Eddy Current and Applications (Important for CBSE Class XII)

Eddy Current and Applications (Important for CBSE Class XII)
If you have covered a lot of ground in EMI, you might have reached the Topic of Eddy Current. EMI essentially deals with the Induced emf that gets generated in conductors because of change in flux (d?/dt). But did you know that sometimes this induced EMF/Induced current can be a Drawback? There are advantages also of Eddy currents, but let's discuss drawbacks first. 

Drawbacks of Eddy Currents:

The disadvantage is because the induced emf that appears in blocks (Not wires !!) of Electrical Machines results in circular loops of current in the metallic block. Block could mean a Cube, Cuboid shaped Conductor) of Electrical Machines (generally we see the drawback of Eddy Currents in Electrical Machines). One such example of a block is the one that resides inside the  Transformer. The metallic block in the Transformer is called the Core of the Transformer. This core is made up of Iron (a metal). All the electrical machines have a core, e.g. a Motor also has a Core, so we do see Eddy currents in the Motor. You might have seen a Transformer near your home or a Motor in your home. That Transformer has an alternating current flowing through it. That alternating current has a magnetic flux associated with it ( Remember Magnetic effects of current). If it's not a Metallic Block one can see Eddy Currents in Metallic Frames too. In the Electrical Machines if eddy currents are getting induced in the core, then the core has an Induced current, which means that core gets heated up. Its a wastage of precious power (watts). The metallic core of the electrical core gets heated up.

How Eddy Current Drawback is handled

One can create slots inside the Metalic Core, and you would notice that Eddy currents reduce. The slots make the size of the Induced Current Loops smaller (small radii of the loop unlike a solid Metallic frame). The other way is to construct the Metallic Core using Thin Layers of Conducting Material separated by Insulating Material. The Insulating material would not let Eddy currents to flow. Insulators offer high resistance to current. Thus we would have fewer eddy currents. More about Eddy currents are covered in Electrical Engineering. One can read about Laminated Cores inside Electrical Machines that reduce the Eddy current. In reality, the core is never a Single Large metallic block but Laminated.

Advantages

Deadbeat Galvanometer-Damping effect of Eddy Current

If you notice a Galvanometer, you will find Eddy currents generated inside the Galvanometer too. The Galvanometer is used for finding out if the circuit has a Current. But we want the Galvanometer to come back to the Original reading, i.e. Equilibrium Position after the current has been withdrawn and not Oscillate for a long, long time. We can introduce Friction inside Galvanometer in such a way that Needle comes to Rest, but If Friction were to be in Picture, we would not get an accuracy of measurement in Galvanometer. The Galvanometer has coils inside it. The Coils are the one that carries the current that needs to be measured. To ensure that the Galvanometer Needle comes back to its original position without oscillating for a long, long time the current measuring Coil is wound over a metallic Frame. This metallic frame is also exposed to the Magnetic flux of the external current entering the Galvanometer coil. The metallic frame then has the Eddy Current generated. This Eddy current will oppose the very cause that created it, i.e. Motion of the Coil that carries the current in a magnetic field. If you don't use a Metallic Frame, you will get a situation where the Needle of Galvanometer keeps on oscillating and. But you want the Galvanometer needle to immediately come back to its original position ( once there is no current). Here is where the eddy current in the Metallic Frame helps. It brings the Needle to rest Instantaneously using the force experienced by the Eddy current Loops in the magnetic field. You can imagine the direction of the Eddy Current Loop such it opposes the induced circular current loops. Such a Galvanometer is called a Dead Beat Galvanometer. The Eddy current here opposes Oscillations, and the effect of eddy current is called Damping.

Separating Metal from Non-Metals

Eddy currents can also be used for separating metals from non Metals. When the Truck unloads the Garbage that contains metal and Non-metal. You can see how the Municipality Truck unloads the Garbage. The Garbage travels down an incline. As it moves, we can keep magnet below the Incline. This magnet will generate the eddy currents, and the Metallic Trash will experience damping and slow down. This is called Motional Drag. The Non-Metallic Trash can continue.

Using Eddy Currents to Find the Cracks in the Conductors

Eddy currents are used for detecting cracks in the surface of the conductive materials. Eddy currents are circular current loops, and hence they also produce Magnetic Field. If the conductive material has cracks, the magnetic field provided at the cracked surface will be different. So we can study the magnetic field on the surface of the Conductive material. Where ever the magnetic surface is cracked, the eddy current will be absent. So this is used in aeroplanes where the surfaces are tested for eddy currents. A crack in an aeroplane can result in a disaster.