ALTERNATING-CURRENT SINGLE-PHASE CIRCUITS
FUNDAMENTAL FREQUENCY
A cycle consists of one complete pattern of change of the AC wave; that is, the period from any point on an AC wave to the next point of the same magnitude and location at which the wave pattern begins to repeat itself. See Figure 4-10.
If the usual alternating voltage or current is plotted against time, it produces the curve in Figure 4-10. A single cycle covers a definite period of time and is completed in 360°. This period of time may be expressed in terms of an angle 2 f t radians. Note that for t T (one period or 360°), 2 radians. Since 360°
2 radians, 180° radians.
Figure 4-10. Sine-Wave Relationships.
The number of cycles completed per second is the frequency of the waveform expressed in Hertz (Hz). Higher frequencies may be expressed in either kiloHertz (kHz) or megaHertz (MHz).
The frequency in Hertz (Hz) is f 1/T. In the previous equations for v(t) and i(t), we can substitute for 2f, to obtain the angular frequency having units of radians per second. Since f 1/T, then 2/T.
v(t) Vpcos(t v) V i(t) Ipcos(t i) A
Application Frequency
AM radio broadcasts 535 to 1,605 kHz FM radio broadcasts 88 to 108 MHz Television (Channels 2-13) 55 to 216 MHz Communication satellites 5 GHz
The mathematical argument of the cosine and sine functions should be expressed in radians or degrees. A common mistake is to mix these units in the argument during a calculation. However, by convention, the phase angle, vor i, is usually expressed in degrees to be more understandable.
A sinusoidal voltage can be assigned a value in three ways:
1. By the maximum (Vmax) or peak value (Vp). This value is used in insulation stress calculations;
2. By the average value (Vavg) which is equal to the average value of v for the pos-itive half (or negative half ) of the cycle. This value is often used in rectification problems. The average value of a cosine waveform over one period is zero.
That is, in one period, there is just as much area above the x-axis as there is below;
3. By the root-mean-square (Vrms) or the effective value (Veff). The term Vrms is generally used. In electricity, the effective value of an alternating current is that value of current which gives the same heating effect in a given resistor as the same value of direct current. Unless some other description is specified, when alternating currents or voltages are mentioned, it is always the rms value that is meant.
PHASORS
If two sine waves of the same frequency do not coincide with respect to time, they are said to be out of phase with each other. In Figure 4-11, the current waveform I, is ° out of phase with the voltage waveform. As shown, it is behind or lagging the voltage waveform by the angle . It reaches its peak value at ° after the voltage waveform reaches its peak. The trigonometric cosine of this angle between the voltage and current is the displacement power factor of the circuit.
Displacement Power Factor DPF cos( v i) cos pf Table 4-1. Application Frequencies.
Here, vand iare the phase angles of the 60 Hz voltage and current wave-forms, respectively. The Displacement Power Factor (DPF) should be identified as either leading or lagging. As will be seen later, current always lags the voltage for an inductive load or circuit. And, for a capacitive load or circuit, the current will always lead the voltage. An easy way to remember this is the saying, “ELI the ICE man.” It says for an inductor L, the voltage (i.e., E emf electromotive force) leads the current. Alternatively, it says that the current lags behind the voltage.
For a capacitor C, the current leads the voltage, which is the same as saying the voltage lags behind the current. When vand i are equal, the DPF 1.0, and the voltage and current are “in phase.”
Figure 4-11. Current Wave Lagging Voltage Wave.
Example: Calculate the displacement power factor if the voltage v(t) and current i(t) are
v(t) 2 226 cos(260t 0°) V i(t) 2 15.6 cos(260t 34.1°) A
Solution: Using the definition above, take the phase angle of the voltage and subtract the phase angle of the current to obtain the displacement power factor angle pf, 0° – (34.1°) 34.1°. The cosine of 34.1° is 0.828. One piece of informa-tion is missing for our answer. Is the power factor leading or lagging? The answer is lagging. Refer to the DPF explanation to see why this is so.
Example: The angle between the voltage v(t) and current i(t) is 25.5°. The voltage is leading the current. What is the displacement power factor?
Solution: The cosine of 25.5° is 0.903. Since the voltage is leading the current, or the current is behind the voltage, then the displacement power factor is lagging.
Figure 4-12. Voltage Phasor.
The mathematical representation of electrical quantities by sinusoidal func-tions is unwieldy and time consuming. The preferred method is to represent currents and voltages by phasor diagrams in which rotating vectors are sub-stituted for cosine waveforms. These rotating vectors, called phasors, are drawn as vectors in the complex plane and considered to rotate in the counter clockwise direction one complete revolution (360°), while the cosine wave passes through one cycle (360°). Thus, the phasors are rotating at an angular frequency, 2f radians per second.
The phasor’s length is defined either as the peak or rms value of the sine wave of the current or voltage. It is extremely important that whatever convention is adopted, it is followed for all calculations. The phase angle indicates the position of the phasor relative to a previously defined reference phasor. The reference pha-sor is usually the phase A line-to-neutral voltage having an angle of 0°. Because power equipment is normally specified by its rms quantities, in the remainder of this chapter, all magnitudes are assumed to be rms. As such, if v(t) or i(t) are given by their peak or maximum amplitude, it will be necessary to convert them to their rms values by dividing by 2 .
One method of phasor notation is I Irms/_ _° meaning the phasor I is at an angle of ° counterclockwise from the positive x-axis (Figure 4-13).
Example: What are the phasors representing –6 cos(t – 30°) and 5 sin(t 10°)?
Solution: The first is 6 /_–__3_0__°__ __ _1_8_0__° 6/_1_5_0_° or 6 /_–__2_1_0_°. The second must first be converted from a sine function to a cosine function. We do this by subtracting 90°. Therefore, 5 /_–__1___0_°____9_0_° 5/_–__1__0_0_°.
Example: If the phasor VS 8.00 /_–__3_8._7__° V, what is vs(t)?
Solution: The answer is vs(t) 8.00 cos(t – 38.7°) V.
Phasors representing currents or voltages can be resolved into vertical and horizontal components, (Figure 4-13) using the information provided in the com-plex number section of Chapter 3. Because phasors are comcom-plex numbers written in polar form, use the methods presented in Chapter 3 to perform addition, sub-traction, multiplication, and division.
Figure 4-13. Phasor Voltage Resolved into Components.
INDUCTANCE
Any conductor which is carrying current is cut by the flux of its own field when the current changes in value. A voltage is thereby induced in the conductor, which, by Lenz’s Law, opposes the change in current in the conductor. If the current is decreasing, the polarity of the induced voltage tries to maintain the current; if the current is increasing, the induced voltage tends to keep the current down. The amount of induced voltage depends upon the change in the number of flux lines cutting the conductor, which in turn, depends upon the rate of change of current in the conductor. The proportionality factor between the induced voltage and the rate of change of current is the inductance, L, of the circuit.
By equipment design, the inductance of a circuit can generally be considered dependent on the current magnitude in the circuit and on the physical character-istics of the circuit. A conductor in the form of a coil cuts more flux lines and has a greater inductance than a straight conductor. Magnetic material versus air in the flux path further concentrates the flux lines to the conductor, allowing the conductor to cut more flux lines, which further increases the inductance.
Inductance is expressed in Henries (H). A more common unit is the milli-Henry (mH), which is one-thousandth of a milli-Henry.
In direct-current circuits inductance has no effect except when current is changing. Consider a pure resistive DC circuit (Figure 4-14a). When a voltage is impressed, the current instantly assumes its steady-state value determined by (V/R), as shown in Figure 4-14b.
Figure 4-14. Direct Current in a Resistance Circuit.
If an inductance is inserted in series with the same resistor, the current does not increase instantly to its steady-state value when the switch is closed. Instead, there is a time delay before the current reaches the same steady-state value as before, shown in Figure 4-15b. The induced voltage in the inductor opposing the rising current causes this effect.
Figure 4-15. Direct Current in an Inductive Circuit.
The larger the value of the circuit inductance, the longer the time required for the current to reach its steady-state value. However, once this value has been reached, the inductance has no further effect and only the resistance limits the magnitude of circuit current.
With alternating current the instantaneous current is always changing, so in an inductive circuit the inductive effect is always present. For every quarter cycle of the line frequency, energy is being stored to or released from the magnetic field.
Inductance has a very definite current-limiting effect on alternating current as contrasted with steady-state direct current. This effect is directly proportional to the magnitude of the inductance L. It is also proportional to the rate of change of current, which is a function of the frequency of the supply voltage. The total opposing, or limiting effect of inductance on current may be calculated by the following equation and is called the inductive reactance
XL 2fL L
where XL inductive reactance in , f frequency in Hz, and L inductance in H.
In a purely inductive AC circuit, the maximum rate of change of current occurs when the current passes through zero. At this instant of zero magnitude but maximum change, there is maximum induced voltage and the voltage wave is at its peak value. When the current reaches its peak value, the rate of change of current is zero and the induced voltage is zero. As shown in Figure 4-16, the cur-rent wave lags the voltage wave by 90°.
Figure 4-16. Phase Relationships in a Circuit of Pure Inductance.
When two inductors are in series, they may be replaced by a single equiva-lent inductor. In this way, they are similar to a resistor. When L1is in series with L2, then LEQ L1 L2. Similarly, inductors in parallel add like resistors in parallel,
LEQ L1 L2 ——1—— —L
—1—
—L
——1L2 L—2
—1 L1 1—
L2
CAPACITANCE
Electric current flow is generally considered to be a movement of negative charges, or electrons, in a conductor. In conducting materials some of the electrons are loosely attached to the atoms so that when a voltage is applied to a closed circuit these electrons are separated from the atoms and their movement constitutes a current flow.
The electrons in an insulator are much more firmly bonded to the atoms than in a conductor. When a voltage is applied to an insulator, the electrons seek to leave the atoms but cannot do so. However, the electrons are displaced by an amount dependent upon the force applied, the voltage difference. When voltage
changes, the displacement also changes. When this electron motion takes place, a displacement current flows through the dielectric and there is a charging-current flow throughout the entire circuit.
Consider the circuit of Figure 4-17a which has a small insulating gap between the ends of the wires. When the switch is closed there is no continuous current flow in the circuit. However, a very small current may be measured with an extreme-ly sensitive instrument for a short time. Electrons move through the circuit to build up an electrical charge across the gap, which is equal to the impressed volt-age VS. Once the charge has been established there is no further electron movement.
Figure 4-17. Capacitive Circuit.
If, instead of a small gap, the area is enlarged by connecting plates to each of the conductors, as in Figure 4-17b, the current required to raise the charge to a given level is increased because a greater movement of electrons is required.
Such devices, consisting of large conducting areas separated by thin insulating materials such as air, mica, glass, etc., are called capacitors. Any two conductors separated by insulation constitute a capacitor, but normally the capacitance effect is negligible unless the components and their arrangement have been specifically designed to provide capacitance.
The capacitance, C, is a function of the physical characteristics of the capaci-tor, such as the plate area, the distance, and the type of insulation between the plates. Capacitance is expressed in Farads. A more common, smaller unit is the microFarad (µF), which is one-millionth of a Farad (F).
Capacitors may be connected in parallel or in series. The total capacitance of capacitors connected in parallel is the sum of the individual capacitances.
CEQ C1 C2 C3
For a series connection, the net capacitance is found by a formula similar to that for parallel resistances.
CEQ ——1—— —C
1
—C
—1—
—C
——2 C2
—1 C1 1—
C2
In a DC circuit, current flows through a capacitor only when the voltage across it changes. In an AC circuit, the voltage is continually changing and current flows through a capacitor as long as the alternating voltage is applied. The current mag-nitude is proportional to the rate of change of voltage. With a sinusoidal voltage, the maximum rate of change occurs when the voltage crosses zero and the peak value of current occurs at this instant. When the voltage is at its peak, its rate of change is zero and the current magnitude is zero. Therefore, there is a 90-degree phase displacement between current and voltage in a capacitor. When the rate of change of voltage is positive, the current must be in the positive direction to sup-ply the increasing positive charge. Therefore, the current leads the voltage in a capacitor. These relationships are shown in Figure 4-18.
Figure 4-18. Phase Relationship in a Circuit of Pure Capacitance.
The current-limiting effect of a capacitor, its reactance, is dependent on two quantities: capacitance and frequency. Charging current increases with increasing capacitance, so with a given voltage the reactance must be inversely proportional to capacitance. Rate of change of voltage is proportional to frequency, hence charging current is also proportional to frequency and reactance is inversely pro-portional. Capacitive reactance may be calculated from the following equation:
XC –—–—1—
2fC —–—1—
C
where XC capacitive reactance in , f frequency in Hz, and C capacitance in F.
Capacitive reactance causes leading current and a leading power factor while inductive reactance causes lagging current and a lagging power factor. Capacitors are often used to balance some of the inductive reactance (e.g., motors and trans-formers) of a circuit and therefore to increase the circuit power factor. They are also used to balance some of the inductive voltage drop in a circuit and therefore increase the available voltage.