Rev. 11/10/00
3
Capacitive Reactance
Basically, reactance is to an AC circuit what resistance is to a DC circuit.
Reactance, however, is affected by the value of the component and the frequency
of the circuit in which it is being used. Capacitive reactance, commonly expressed
as Xc, is inversely proportional to the value of the capacitor and the frequency.
The formula is Xc = 1/2
π
fC, where Xc is calculated by dividing one by (two times
the value of Pi times the frequency times the capacitance); Pi (
π
) = 3.1416.
Therefore, we can observe that Xc increases as frequency or capacitance decreases
and Xc decreases as frequency or capacitance increases. Larger gauge wires and
longer runs of wire have more capacitance on them which calculates to lower
capacitive reactance. As the frequency increases, the capacitive reactance also
decreases. Lower reactance, like lower resistance, appears more like a short as the
value approaches zero.
Inductance
An inductor is a coil of wire that can be used to increase the inductance in a
circuit. Inductance is resistance to any change in current flow and slows down the
rate of change. Inductance is the result from a magnetic field that surrounds a wire
when current flows through the wire. The number of turns in the coil and the rate
of the change of current through the coil determines the inductance. Inductance is
measured in henrys, though millihenrys is the common range of measurements.
Telephone circuits use load coils as inductors to increase the circuit inductance.
Increasing inductance counteracts capacitance thereby stabilizing current flow
(voice signals). A cable equipped with load coils is said to be a loaded cable. The
most common loading scheme is called H88 because 88mH coils are used and
they are spaced at 6,000 foot intervals.
Inductive Reactance
Inductive reactance, like capacitive reactance, is affected by the value of the
component and the frequency of the circuit in which it is being used. However,
inductive reactance, commonly expressed as Xl, is directly proportional to the
value of the inductor and the frequency. The formula is Xl = 2
π
fL, where Xl is
calculated by multiplying (two times the value of Pi times the frequency times the
inductance); Pi (
π
) = 3.1416. Therefore, we can observe that Xl increases as
frequency or capacitance increases and Xl decreases as frequency or capacitance
decreases. Higher reactance, like higher resistance, appears more like an open as
the value approaches infinity.
