Susceptanssi

Susceptanssi, or susceptance in English, is the imaginary part of a circuit’s admittance. It is denoted by B and measured in siemens. In alternating current analysis, the complex admittance Y that relates current I to voltage V is written as Y = G + jB, where G is conductance (the real part) and B is the susceptance (the imaginary part).

For reactive elements, susceptance depends on frequency. A capacitor with capacitance C has Yc = j ω C, so B = ω C. An inductor with inductance L has YL = 1/(j ω L) = - j /(ω L), so B = -1/(ω L). A pure resistor has B = 0 at all frequencies. The frequency ω equals 2πf, where f is the AC frequency.

The sign of B indicates the type of reactive behavior: positive B is capacitive (current leads voltage by 90 degrees), while negative B is inductive (current lags voltage). In a network, the susceptance forms part of the total admittance Y = G + jB, and relates to reactive power: reactive power Q is often expressed in relation to B by Q = - B V^2 for a given RMS voltage V, reflecting the standard convention that inductive elements absorb reactive power and capacitive elements supply it.

Applications of susceptance include AC circuit analysis, power-flow calculations in electrical grids, and the modeling of shunt elements in transmission-line and filter design. It is a key concept for understanding how reactive components influence voltage, current, and phase relationships in alternating current systems.