currents. The setup operates like an eddy current brake, a device commonly used in heavy vehicles to dissipate kinetic energy by generating eddy currents. A set of simple experiments is proposed to measure eddy current losses and to relate them to various relevant parameters. Typical results for each of the experiments are presented, and comparisons with theoretical predictions are included. The experiments, which are devoted to first-year undergraduate students, deal also with other pedagogically relevant topics in electricity and magnetism, such as basic laws, electrical measurement techniques, the sources of the magnetic field and others.
Print publication: Issue 4 (July 2004)
Received 6 February 2004
Published 20 April 2004
2) Gmr and eddy current sensors in use of stress measurement
Werner ricken and wolf jurgen becker. Department of Electrical Engineering, Measurement Technology, University of Kassel, Wilhelmshoher Allee 71, 34109 Kassel, Germany.
Abstract-The use of high sensitive magnetic sensors like magnetoresistive sensors allows novel applications in non-destructive testing (NDT), i.e. monitoring the conditions of reinforced concrete constructions. Using novel magnetoresistive sensors, a stress measurement of prestressed steel bars in concrete is realizable. The giant magnetoresistive sensor (GMR sensor) and the eddy current sensor are introduced here for high sensitive stress measurement. The combination of the eddy current technique with the magnetoelastic stray field measurement basically improves the mechanical stress measurement.
3) Improvement of the properties of an eddy current magnetic shield with active compensation
J Malmivuo et al 1987 J. Phys. E: Sci. Instrum. 20 151-164
Electron. Lab., Tampere Univ. of Technol., Finland
Abstract. The authors have improved the properties of their eddy current shield by equipping it with an active compensation system to cancel the static and low-frequency magnetic field. The performance of the compensation system is measured very accurately. These measurements and the measurement of biomagnetic fields show that an eddy-current shield with an active shielding is a practical solution for clinical biomagnetic measurements because of its low cost and the small space it needs.
4) MWM eddy current sensors for monitoring of crack initiation and growth during fatigue tests and in service
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Vladimir Zilberstein, Darrell Schlicker, Karen Walrath, Volker Weiss and Neil Goldfine
JENTEK Sensors, Inc., 110-1 Clematis Avenue, Waltham, MA 02453-7013, USA
Abstract-A new surface-mountable (conformable foil) eddy-current sensor called the Meandering Winding Magnetometer-Array (MWM™-Array) has the capability to monitor crack initiation and growth in fatigue test coupons. Fatigue tests with the MWM-Array mounted on the surface inside a 6.4-mm hole in an Al 2024-T3 tension–tension fatigue specimen, demonstrated the capability to detect cracks with l<50 μm (a<25 μm). This provides a new capability to monitor fatigue tests of coupons and components to determine the number of cycles to crack initiation and to study ‘short crack’ growth. Also, MWM conductivity measurements correlate with crack length, permitting crack length monitoring. Ongoing research is focused on depth monitoring as well. Numerous applications are under evaluation for permanently mounting MWM-Arrays in difficult-to-access locations on commercial and military aircraft. The motivation for permanently mounting MWM eddy current sensors is either (1) to replace an existing inspection that requires substantial disassembly and surface preparation (e.g. inside the fuel tank of an aircraft), or (2) to take advantage of early detection and apply less invasive life-extension repairs, as well as reduce interruption of service when flaws are detected. Implementation of permanently mounted MWM-Arrays and sensors is expected to improve fleet management practices and modify damage tolerance assumptions.
5) An Integral Computational Model for Crack Simulation and Detection via Eddy Currents
Authors: Albanese R ,rubinacci g, Villone F.
Abstract:
In this paper an innovative technique is described to solve the electromagnetic problem in the presence of a cracked conductor. Both the direct problem (given the crack, compute the scattered field) and the inverse problem (given the external measurements, obtain the crack position and shape) are dealt with. The features of an integral formulation in terms of a two-component electric vector potential expanded over edge elements are fully exploited. The resulting method proves to be extremely efficient, also thanks to the binary nature of the unknown. In this paper we restrict our attention to a class of problems, namely the eddy current testing for thin cracks in non-magnetic metallic plates, but the method can be extended to more general cases.
Applications
Electrical
Eddy currents are used to great effect in movement-to-electricity converters such as electrical generators and dynamic microphones.
Repulsive effects/levitation
Superconductors allow perfect, lossless conduction, which creates perpetually circulating eddy currents that are equal and opposite to the external magnetic field, thus allowing magnetic levitation. For the same reason, the magnetic field inside a superconducting medium will be exactly zero, regardless of the external applied field.
In addition, in a fast varying magnetic field the induced currents, in good conductors, particularly copper and aluminium, exhibit diamagnetic-like repulsion effects on the magnetic field, and hence on the magnet and can create repulsive effects and even stable levitation, albeit with reasonably high power dissipation due to the high currents this entails.
They can thus be used to induce a magnetic field in aluminum cans, which allows them to be separated easily from other recyclables.
Side Effects
Eddy currents are the root cause of the skin effect in conductors carrying AC current.
METAL DETECTORS
Metal detectors use electromagnetic induction to detect metal. Uses include de-mining (the detection of land mines), the detection of weapons such as knives and guns, especially at airports, geophysical prospecting, archaeology and treasure hunting. Metal detectors are also used to detect foreign bodies in food, and in the construction industry to detect steel reinforcing bars in concrete and pipes and wires buried in walls and floors.
In its simplest form, a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field. If a piece of electrically conductive metal is close to the coil, eddy currents will be induced in the metal, and this produces an alternating magnetic field of its own. If another coil is used to measure the magnetic field (acting as a magnetometer), the change in the magnetic field due to the metallic object can be detected.
Adjustable-speed drive
Adjustable speed drive (ASD) or variable-speed drive (VSD) describes equipment used to control the speed of machinery. Many industrial processes such as assembly lines must operate at different speeds for different products. Where process conditions demand adjustment of flow from a pump or fan, varying the speed of the drive may save energy compared with other techniques for flow control.
Where speeds may be selected from several different pre-set ranges, usually the drive is said to be “adjustable” speed. If the output speed can be changed without steps over a range, the drive is usually referred to as “variable speed”.
Adjustable and variable speed drives may be purely mechanical, electromechanical, hydraulic, or electronic.
Electricity meter
An electric meter or energy meter is a device that measures the amount of electrical energy supplied to or produced by a residence, business or machine.
Eddy current brake
An eddy current brake of a German ICE 3 in action.
An eddy current brake, like a conventional friction brake, is responsible for slowing an object, such as a train or a roller coaster. Unlike friction brakes, which apply pressure on two separate objects, eddy current brakes slow an object by creating eddy currents through electromagnetic induction which create resistance, and in turn either heat or electricity.
Proximity sensor
A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic or electrostatic field, or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. The object being sensed is often referred to as the proximity sensor’s target. Different proximity sensor targets demand different sensors. For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor requires a metal target.
Applications
BIBLIOGRAPHY
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- www.wikipedia.com
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