Electrical Impedance (Z), is the total opposition
that a circuit presents to alternating current. Impedance is
measured in ohms and may include resistance (R), inductive reactance
(XL), and capacitive reactance (XC). Capacitive reactance can
usually not present in eddy current testing so this term is
not included the equation.
total impedance is not simply the algebraic sum of resistance
and inductive reactance. Since the inductive reactance is 90
degrees out of phase with the resistance and, therefore, their
maximum values occur at different times, vector addition must
be used to calculate impedance. This is illustrated in the image
to the right.
If the amount of resistance is represented by
the length of the horizontal line and the amount of inductive
reactance is represented by the length of the vertical line;
then, the amount of impedance is represented by the length of
the diagonal line. Since the lines form a right triangle, the
Pythagorean theorem can be used to find the length (value) of
the impedance line.
The Pythagorean theorem is written: c2=a2+b2.
For this application the variable, a is equal
to resistance, b is equal to inductive reactance, and c is equal
to the impedance. So, the equation becomes:
When this equation is rewritten to solve for Z,
the impedance equation is produced in the form presented .
Calculate the impedance
when the resistance is 0.6 ohms and the inductive reactance
is 0.4 ohms.
Simply plug in the values
and solve for Z.