The term eddy means circular or spiral movement or whirlpool. The current can be induced not only in a conducting wire or coil, but also in a conducting sheet or block. The nature of such current is circulatory and hence the name eddy current.
When a metal (like soft iron block) is placed in a varying magnetic field, it induces current in the metal. This induced current is called eddy current or Foucault’s current as discovered by Foucault. Its direction is given by Lenz’s law or Fleming right hand rule.
Hence, the induced circulating current produced in a conducting sheet or block due to change in magnetic flux with it is called eddy current.
Since the resistance of conductor (metal) is low so eddy current is quite large even though induced emf is small. That is why the eddy currents can produce considerable heating and magnetic effects. \[\text{Eddy current }(I)=\frac{\text{Induced emf}}{\text{Resistance of sheet}}\]\[\therefore I=\frac{\varepsilon}{R}\] From Faraday’s laws of electromagnetic induction, \[\varepsilon=-\frac{d\phi}{dt}\] Hence, \[I=-\frac{d\phi/dt}{R}\] The eddy currents flow in closed paths in planes perpendicular to the lines of induction in the mass of the metal.
Undesirable Effects of Eddy Current
Due to eddy currents, heat is produced in the interior of iron cores of the rotating armatures of electric motors, dynamo and in the cores of transformers, which experience flux changes when they are in use. Hence, the production of eddy currents in a metallic block leads to the loss of electric energy in the form of heat. The heat produced due to eddy currents break the insulation used in electric machines or appliances.
To reduce the energy loss due to eddy currents, the cores are made of thin sheets of iron called laminations. The iron sheets are insulated from each other by thin papers, varnish, etc. to increase the resistance of eddy currents and hence the heating effects are minimised.
Large resistance between the thin sheets confine the eddy currents to the individual sheets. Eddy currents cannot be removed completely but only they can be minimised. Eddy currents also cause unwanted damping effects.
Uses of Eddy Currents
Diathermy
Eddy currents are used for the localised heating of tissues in human body. This method of tratment is called diathermy.
Electromagnetic Brakes
Eddy currents are used in electric brakes to control the speed of electric trains. The metal drums are coupled with the wheels of the electric train. When a strong magnetic field is applied to a rotating drum, large eddy currents are set up in it which oppose the motion of the drum and the drum gets stopped.
Induction Furnace
It is based on the heating effect of eddy currents. In this process, a metallic block is placed in a high frequency changing magnetic field. Strong eddy currents are developed in the block and hence large heat is developed which can melt the block. This process of induction furnace is used to separate metals from their ores and to make some alloys.
Eddy current damping or Dead beat galvanometer
According to Lenz’s law, eddy currents always flow in such a direction as to oppose the motion which has produced them. So they can reduce the oscillations of a vibrating system. An example of such use of eddy currents is a moving coil galvanometer.
The coil of the galvanometer is generally wound in the frame. As the coil swings in the magnetic field of the galvanometer, eddy currents are induced in the frame. These eddy currents oppose the motion of the coil and hence the pointer attached to it. Consequently, the pointer quickly attains the final position without overshooting or oscillating violently.
In this way, eddy currents reduce the oscillations of the pointer. This is known as electromagnetic damping or eddy current damping. Such a galvanometer is called dead beat galvanometer.
Speedometer
Speedometer is a device that measures the instantaneous speed of a land vehicle. In a speedometer, a small magnet is attached to the axle of the wheel. This magnet rotates according to the speed of the vehicle in an aluminium drum. Due to the rotation of the magnet, magnetic flux linked with the aluminium drum changes and hence eddy currents are produced in it. A pointer attached to the drum is deflected in the direction of rotation of the drum. The speed is measured which corresponds to the deflection of the pointer.
Some other uses of eddy current
- It is used to record the consumption of electricity in energy meters.
- In coin vending machines, eddy currents are used to detect counterfeit coins, or slugs.
- It is used in sheet resistance measurement.
- It is used in coating thickness measurements.
- It is used in traffic detection systems.
- It is used in safety hazard and defect detection applications.
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