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Concept
Susceptance
Summary
In electrical engineering, susceptance (B) is the imaginary part of admittance (Y = G + jB), where the real part is conductance (G). The reciprocal of admittance is impedance (Z = R + jX), where the imaginary part is reactance (X) and the real part is resistance (R). In SI units, susceptance is measured in siemens (S).
The term was coined by C.P. Steinmetz in a 1894 paper.
In some sources Oliver Heaviside is given credit for coining the term, or with introducing the concept under the name permittance.
This claim is mistaken according to Steinmetz's biographer.
The term susceptance does not appear anywhere in Heaviside's collected works, and Heaviside used the term permittance to mean capacitance, not susceptance.
The general equation defining admittance is given by
where
The admittance (Y) is the reciprocal of the impedance (Z), if the impedance is not zero:
and
where
The susceptance is the imaginary part of the admittance
The magnitude of admittance is given by:
And similar formulas transform admittance into impedance, hence susceptance (B) into reactance (X):
hence
The reactance and susceptance are only reciprocals in the absence of either resistance or conductance (only if either R = 0 or G = 0, either of which implies the other, as long as Z ≠ 0, or equivalently as long as Y ≠ 0).
In electronic and semiconductor devices, transient or frequency-dependent current between terminals contains both conduction and displacement components. Conduction current is related to moving charge carriers (electrons, holes, ions, etc.), while displacement current is caused by time-varying electric field. Carrier transport is affected by electric field and by a number of physical phenomena, such as carrier drift and diffusion, trapping, injection, contact-related effects, and impact ionization. As a result, device admittance is frequency-dependent, and the simple electrostatic formula for capacitance, is not applicable.
Official source
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