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This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved. 103 13 Alternating Current Objectives After studying this chapter, you will be able to answer these questions: 1. How does an ac generator differ from the dc generator? 2. What is a sinusoidal (sine) waveform? 3. What is meant by phase relationships? 4. How can dc current be compared with ac current? Important Words and Terms The following words and terms are key concepts in this chapter. Look for them as you read this chapter. average value cycles per second effective value frequency hertz (Hz) peak-to-peak value peak value period phase displacement root-mean-square (rms) value sine wave single-phase generator sinusoidal waveform slip ring three-phase generator vector In the chapter on direct current generators, we learned an alternating current was generated in the rotating armature. However, due to the commutator, the alternating current (ac) appeared as direct current (dc) in the output of the generator. The most common form of electrical current used is alternating current. It is used in industry to run motors that operate machines. It is used in the home for lighting, heating, cooling, cooking, and entertainment purposes. Many forms of electrical power plants are located throughout the United States. A power generating plant converts various forms of energy into electrical energy. A steam plant burns coal to make steam. Many power generation facilities create steam to drive turbines (a fan-like assembly used to catch moving steam), which then rotates a generator. Steam can be created by any fossil fuel: coal, oil, and natural gas. The energy of falling water is captured in a hydroelectric plant and changed via turbines and generators into electrical energy. Nuclear power plants heat water 104 Electricity using the heat generated by the fission of uranium fuel, Figure 13-1. The steam from the heated water drives a turbine that is joined to a generator and rotates the armature. The ac generator is similar to the simple dc generator with one exception. The commutator has been replaced with slip rings, Figure 13-2. A slip ring is connected to each end of the coil. The brushes are in constant sliding contact with the rings providing a way to pick up the electrical energy generated in the rotating armature. Note that slip rings remain in contact with each coil connection—no switching (commutation) takes place. The changing polarity and changing direction of current in the rotating armature has already been explained in the unit on the principles of the generator. The two important things to remember in the case of the ac generator is that: 1. The current in an external circuit connected to the brushes of the generator is an alternating current periodically rising and falling and changing direction. Figure 13-1. The Calvert Cliffs nuclear power plant. (Constellation Nuclear LLC) Magnetic field N S Brush Slip rings Brush Figure 13-2. The alternating current generator has slip rings instead of a commutator. 2. The same rising and falling and changing of current is also found in the armature. Vectors and Sinusoidal Waveforms Engineers and technicians have a convenient way of representing a force. It is represented by means of a vector. A vector is simply an arrow. The length of the arrow represents the magnitude (level or amount) of the force. The direction that arrow is pointing is the direction in which the force is acting. Sometimes this technique, used to combine variables, is called graphical mathematics. The vector can be used to represent the magnitude of a generated voltage and the direction in which electromotive force is acting or causing a current to flow. To better understand the use of vectors, refer to Figure 13-3 and the generation of the sinusoidal or sine waveform. For example, let’s assume the generator can produce a voltage of 50 volts. (Scale Chapter 13 105 Alternating Current Position 2 50 volts 2 X X Z + Z 135° 45° Position 1 Position 3 Base line Position 1 1 3 1/8 1 1/4″ = 50 V 3/8 50 volts 4 Position 4 Time for one revolution = 1/60 sec. Figure 13-3. The development of a sinusoidal, or sine, waveform. Position l: 0 Degrees of Rotation. Conductor not cutting through the magnetic field. No voltage generated. Position 2: 90 Degrees of Rotation. Conductor in center of the magnetic field. Maximum voltage. Positive polarity generated. 1/4 inch = 10 volts.) The vector rotates around point 0 in a counterclockwise direction. This can be compared to the rotating armature of the generator. Figure 13-4 shows the rotating armature in four positions. Now follow the explanation using both Figures 13-3 and 13-4. E N 0° N S + 0 – N – + Time 0 180° – Position 2 Position 1 Position 3 E N 270° S + 0 90° Time E S E S + 0 – Position 4 N Time 360° S + 0 Time – Position 1 Figure 13-4. Compare the pictures of the vector and the rotating armature coil of the ac generator. Time 106 Electricity Position 3: 180 Degrees of Rotation. Conductor not cutting through the magnetic field. No voltage generated. Position 4: 270 Degrees of Rotation. Conductor in center of the magnetic field. Maximum voltage generated. Negative polarity generated. Position 1: 360 Degrees of Rotation (0 Degrees). Conductor not cutting through the magnetic field. No voltage generated. The positive voltage is a force in one direction and the negative voltage is a force in the opposite direction. Current also corresponds to the direction of the force or voltage. Assuming that the armature is revolving at a speed of 60 revolutions per second, the elapsed time for one revolution would be one-sixtieth (1/60) of a second. Refer to Figure 13-3 again. Note that one complete waveform occurs in 1/60 of a second. Therefore, the voltage starting at zero rises to maximum force in one direction (Position 2) then at the end of 180° rotation returns to zero, (Position 3). In the next 90° of rotation the voltage rises to maximum but in the opposite direction (Position 4). In the last 90° the voltage returns to zero (Position 1). The waveform in Figure 13-3 is a graph of the instantaneous voltages and represents the rise and fall of the voltage during one revolution. Instantaneous values are found when voltage (or current) levels are determined at precise instances of the waveform. Points X and Z have been plotted. The waveform is the result of a line drawn through several plotted points. Point X is taken at 45° revolution and at 1/8 of the time base distance. Point 2 is taken at 90° revolution and 1/4 of the time base. Point Z at 135° revolution and 3/8 of the time base. Many points can be plotted to get a very accurate curve. The curve is known as the sinusoidal waveform and History Hit! Heinrich Rudolph Hertz (1857–1894) Hertz was a German physicist who became a university professor. Much of his research centered along radio waves. In fact, he was the first to demonstrate the existence of such waves. Unfortunately, he died before Guglielmo Marconi turned radio wave transmission into global communication. Hertz is, however, honored for his contribution to electronic communication by having the unit of frequency named in his honor. represents an alternating current or voltage. We simplify the word sinusoidal by calling the waveform a sine wave. The term sine wave will give you an introduction to trigonometry. It shows that the instantaneous voltage developed at any point on the sine wave depends upon the degrees of rotation of the vector. Frequency The chain of events described in the previous examples represents one cycle of operation. You will notice that from position 4 (270 degrees of rotation), the vector returns to position 1 (0 degrees of rotation) and the whole sequence of events repeats over and over for each revolution of the vector. The sine wave is a graphic picture of the instantaneous voltages during one cycle of generation. If the time duration of one cycle is 1/60 of a second, then the frequency of the generated voltage is 60 cycles per second. The measurement of cycles per second has been replaced by the term hertz, abbreviated Hz. One hertz equals one cycle per second. Electrical power generation in the United States is based upon the 60 Hz frequency standard. Chapter 13 107 Alternating Current Period Voltage A It is important to become familiar with the time duration of a wave, which is called its period. The period of a wave is the inverse of the frequency. A 60 Hz wave has a period of 1/60 of a second. Therefore an equation can be written which tells us that: period (in seconds) = 270° 0° 90° 360° 180° cannot be considered as equal to its peak voltage, because the peak is reached at only one point during the half cycle. One can find an average value (voltage, current, or power) represented by a sine wave by the formula: Eaverage = 0.637 × Epeak 1 frequency (in hertz) Math Manipulation! Find the period of a 1000 Hz wave. Its period would be: period = Voltage B 108 Electricity 1 1000 Hz or 0.001 second (1 ms) The generator discussed in Chapter 12 would be known as a single-phase generator. It produces electricity of only one phase. It is possible to place a second coil in the generator that would produce another voltage. Usually this coil is so spaced that the second voltage is 90° out of step (out of phase) with the first voltage. The waveform output of this generator is shown in Figure 13-5. Voltage A is 90° out of phase with voltage B, or there is a 90° phase displacement (difference) between the two voltages. Such a generator is a two-phase generator. The output circuits can be connected to use voltage A, voltage B, or both voltages in series. This type of generator is not very common. A third very common type of alternator or generator is three-phase. A three-phase generator has coils spaced 120° apart and three separate voltages are generated. Each voltage has a phase displacement of 120°. or Figure 13-5. The waveform of a two-phase alternating current. The output wave is shown in Figure 13-6. Voltage B is 120° out of phase with voltage A, and voltage C is 120° out of phase with voltage B. The term phase is sometimes written with the symbol φ. Examples of such use are: 1φ equals single phase and 3φ equals three phase. Average Values A closer look at the sine wave for one-half cycle or one alternation discloses that the voltage is continuously varying. It starts at zero, rises to a maximum or peak value, and then returns to zero, Figure 13-7. The value here is a voltage, but it could be a voltage, current, or power value. The overall value of this voltage waveform A B C A B C Epeak = 1.57 × Eaverage Math Manipulation! What is the average value of an alternating voltage that has a peak voltage of 50 volts? Eaverage = 0.637 × Epeak Eaverage = 0.637 × 50 V Eaverage = 31.85 V Carefully note that the average value is computed as the average value of one-half cycle. The average value of a complete cycle would be zero. Remember, one complete cycle has an equal amount of waveform above the zero reference line (base line) and below it. In this way, the Effective Values A more meaningful value of an alternating current is its effective value. To determine the effective value, the alternating current is compared to a direct current. In other words, if a direct current will produce a certain power, what equivalent alternating current will produce the same power or work performed in an electrical circuit? If a load, such as a resistor, were connected across an ac circuit, the instantaneous currents between zero and peak could be taken and the effective power could be found. As power equals: P = I 2R all instantaneous current values could be squared and added together. Dividing the sum by the number of current values used gives the average of the squared currents. The square root of this value is known as the root-mean-square (rms) value. Effective values and rms values are the same thing. Either term can be used. It is found to be 0.707 times the peak value. Eeff (rms) = 0.707 × Epeak or Epeak = 1.414 × Eeff (rms) Peak value Peaktopeak value 2 × Epeak = Ep-p Epeak = Figure 13-6. The waveform of a three-phase alternating current. positive alternation cancels out the negative alternation and the net result is zero. Ep-p 2 Figure 13-7. A waveform showing the instantaneous peak voltage (or current) value. Math Manipulation! Find the effective value of a 20 ampere peak current. Eeff = 0.707 × Epeak Eeff = 0.707 × 20 A Chapter 13 109 Alternating Current 110 Electricity Eeff = 14.14 A A 20 ampere peak alternating current has an effective value equal to 14.14 amps of direct current. Remember the importance of the rootmean-square (rms), or effective, value of the voltage or current. These values provide the same work (heating effect) in a resistive circuit as dc. For this reason, most meters are designed to provide you rms, or effective, values for all ac measurements (voltage and current). These values are used for all home and most industrial applications of ac. A summary of these formulas for average and effective values is as follows: Eeff = 0.707 × Epeak Epeak = 1.414 × Eeff Eavg = 0.637 × Epeak Epeak = 1.57 × Eavg Eeff = 1.11 × Eavg Eavg = 0.9 × Eeff Peak-to-Peak Values Another value of an ac wave you should recognize is its peak-to-peak value. Study Figure 13-7 and note these values. The peak-to-peak value of a waveform always appears on an oscilloscope, which is one of the basic instruments used in observing and measuring ac waves. The oscilloscope, generally referred to as “the ‘scope,” is a graphical voltmeter. Basically, the oscilloscope will display waveforms of both dc and ac over a variable time base. This means the waveform can be spread out horizontally for very fast or very slow signals. Additionally, the waveform displayed can be adjusted vertically (magnitude) for very small or very large signals. Not a basic test instrument for the beginner, an oscilloscope is widely used in phase relationships, Figure 13-8. An oscilloscope used in many industrial applications. (Knight Electronics) electronic communication circuits, and other specialized industrial applications. An oscilloscope is shown in Figure 13-8. Quiz—Chapter 13 Write your answers to these questions on a separate sheet of paper. Do not write in this book. 1. On graph paper, construct a sine wave. Show the rotating vector and plot points at 30 degree intervals of rotation. 2. A dc generator uses a commutator, but the ac generator has ______. 3. A vector represents the ______ and ______ of a force. 4. Frequency of an alternating waveform is measured in ______. 5. A certain electric current has a frequency of 400 Hz. What is the time interval or period of one cycle? 6. What is the effective value of a 150 peak voltage? 7. What is the peak voltage of the electricity in your home that is rated at 120 volts? 8. Meters usually are read in ______ voltage and current values. 9. On graph paper draw a single-phase wave, a two-phase wave, and a threephase wave. 10. What is the rms of an average voltage of 90 volts? Many modern test instruments, like this power analyzer, can display their results graphically. (Fluke Corp.)