T.O. 33B-1-1
4-19
consider only the inductance of a single circuit element, specifically the coil used to sense changes in eddy current
flows in test specimens. This inductance is called self-inductance (L).
4.3.2.2.1
Self Inductance.
Self-inductance (L) is expressed in henrys. A henry is the inductance by which one volt is produced across a coil when
the inducing current is changed at the rate of one ampere per second. A formula for self-inductance expressed in these
terms is as follows:
L = E / (DI / T)
Where:
L
=
Inductance (henrys)
E
=
Induced Electromotive Force (volts)
DI = Change in Current (amperes)
T
=
Time (seconds)
4.3.2.2.1.1
Because the henry is such a large unit, inductance is more commonly expressed in terms of millihenrys (1/1000 henry)
or microhenrys (1/1,000,000 henry). Typical coils used in eddy current inspection have self-inductances in the range of
10 to several hundred microhenrys.
4.3.2.2.1.2
The inductance of a coil depends upon the number of turns in the coil, the size of the coil, the permeability of the
material within the coil (i.e., the core of the coil), and total magnetic flux through the coil. An alternate method of
expressing self-inductance (L) is:
L = n f / I
Where:
L
=
Inductance (henrys)
n
=
Number of turns in coil
f
=
Magnetic flux (webers)
I
=
Current through coil (amperes)
4.3.2.2.2
Inductive Reactance.
The measure of the amount of opposition or resistance (ohms) to alternating current flow due to inductance in a coil is
called inductive reactance. Inductive reactance is dependent upon the value of the inductance of the coil and the
frequency of the alternating current. The inductive reactance increases as the inductance or frequency increases. This
can be stated by the following equation:
XL = 2 p f L
Where:
XL =
Inductive reactance (ohms)
f
=
frequency (Hertz)
L
=
Inductance (henrys)
4.3.2.2.2.1
The inductive reactance results from the electromotive force generated across a coil by the alternating current. The
instantaneous value of this induced voltage increases and decreases as the rate of change of the applied alternating
current increases and decreases as shown in Figure 4-14. The voltage is at its maximum value when the rate of current
change is at its maximum; this occurs when the current value is at zero. Conversely, the voltage is zero when the rate
of current change is zero; this occurs when the current is at its maximum value. Considering 360 degrees to be one
complete cycle, the induced voltage leads the current (i.e., is out of phase with the current) by 90 degrees as illustrated
in Figure 4-14. The induced voltage is in opposition to the electromotive force applied to the coil, reducing the
amplitude of the resultant current.
