Induced circulating or whirling current produced in a conductor by itself due to change in its own magnetic flux linked with a conductor is called eddy current.
This current was discovered by Foucault, so it is also known as Foucault Current. The direction of these currents are in accordance with the Lenz law.
It has some advantage and so used in following applications –
Induction furnace is based on the heating effect of eddy currents.
The metallic block which requires to melt is placed in a high frequency changing or alternating magnetic field. Strong eddy currents are induced in the block. Due to the high resistance of the metal block a large amount of heat is produced in it. This heat ultimately melts the metallic block.
An induction furnace is used for the following purposes –
- In melting process for extraction of metals from their ores.
- In melting process to make alloys of metals.
- In foundries for making castings.
Speedometer is a device used to measure the instantaneous speed of a vehicle.
In a speedometer, a small magnet is attached to the axle of the wheel. This magnet is surrounded by an Aluminium drum. The magnet rotates according to the speed of the vehicle inside the 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 is attached to the drum which deflects and indicates the instantaneous speed of vehicle.
When a current is passed through the coil of a galvanometer the pointer attached to the coil gets deflected in the magnetic field. The coil with pointer keeps on oscillating for a long time. In order to stop the motion of the pointer in a shorter interval of time, the coil is wound on a non ferrous metallic frame made of Aluminium or Copper.
When a current is passed through such coil the coil along with the metallic frame oscillates. Due to rotational motion the magnetic flux linked with the metallic frame changes and hence eddy currents are produced in it.
According to Lenz law, these currents oppose the motion of the frame and hence dampen the oscillations of coil and frame. Thus, the coil comes to rest in a shorter interval of time. This type of damping is called Electromagnetic damping.
This type of galvanometer is called Dead beat galvanometer.
Electromagnetic braking system are used in trains. It is attached to the axle of wheel. Due to rolling of wheels, relative motion between the magnet and rails induces eddy currents in the rails.
By using solenoids, these currents are used for braking purpose.
Concept of eddy currents is also used in energy meters to record the electric energy consumption. Induced currents are produced in Aluminium disc used in these meters due to variation in magnetic field. Aluminium disc rotates due to eddy currents produced in it and its revolution indicates the energy consumption.
Eddy currents are used to rotate the short circuited rotor of an induction motor. Ceiling fans have also induction motors which run on single phase alternating current. Eddy currents in the rotor interact with the magnetic field of the rotor producing rotating force on the rotor.
Eddy currents produced in copper pots are used for cooking food. This process of cooking is known as induction cooking.
In medical science, the process of producing heating effect due to eddy currents for localized heating of tissues for treatment of disease is called inductothermy.
In this process, a coil of many turns is placed around the affected part of human body without touching the body part. An alternating current of high frequency is then passed through the coil which produces heating effect.
It may be noted that inductothermy is a far better treatment process than the electrode diathermy.
Undesirable effects of these currents are listed below –
- Production of these currents in a metallic block leads to the loss of electric energy in the form of heat.
- The heat produced due to eddy currents breaks the insulation used in the electrical machines or appliances.
- These currents may cause unwanted damping effect.
Metallic cores are used in A. C. electrical devices like transformer, dynamo, choke etc. Due to changing magnetic field, large eddy currents are produced in the core which cause large amount of heat in the core.
These sheets are electrically insulated and are called lamination. Insulation breaks the paths of the eddy currents and keeps the eddy currents restricted to the individual sheets. Thus, paths of the eddy currents are broken so that the loss due to eddy currents are reduced to a great extent.