How to Conduct an Eddy Current Test

An eddy current is a type of current that is produced due to changes in the magnetic field. In other words, when a magnetic flux is flowing inside a coil, eddy currents are subsequently formed. These currents are created by electromagnetic induction. They are always present in closed loops. Eddy currents produce a magnetic field that opposes a device’s original magnetic field. This leads to a rise in the temperature inside transformers, creating power losses. The eddy current testing process is used to check for flaws in metals that are difficult to find with the naked eye.

Eddy Current Testing

Eddy current testing uses the method of electromagnetic induction to check the presence of eddy currents in a product or metal. Moreover, conducting an eddy current test shows the potential damage caused by eddy currents. 

In this method, a coil specially designed for testing is supplied with currents. When placed on the material’s surface, the current produces an opposite magnetic field, leading to eddy currents. 

The automobile and aerospace sectors frequently conduct eddy current tests on their materials.

How Does an Eddy Current Test Work?

The process of eddy current testing mainly depends on the phenomenon of electromagnetic induction: 

• When a current is passed through any conducting coil, a magnetic field is developed that opposes the current

• If this coil is brought closer to any other conducting coil, this change permeates the coil

• Thus, an eddy current is created inside that coil by the concept of Lenz’s law. This induces a magnetic field inside the second coil

• The magnetic fields of both the coils oppose each other. Thus, we see some deviation in the current and the voltage reading in the galvanometer

• Any change in this system will cause a change in the magnitude of the induced current. This includes the thickness of the coil and the distance between the coils

Permeability

This method is used to calculate the magnitude of the coil being magnetised. The higher the permeability, the lower the level of penetration. 

In non-magnetic metals like steel or aluminium, the permeability is very low. On the other hand, magnetic metals will have much higher permeability. The level of penetration can be changed by altering the frequency of the current. 

Advantages of Eddy Current Testing

We conduct an eddy current test to check the presence of an eddy current. This practice has many advantages, such as:

• We can check for cracks that appear on the surface of the metal, even if they are very small

• Defects can also be detected in different layers of the coil, even if the coil has coatings

• The eddy current testing process can tell us how efficient a machine is

• The electrical conductivity of the systems can be measured

• This method is non-contact. So, it can help us check surfaces that have a very high temperature or even those that are underwater

Limitations of Eddy Current Testing

The eddy current testing process also has a few limitations:

• This test can only be performed on conductive surfaces

• The level of penetration will be different in every case

• The flaws in the material cannot be detected if the testing object is kept parallel

• As every material has different permeability, it becomes difficult to perform the test

Eddy Current Applications

The main examples of equipment that makes use of eddy currents are:

• Gym equipment

• Roller coasters

• High-speed trains 

• Mobile speedometers

• Induction furnaces 

• Galvanometers    

• Trains that employ brakes

Conclusion

An eddy current is based on the principle of electromagnetic induction. It is a current that opposes the magnetic current flowing inside the coil. Eddy currents can be affected by the presence of cracks or damage in the coil. They are used in many devices, such as high-speed trains, induction furnaces, and galvanometers. 

We can test these currents using the eddy current testing process that has been discussed in this article. The limitations of the test should be kept in mind.