Eddy currents

                Eddy current

Eddy currents are the currents induced in solid metallic masses when changing magnetic flux passes through them. Changing magnetic fields set up current loops in the metallic masses. These loops are irregularly shaped. 

  Eddy currents also oppose the change in magnetic flux, so their direction is given by Lena's law. 

 Experiments to demonstrate Eddy currents : 

Experiment 1. Shows a copper plate free to swing between the poles of a strong magnet. The oscillations of the plate are highly damped and the plate comes to rest in a short time. As the plate moves in and out of the magnetic field, the magnetic flux linked with it changes. This induces Eddy currents in the plate and opposes it's motion in the magnetic field. 

Experiment 2. Now take the pendulum of a flat copper plate with narrow slots cut across it. As rhe plate is set into oscillation, Eddy currents are set up in the plate. But this plate swings for longer duration than the plate without slots. This is because the loop has much larger paths for the electrons to travel. Larger paths offer more resistance to electrons and so the Eddy currents are sufficiently reduced. As a result, the opposition to the oscillations becomes very small. 

Undesirable effects of Eddy currents. 

Eddy currents are produced inside the iron cores of electric motors, dynamos and transformers, which experience flux changes, when they are in use. Eddy currents cause un necessary heating and wastage of power. 

Minimisation

The Eddy currents can be reduced by using laminated core which instead of a single solid mass consists of thin sheets of metal, insulated from each other bys thin layer of varnish, the planes of the sheets are placed perpendicular to the direction of the currents that would be set up by 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. 

Applications of Eddy currents. 

Eddy currents are used to advantage in some applications described below : 

1. Induction furnace.                                        If a metal specimen is placed in a rapidly changing magnetic field (produced by high frequency a.c. ), 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.  

Some galvanometers have a fixed core made of non - magnetic metallic material. When the coil oscillates, the Eddy currents generated in the core oppose the motion and bring the coil to rest quickly. 

3. Electric brakes. 

A strong magnetic field is applied to the rotating drum attached to the wheel. Eddy currents set up in the drum exert a torque on the drum so as to stop the train. 

4. Electric power meters. 

In an analogue type electric power meter, an aluminium disc rotates due to Eddy currents. These currents are induced by magnetic fields produced by sinusoidally varying currents in a coil.