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The tuned circuit is a fundamental building block among electrical and analog electronic circuits. Understanding. how it functions provides insight to power quality, communications, controls, electronic equipment, and many other systems found in industrial manufacturing plants. This article provides a brief tutorial on tuned circuit theory and operation.
RCL circuits
Recent articles in PLANT ENGINEERING magazine have discussed capacitance and inductance as separate entities (see January and May 2004 issues of PLANT ENGINEERING magazine, Plant Electronics). They explain how voltage and current behave in circuits containing only one or the other. However, many practical applications of these devices make use of the interactions between them. Although capacitance and reactance are presented as "pure" in theory, it is virtually impossible to have only one in an actual circuit.
Voltage and current in a pure inductive or a pure capacitive circuit is 90 degrees out-of-phase with voltage and current through a resistive load. Because current in a capacitive load leads the current through a resistive load, and the current in an inductive load lags the current in a resistive load, the current in a pure inductance will be 180 degrees out-of-phase with the current in a pure capacitance (Fig. 1).
When used together in a circuit, a capacitor stores the charge while the magnetic field around an inductor is collapsing. The inductor's magnetic field expands while the capacitor discharges.
In addition, the effect of frequency on the opposition to the flow of current through each device is exactly opposite. Higher frequency currents flow more easily through capacitors than do lower frequency currents. Lower frequency currents flow more easily through inductors than do the higher frequency currents. This opposition is similar to resistance in a dc circuit, but it behaves differently when ac is present. This resistance to ac is called reactance. It can be thought of as ac resistance.
Opposition to current flow that is caused by inductance is called inductive reactance (X^sub L^); opposition to current flow that is caused by capacitance is called capacitive reactance (X^sub C^). Combining the effects of capacitive reactance, inductive reactance, and resistance, the total opposition to the flow of ac through a circuit is obtained. This opposition is called impedance (Z) of a circuit. X^sub L^ X^sub C^ Z, and...