T.O. 33B-1-14-19consider only the inductance of a single circuit element, specifically the coil used to sense changes in eddy currentflows in test specimens. This inductance is called self-inductance (L).4.3.2.2.1 SelfInductance.Self-inductance (L) is expressed in henrys. A henry is the inductance by which one volt is produced across a coil whenthe inducing current is changed at the rate of one ampere per second. A formula for self-inductance expressed in theseterms is as follows:L = E / (DI / T)Where:L = Inductance (henrys)E = Induced Electromotive Force (volts)DI = Change in Current (amperes)T = Time (seconds)4.3.2.2.1.1Because 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 of10 to several hundred microhenrys.4.3.2.2.1.2The inductance of a coil depends upon the number of turns in the coil, the size of the coil, the permeability of thematerial within the coil (i.e., the core of the coil), and total magnetic flux through the coil. An alternate method ofexpressing self-inductance (L) is:L = n f / IWhere:L = Inductance (henrys)n = Number of turns in coilf = Magnetic flux (webers)I = Current through coil (amperes)4.3.2.2.2 InductiveReactance.The measure of the amount of opposition or resistance (ohms) to alternating current flow due to inductance in a coil iscalled inductive reactance. Inductive reactance is dependent upon the value of the inductance of the coil and thefrequency of the alternating current. The inductive reactance increases as the inductance or frequency increases. Thiscan be stated by the following equation:XL = 2 p f LWhere:XL = Inductive reactance (ohms)f = frequency (Hertz)L = Inductance (henrys)4.3.2.2.2.1The inductive reactance results from the electromotive force generated across a coil by the alternating current. Theinstantaneous value of this induced voltage increases and decreases as the rate of change of the applied alternatingcurrent increases and decreases as shown in Figure 4-14. The voltage is at its maximum value when the rate of currentchange is at its maximum; this occurs when the current value is at zero. Conversely, the voltage is zero when the rateof current change is zero; this occurs when the current is at its maximum value. Considering 360 degrees to be onecomplete cycle, the induced voltage leads the current (i.e., is out of phase with the current) by 90 degrees as illustratedin Figure 4-14. The induced voltage is in opposition to the electromotive force applied to the coil, reducing theamplitude of the resultant current.
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