Specifications

Reactance takes two forms - inductive (X
L
) and capacitive (Xc). By definition, X
L
=2πfL and
Xc=1/(2πfC), where f is the frequency of interest, L is inductance, and C is capacitance. 2πf can be
substituted for by the angular frequency (ω:omega) to represent X
L
=ωL and Xc=1/(ωC). Refer to
Figure 1-3.
Figure 1-3. Reactance in two forms - inductive (X
L
) and capacitive (Xc)
A similar reciprocal relationship applies to susceptance and admittance. Figure 1-4 shows a typical
representation for a resistance and a reactance connected in series or in parallel.
The quality factor (Q) serves as a measure of a reactance’s purity (how close it is to being a pure
reactance, no resistance), and is defined as the ratio of the energy stored in a component to the
energy dissipated by the component. Q is a dimensionless unit and is expressed as Q=X/R=B/G.
From Figure 1-4, you can see that Q is the tangent of the angle θ. Q is commonly applied to induc-
tors; for capacitors the term more often used to express purity is dissipation factor (D). This quanti-
ty is simply the reciprocal of Q, it is the tangent of the complementary angle of θ, the angle δ shown
in Figure 1-4 (d).
Figure 1-4. Relationships between impedance and admittance parameters
1-2