Admittance

Quick Answer

Admittance is the reciprocal of impedance, measured in siemens. Its real part is conductance and its imaginary part susceptance, so it states how readily a network passes alternating current rather than how strongly it opposes it. Admittances of parallel branches add, which is why parallel AC circuits are worked this way.

Series and parallel each have a quantity that adds. Impedances in series add, because one current runs through the lot and the voltages stack up. Parts in parallel share a voltage instead and their currents stack up, so what adds there is the reciprocal. It comes up often enough to be given its own name and its own symbol. A node in an AC analysis is written as a sum of the admittances touching it, exactly as the DC version sums conductances.

One point catches people out. The conductance of a network is not one over its resistance unless the reactance is zero. Invert a complex impedance and the real part that falls out depends on the reactance as well, so a lossy inductor's conductance changes with frequency even with its winding resistance held constant. Only for a pure resistance do the DC habit and the AC definition agree.

Susceptance, the imaginary part, carries its sign the other way round from reactance: a capacitor's reactance is negative and its susceptance positive. Power factor correction is the everyday use. A shunt capacitor adds positive susceptance at the node to cancel an inductive load's negative one, and the supply is then left looking at an admittance that is purely real.