Eddy Current

Question 1

What is Impedance, and why does it change?

Answer 1

Impedance is the Ratio of the Electrical field strength to the Magnetic field strength.

The impedance changes when the magnetic field is distorted due to defects.

Question 2

How is the Impedance of the coil created?

Answer 2

The alternating current interacts against the eddy current to create the Impedance.

Question 3

How does the eddy current work?
Describe the steps in only 6 steps.

Answer 3

An alternating current passes through a coil, generating a magnetic field.
When the coil is placed near a conductive material, eddy currents are induced in the material.
The eddy currents create their own magnetic field, which opposes the original field.
This opposition changes the impedance of the coil.
A receiver detects and measures the impedance change.
The change in impedance indicates the presence of flaws or changes in the material.

Question 4

What is the definition of Eddy Current?

Answer 4

Eddy current is defined as a circulating electrical current induced in a conducting article by an
alternating magnetic field. As the magnetic field alternates, so does the eddy current (reverses).

Question 5

What is the principle of Eddy Current?

Answer 5

The path of the eddy current is distorted by the presence of a defect or inclusion. The change in
impedance in the coil can be measured and used to give an indication of defects or difference in
physical, chemical and metallurgical structure.
The induced eddy current is concentrated near the surface of the specimen, resulting in the so-called
‘skin effect’.

Question 6

List the Applications of Eddy Current

Answer 6

 Metal sorting.
 Detection of cracks, voids, inclusions.
 Determination of coating thickness.
 Measurement of thickness of non-conducting films on electrically conducting base materials.

Question 7

List the Factors that affect the Magnitude of Eddy Current.

Answer 7

 The magnitude and frequency of the alternating current.
 The electrical conductivity, magnetic permeability, and shape of the specimen.
 The relative position of coil and specimen.
 The presence of discontinuities or inhomogeneities in the specimen.

Question 8

What are the three types of Test coils and probes?

Answer 8

(a) The concentric or encircling coil - completely surrounds the specimen.
(b) The point probe or surface coil - a small coil that can be placed near the surface of
the specimen.
(c) The inside or bobbin type coil - can move through tube or pipe.

Question 9

What are 4 adjustments you can make on the LCD screen?

Answer 9

Grain, Rotational angle, Horizontal gain, and Vertical gain