Eddy current:
Current can be induced, not only in conducting coils but also in conducting sheets or blocks. Whenever the magnetic flux linked with a metal sheet or block, changes an EMF is induced in it. The induced current flow in closed paths in planes perpendicular to the lines of force throughout the body of the metal. These currents look like eddies or whirls-pools in water and so they are known as eddy currents.
Eddy currents are the currents induced in solid metallic masses when the magnetic flux threading through them changes
Eddy currents also oppose the change in magnetic flux, so their direction is given by Lenz's law.
Experiment to demonstrate eddy currents:
Experiment 1:Take a pendulum having its bob in the form of a flat copper plate. As shown in fig it is free to oscillate between the pole pieces of an electromagnet. In the absence of any magnetic field, the pendulum swings freely. As the electromagnet gets switched on the oscillations of the pendulum get highly damped and soon it comes to rest. This is because as the copper plate moves in between the pole pieces of the magnet, magnetic flux threading through it changes. S eddy currents are set up in it which according to Lenz's law, oppose the motion of the copper plate in the magnetic field. Eddy currents flow anticlockwise as the plate swings into the field and clockwise as the plate swings of the field.
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| Eddy current damp the oscillation of a copper plate in a magnetic field |
Undesirable effects of eddy currents:
Eddy current is produced inside the iron cores of the rotating armatures of electric motors and dynamos, and also in the cores of transformers, which experience flux changes when the are in use. Eddy currents cause unnecessary heating and wastage of power. The heat produced by eddy currents may even damage the insulation of the coil.
Minimization:
The eddy current can be reduced by using laminated core which instead of a single solid mass consists of thin sheets of metal, insulated from each other by a thin layer of varnish, as shown in fig. The planes of the sheets are placed perpendicular to the direction of the currents that would be set up but the emf induced in the material. The insulation between the sheets then offers high resistance to the induced emf and the eddy currents are substantially reduced.
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| (a)Solid core (b)Laminated core |
Application of eddy currents:
[1]Induction furnace: If a metal specimen is placed in a rapidly changing magnetic field (produced by high-frequency AC) very large eddy currents are set up. The heat produced is sufficient to even melt the metal. This process is used in the extraction of some metals from their ores.
[2] Electromagnetic damping: When a current is passed through a galvanometer, its coil suffers few oscillations before coming to rest in the final position. As the coil moves in the magnetic field, the induced current is set in the coil are damped. This is called electromagnetic damping. The electromagnetic damping can be further increased by winding the coil on a light copper or aluminum frame. As the frame moves in the magnetic field , eddy currents are set up in the frame which resists the motion of the coil. This is how a galvanometer is rendered dead beat i.e. the coil does not oscillate-it deflect and stays in the final position immediately.
[3] Electric breaks: A strong magnetic field is applied to the rotating drum attached to the wheel. Eddy currents set up in drum exert a torque on the drum so a stop the train.
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| Eddy current brake rely based on electromagnetism to stop the train |
Eddy Current Braking Systems in the Real World:
So, how do braking systems using eddy currents work in the real world? One design developed and tested by a German railway company uses a linear array of eight electromagnets fitted between the wheels, at a distance of about 7 mm from the rail. Train operators can turn on these magnets when they want to slow down, which causes the magnets to generate a magnetic field that expands into the rail. Because the rail is stationary, it will experience a concentrated magnetic field moving in at high velocity, and strong eddy currents will develop. These eddy currents are a result of the rail resisting the enforced change in magnetic flux: They flow in such a direction that the rail generates its own magnetic field, which tries to counteract (expel) the applied one. The two magnetic fields repel each other and a braking force results — meaning the train will come to a frictionless stop.
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| Eddy current brake on train |
[1]Inductothermy: Eddy currents can be used to heat localized tissues of the human body. This branch is called inductothermy.
[2]Energy meters: In energy meter used for measuring electric energy, the eddy currents induced in an aluminum disc are made use of.
[3]Speedometers: In a speedometer, a magnet rotates with the speed of the vehicle. The magnet is placed inside an aluminum drum which is carefully pivoted and held in position by a hairspring. As the magnet rotates, eddy currents are set up in the drum which opposes the motion of the magnet. A torque is exerted on the drum in the opposite direction which deflects the drum through an angle depending on the speed of the vehicle.
[4]Induction motor: In an A.C. induction motor, a rotating magnetic field is produced by two single-phase alternating currents having a phase difference of 90. A metallic rotor is placed in the magnetic field. The eddy current setup in the rotor tends to oppose the relative motion between the rotating magnetic field and the rotor. As a result of the rotor also starts rotating about its axis.
Knowledge points:
- Eddy currents are basically the induced current set up inside the body of conductor whenever the magnetic flux linked with it changes.
- Eddy currents tend to follow the path of least resistance inside a conductor S they form irregularly shaped loops. However, their direction is not random but guided by Lenz's law
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| Lenz's law application |
- Eddy currents can be induced in biological tissues, For example, the cavity of the eye is filled with a conducting fluid. A large transient magnetic field of 1 T alternating at a frequency of 60 HZ then induces such large current in the retina that it produces a sensation of intense brightness.