BASIC AC REACTIVE COMPONENTS
There are many natural causes of capacitance in AC power circuits, such as transmission lines, fluorescent lighting, and computer monitors. Normally, these are counteracted by the inductors previously discussed. However, where capacitors greatly outnumber inductive devices, we must calculate the amount of capacitance to add or subtract from an AC circuit by artificial means.
The variation of an alternating voltage applied to a capacitor, the charge on the capacitor, and the current flowing through the capacitor are represented by Figure 3. The current flow in a circuit containing capacitance depends on the rate at which the voltage changes. The current flow in Figure 3 is greatest at points a, c, and e. At these points, the voltage is changing at its maximum rate (i.e., passing through zero). Between points a and b, the voltage and charge are increasing, and the current flow is into the capacitor, but decreasing in value. At point b, the capacitor is fully charged, and the current is zero. From points b to c, the voltage and charge are decreasing as the capacitor discharges, and its current flows in a direction opposite to the voltage. From points c to d, the capacitor begins to charge in the opposite direction, and the voltage and current are again in the same direction.
Figure 1 Voltage, Charge, and Current in a Capacitor
At point d, the capacitor is fully charged, and the current flow is again zero. From points d to e, the capacitor discharges, and the flow of current is opposite to the voltage. Figure 1 shows the current leading the applied voltage by 90°. In any purely capacitive circuit, current leads applied voltage by 90°.
Capacitive reactance is the opposition by a capacitor or a capacitive circuit to the flow of current. The current flowing in a capacitive circuit is directly proportional to the capacitance and to the rate at which the applied voltage is changing. The rate at which the applied voltage is changing is determined by the frequency of the supply; therefore, if the frequency of the capacitance of a given circuit is increased, the current flow will increase. It can also be said that if the frequency or capacitance is increased, the opposition to current flow decreases; therefore, capacitive reactance, which is the opposition to current flow, is inversely proportional to frequency and capacitance. Capacitive reactance XC, is measured in ohms, as is inductive reactance. Equation (8-3) is a mathematical representation for capacitive reactance.
Equation (8-4) is the mathematical representation of capacitive reactance when capacitance is expressed in microfarads (µF).
Example: A 10µF capacitor is connected to a 120V, 60Hz power source (see Figure 2). Find the capacitive reactance and the current flowing in the circuit. Draw the phasor diagram.
Figure 2 Circuit and Phasor Diagram
No circuit is without some resistance, whether desired or not. Resistive and reactive components in an AC circuit oppose current flow. The total opposition to current flow in a circuit depends on its resistance, its reactance, and the phase relationships between them. Impedance is defined as the total opposition to current flow in a circuit. Equation (8-6) is the mathematical representation for the magnitude of impedance in an AC circuit.
The current through a certain resistance is always in phase with the applied voltage. Resistance is shown on the zero axis. The current through an inductor lags applied voltage by 90° inductive reactance is shown along the 90° axis. Current through a capacitor leads applied voltage by 90° capacitive reactance is shown along the -90° axis. Net reactance in an AC circuit is the difference between inductive and capacitive reactance. Equation (8-7) is the mathematical representation for the calculation of net reactance when XL is greater than XC.
Equation (8-8) is the mathematical representation for the calculation of net reactance when XC is greater than XL.
Impedance is the vector sum of the resistance and net reactance (X) in a circuit, as shown in Figure 5. The angle Θ is the phase angle and gives the phase relationship between the applied voltage and the current. Impedance in an AC circuit corresponds to the resistance of a DC circuit. The voltage drop across an AC circuit element equals the current times the impedance. Equation (8-9) is the mathematical representation of the voltage drop across an AC circuit.
The phase angle Θ gives the phase relationship between current and the voltage.
Impedance is the resultant of phasor addition of R and XL. The symbol for impedance is Z. Impedance is the total opposition to the flow of current and is expressed in ohms. Equation (8-10) is the mathematical representation of the impedance in an RL circuit.
Example: If a 100 Ω resistor and a 60 Ω XL are in series with a 115V applied voltage (Figure 3), what is the circuit impedance?
Figure 3 Simple R-L Circuit
In a capacitive circuit, as in an inductive circuit, impedance is the resultant of phasor addition of R and XC. Equation (8-11) is the mathematical representation for impedance in an R-C circuit.
Example: A 50 Ω XC and a 60 Ω resistance are in series across a 110V source (Figure 4). Calculate the impedance.
Figure 4 Simple R-C Circuit
Impedance in an R-C-L series circuit is equal to the phasor sum of resistance, inductive reactance, and capacitive reactance (Figure 5).
Figure 5 Series R-C-L Impedance-Phasor
Equations (8-12) and (8-13) are the mathematical representations of impedance in an R-C-L circuit. Because the difference between XL and XC is squared, the order in which the quantities are subtracted does not affect the answer.
Example: Find the impedance of a series R-C-L circuit, when R = 6 Ω, XL = 20 Ω, and XC = 10 Ω.(Figure 6).
Figure 6 Series R-C-L Impedance-Phasor
Impedance in a parallel R-C-L circuit equals the voltage divided by the total current. Equation (8-14) is the mathematical representation of the impedance in a parallel R-C-L circuit.
Total current in a parallel R-C-L circuit is equal to the square root of the sum of the squares of the current flows through the resistance, inductive reactance, and capacitive reactance branches of the circuit. Equations (8-15) and (8-16) are the mathematical representations of total current in a parallel R-C-L circuit. Because the difference between IL and IC is squared, the order in which the quantities are subtracted does not affect the answer.
Example: A 200 Ω resistor, a 100 Ω XL, and an 80 Ω XC are placed in parallel across a 120V AC source (Figure 7). Find: (1) the branch currents, (2) the total current, and (3) the impedance.
Figure 7 Simple Parallel R-C-L Circuit
Any device relying on magnetism or magnetic fields to operate is a form of inductor. Motors, generators, transformers, and coils are inductors. The use of an inductor in a circuit can cause current and voltage to become out-of-phase and inefficient unless corrected.
In an inductive AC circuit, the current is continually changing and is continuously inducing an EMF. Because this EMF opposes the continuous change in the flowing current, its effect is measured in ohms. This opposition of the inductance to the flow of an alternating current is called inductive reactance (XL). Equation (8-1) is the mathematical representation of the current flowing in a circuit that contains only inductive reactance.
The value of XL in any circuit is dependent on the inductance of the circuit and on the rate at which the current is changing through the circuit. This rate of change depends on the frequency of the applied voltage. Equation (8-2) is the mathematical representation for XL.
The magnitude of an induced EMF in a circuit depends on how fast the flux that links the circuit is changing. In the case of self-induced EMF (such as in a coil), a counter EMF is induced in the coil due to a change in current and flux in the coil. This CEMF opposes any change in current, and its value at any time will depend on the rate at which the current and flux are changing at that time. In a purely inductive circuit, the resistance is negligible in comparison to the inductive reactance. The voltage applied to the circuit must always be equal and opposite to the EMF of self-induction.
As previously stated, any change in current in a coil (either a rise or a fall) causes a corresponding change of the magnetic flux around the coil. Because the current changes at its maximum rate when it is going through its zero value at 90 (point b on Figure 1) and 270° (point d), the flux change is also the greatest at those times. Consequently, the self-induced EMF in the coil is at its maximum (or minimum) value at these points, as shown in Figure 1. Because the current is not changing at the point when it is going through its peak value at 0° (point a), 180° (point c), and 360° (point e), the flux change is zero at those times. Therefore, the self-induced EMF in the coil is at its zero value at these points.
Figure 8 Current, Self-Induced EMF, and Applied Voltage in an Inductive Circuit
According to Lenzs Law (refer to Module 1, Basic Electrical Theory), the induced voltage always opposes the change in current. Referring to Figure 1, with the current at its maximum negative value (point a), the induced EMF is at a zero value and falling. Thus, when the current rises in a positive direction (point a to point c), the induced EMF is of opposite polarity to the applied voltage and opposes the rise in current. Notice that as the current passes through its zero value (point b) the induced voltage reaches its maximum negative value. With the current now at its maximum positive value (point c), the induced EMF is at a zero value and rising. As the current is falling toward its zero value at 180° (point c to point d), the induced EMF is of the same polarity as the current and tends to keep the current from falling. When the current reaches a zero value, the induced EMF is at its maximum positive value. Later, when the current is increasing from zero to its maximum negative value at 360° (point d to point e), the induced voltage is of the opposite polarity as the current and tends to keep the current from increasing in the negative direction. Thus, the induced EMF can be seen to lag the current by 90°.
The value of the self-induced EMF varies as a sine wave and lags the current by 90°, as shown in Figure 8. The applied voltage must be equal and opposite to the self-induced EMF at all times; therefore, the current lags the applied voltage by 90° in a purely inductive circuit.
If the applied voltage (E) is represented by a vector rotating in a counterclockwise direction (Figure 8b), then the current can be expressed as a vector that is lagging the applied voltage by 90°. Diagrams of this type are referred to as phasor diagrams.
Example: A 0.4 H coil with negligible resistance is connected to a 115V, 60 Hz power source (see Figure 9). Find the inductive reactance of the coil and the current through the circuit. Draw a phasor diagram showing the phase relationship between current and applied voltage.
Figure 9 Coil Circuit and Phasor Diagram
In the chapters on inductance and capacitance we have learned that both conditions are reactive and can provide opposition to current flow, but for opposite reasons. Therefore, it is important to find the point where inductance and capacitance cancel one another to achieve efficient operation of AC circuits.
Resonance occurs in an AC circuit when inductive reactance and capacitive reactance are equal to one another: XL = XC. When this occurs, the total reactance, X = XL - XC becomes zero and the impendence is totally resistive. Because inductive reactance and capacitive reactance are both dependent on frequency, it is possible to bring a circuit to resonance by adjusting the frequency of the applied voltage. Resonant frequency (fRes) is the frequency at which resonance occurs, or where XL = XC. Equation (8-14) is the mathematical representation for resonant frequency.
In a series R-C-L circuit at resonance the net reactance of the circuit is zero, and the impedance is equal to the circuit resistance; therefore, the current output of a series resonant circuit is at a maximum value for that circuit and is determined by the value of the resistance.
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 the net current flow through the circuit is at minimum because of the high impendence presented by XL and XC in parallel.