RLC Series Circuits with AC

LEARNING OBJECTIVES

By the end of the section, you will be able to:

  • Describe how the current varies in a resistor, a capacitor, and an inductor while in series with an ac power source
  • Use phasors to understand the phase angle of a resistor, capacitor, and inductor ac circuit and to understand what that phase angle means
  • Calculate the impedance of a circuit


The ac circuit shown in Figure 12.3.1, called an RLC series circuit, is a series combination of a resistor, capacitor, and inductor connected across an ac source. It produces an emf of

(Figure 12.3.1)



Figure 12.3.1 (a) An RLC series circuit. (b) A comparison of the generator output voltage and the current. The value of the phase difference
depends on the values of R, C, and L.



Since the elements are in series, the same current flows through each element at all points in time. The relative phase between the current and the emf is not obvious when all three elements are present. Consequently, we represent the current by the general expression

where

is the current amplitude and

is the phase angle between the current and the applied voltage. The phase angle is thus the amount by which the voltage and current are out of phase with each other in a circuit. Our task is to find

and



A phasor diagram involving

and

is helpful for analyzing the circuit. As shown in Figure 12.3.2, the phasor representing

points in the same direction as the phasor for

its amplitude is

The

phasor lags the

phasor by

and has the amplitude

The phasor for

leads the

phasor by

and has the amplitude


(Figure 12.3.2)



Figure 12.3.2 The phasor diagram for the RLC series circuit of Figure 12.3.1.



At any instant, the voltage across the

combination is

the emf of the source. Since a component of a sum of vectors is the sum of the components of the individual vectors—for example,

—the projection of the vector sum of phasors onto the vertical axis is the sum of the vertical projections of the individual phasors. Hence, if we add vectorially the phasors representing

and then find the projection of the resultant onto the vertical axis, we obtain

The vector sum of the phasors is shown in Figure 12.3.3. The resultant phasor has an amplitude

and is directed at an angle

with respect to the

or

phasor. The projection of this resultant phasor onto the vertical axis is

We can easily determine the unknown quantities

and

from the geometry of the phasor diagram. For the phase angle,

and after cancellation of

this becomes

(12.3.1)

Furthermore, from the Pythagorean theorem,

(Figure 12.3.3)



Figure 12.3.3 The resultant of the phasors for
and
is equal to the phasor for
The
phasor (not shown) is aligned with the
phasor.



The current amplitude is therefore the ac version of Ohm’s law:

(12.3.2)

where

(12.3.3)

is known as the impedance of the circuit. Its unit is the ohm, and it is the ac analog to resistance in a dc circuit, which measures the combined effect of resistance, capacitive reactance, and inductive reactance (Figure 12.3.4).

(Figure 12.3.4)



Figure 12.3.4 Power capacitors are used to balance the impedance of the effective inductance in transmission lines.



The

circuit is analogous to the wheel of a car driven over a corrugated road (Figure 12.3.5). The regularly spaced bumps in the road drive the wheel up and down; in the same way, a voltage source increases and decreases. The shock absorber acts like the resistance of the

circuit, damping and limiting the amplitude of the oscillation. Energy within the wheel system goes back and forth between kinetic and potential energy stored in the car spring, analogous to the shift between a maximum current, with energy stored in an inductor, and no current, with energy stored in the electric field of a capacitor. The amplitude of the wheel’s motion is at a maximum if the bumps in the road are hit at the resonant frequency, which we describe in more detail in Resonance in an AC Circuit.

(Figure 12.3.5)



Figure 12.3.5 On a car, the shock absorber damps motion and dissipates energy. This is much like the resistance in an RLC circuit. The mass and spring determine the resonant frequency.



Problem-Solving Strategy: AC Circuits

To analyze an ac circuit containing resistors, capacitors, and inductors, it is helpful to think of each device’s reactance and find the equivalent reactance using the rules we used for equivalent resistance in the past. Phasors are a great method to determine whether the emf of the circuit has positive or negative phase (namely, leads or lags other values). A mnemonic device of “ELI the ICE man” is sometimes used to remember that the emf (E) leads the current (I) in an inductor (L) and the current (I) leads the emf (E) in a capacitor (C).

Use the following steps to determine the emf of the circuit by phasors:

  1. Draw the phasors for voltage across each device: resistor, capacitor, and inductor, including the phase angle in the circuit.
  2. If there is both a capacitor and an inductor, find the net voltage from these two phasors, since they are antiparallel.
  3. Find the equivalent phasor from the phasor in step 2 and the resistor’s phasor using trigonometry or components of the phasors. The equivalent phasor found is the emf of the circuit.


EXAMPLE 12.3.1

An
Series Circuit

The output of an ac generator connected to an

series combination has a frequency of

and an amplitude of

If

and

what are (a) the capacitive reactance, (b) the inductive reactance, (c) the impedance, (d) the current amplitude, and (e) the phase difference between the current and the emf of the generator?



Strategy

The reactances and impedance in (a)–(c) are found by substitutions into Equation 12.2.1 , Equation 12.2.6, and Equation 12.3.3, respectively. The current amplitude is calculated from the peak voltage and the impedance. The phase difference between the current and the emf is calculated by the inverse tangent of the difference between the reactances divided by the resistance.



Solution

a. From Equation 12.2.1, the capacitive reactance is



b. From Equation 12.2.6, the inductive reactance is



c. Substituting the values of

and

into Equation 12.3.3, we obtain for the impedance



d. The current amplitude is



e. From Equation 12.3.1, the phase difference between the current and the emf is



Significance

The phase angle is positive because the reactance of the inductor is larger than the reactance of the capacitor.



CHECK YOUR UNDERSTANDING 12.3

Find the voltages across the resistor, the capacitor, and the inductor in the circuit of Figure 12.3.1 using

as the output of the ac generator.



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