RESONANCE
Basic AC Reactive Components
Parallel Resonance
Resonance in a parallel R-C-L circuit will occur when the reactive current in the inductive
branches is equal to the reactive current in the capacitive branches (or when XL = XC). Because
inductive and capacitive reactance currents are equal and opposite in phase, they cancel one
another at parallel resonance.
If a capacitor and an inductor, each with negligible resistance, are connected in parallel and the
frequency is adjusted such that reactances are exactly equal, current will flow in the inductor and
the capacitor, but the total current will be negligible. The parallel C-L circuit will present an
almost infinite impedance. The capacitor will alternately charge and discharge through the
inductor. Thus, in a parallel R-C-L, as in Figure 10, the net current flow through the circuit is
at minimum because of the high impendence presented by XL and XC in parallel.
Summary
Resonance is summarized below.
Resonance Summary
Resonance is a state in which the inductive reactance equals the capacitive
reactance (XL = XC) at a specified frequency (fRes).
Resonant frequency is:
fRes
1
2p LC
R-C-L series circuit at resonance is when net reactance is zero and circuit
current output is determined by the series resistance of the circuit.
R-C-L parallel circuit at resonance is when net reactance is maximum and
circuit current output is at minimum.
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