1. The principles of eddy current testing depend on the process of electromagnetic induction. 2. This process includes a test coil through which vary or alternating current (AC) is passed. 3. A varying current flowing in a test coil produces a varying electromagnetic field around the coil, As shown in this diagram

Amperage * # of coils= strength of magnetic field

4. The electromagnetic field produced around the coils is directly proportional to the magnitude of applied current, rate of change in current or frequency and the coil parameters. 5. Coil parameters include: Inductance, diameter, length, and thickness, number of turns of wire and core material. 6. Electrical current is defined as the movement of electrons through a conductor 7. The unit of current is the ampere

a. Ampere symbol is “A”

b. An amp = Coulomb/sec.

c. Coulomb= 6.25x1018 Electrons

8. A Conductor is any material that is capable of carrying electrical current, such as copper or iron. 9. Wood and plastic are not conductors.

10. Whether a material can conduct electricity or not depends on the structure of the individual atoms in the material d. 3 or less valence electrons makes for a good conductor 11. Ferromagnetic: Very little to some ability to take on a magnetic field e. Diamagnetic- essentially an insulator

f. Paramagnetic- Only magnetically attracted when an external magnetic force applied. i. Austenitic stainless steel

12. Voltage is the push

Electromotive Force

1. Electromagnetic force is the electrical energy derived from mechanical, chemical, or other form of energy that must be applied across the material to force the electrons to move 2. The unit of the electromagnetic force is called the VOLT 3. Resistance is the opposition to current flow.

Resistance

1. In an alternating current circuit containing only resistance, the resistance simply limits the amount of current that flows through the circuit. 2. It does not change the phase relationship between the voltage and the current. 3. The current is exactly in phase with the voltage.

4. The unit of resistance is the ohm

a. Ohm is defined as the resistance through which electromotive force of 1 Volt will produce a current of 1 Ampere b. Ohms’ law is expressed as follows:

i. E=I*R

Where E is Voltage (volt), I is current (Ampere), and R is resistance (Ohm) 5. Resistance =( specific resistance*the length)/ CSA

c. Where resistance is in ohms

d. Specific resistance in ohms/circular mil-foot

e. Area is in circular mils

f. Length is in feet

6. Coil resistance is determined by the length of wire used to wind a coil. 7. The specific resistance is determined by the wire type and the cross-sectional area of the wire. Sine Wave

1. A sine wave is the form commonly produced by alternating current generators. 2. Since on turn (3600 rotation) of the generator coil produces one cycle of the sine wave, the sine wave can be marked into corresponding degrees of rotation, as shown in the following figure.

Frequency

1. The frequency of an alternating current is defined as the number of cycles of current that occur in one second 2. The unit of frequency is the Hertz

a. Symbol is Hz

3. One hertz is equal to one cycle per second. Current at 60 cycles per second has a frequency of 60 Hz.

Inductive Reactance

1. The opposition to changes in alternating current flow through a coil is called inductive reactance and is designated by the letters XL a. XL=WL

b. XL is the inductive reactance (ohm)

c. L is the inductance (Henries)

d. W equals 2(pi)f

Impedance

2. Impedance in an alternating current circuit is the total opposition to the current flow through the circuit. 3. The impedance

Phase Angle

1. The phase angle between the resistance...