Saturday, December 18, 2010

Eddy current Theory

Eddy current theory is based on electromagnetic induction. That means, a variable magnetic field can induce an electric current on a conductor which is separate to the source of the magnetic field, and also vice versa, which is an electric current generates a magnetic field. Similar theory is used for the magnetic particles inspection as well.
Electromagnetic induction is used in many applications such as, Transformers, Electric motors, Generators, etc...

In the field of Non destructive testing (NDT), we manipulate with this theory which I mentioned above, and try to find surface defects or/and shallow internal defects of ferromagnetic materials.

In Eddy current NDT method, we have a specially made coil in place of the magnet. This coil generates an electromagnetic field with use of an Alternating Current (AC). Because of the AC current, we don’t need to move this coil (which is in place of the magnet) here and there in order to have a variable magnetic field (magnetic flux). With respect to the magnetic field, it will simulate a movement kind of a situation virtually.

Let’s go through these pictures I prepared for better understanding of the eddy current theory.

1. In place of the earlier discussed magnet, we use a coil.

 2. We apply an AC current in order to have a variable magnetic flux. Magnetic field will change it’s direction with respect to the frequency of the AC.

3. Now we use a test piece with no defects.

4. Note that there will be eddy currents appear in the places where magnetic field crosses over the test piece. These eddies will also change their directions (clockwise and counter-clockwise) with respect to the frequency of the AC.
5. Now let’s forget about the coil and its magnetic field and concentrate only on the current Eddies.  
6. If there is an electric current on a ferromagnetic material, a magnetic field should be generated around it.  

7. This newly generated magnetic field reacts with the coil we have been using and generate a current (which is less powerful), opposite to coil’s default AC’s current at a “t” time. Therefore, the ammeter which is placed at the coil will show bit less electric current than what it conduct while it is away from the test piece. 
8. What happens when there is a defect, a crack for example, at the test piece as shown?
Eddy currents will travel around the crack (Cracks are open spaces which do not conduct electricity).

9. These new shaped current eddies will generate its own magnetic field which is different from the earlier one.

10. That will show a change on the ammeter. Therefore our NDT technician will finds out that there is something wrong on that exact place.