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Alternating Current by Robert Cecci CONTENTS Study Unit Alternating Current By Robert Cecci CONTENTS INTRODUCTION SECTION 1: INTRODUCTION TO ALTERNATING CURRENT 1 Uses of Alternating Current 2 Alternating Current Versus Direct Current 2 Generation of Alternating Current 4 Elementary Alternator 6 Generation of Voltage Cycle 7 Sine Wave 8 A Cycle of Alternating Current 8 Frequency 9 Time Measured in Degrees 10 SECTION 2: CHARACTERISTIC VALUES OF THE AC CYCLE 14 Types of Characteristic Values 14 Instantaneous Values 15 Maximum and Peak-to-Peak Values 16 Average Value 17 Effective Value 17 Importance of Effective Value 20 SECTION 3: SINGLE-PHASE ALTERNATING CURRENT 23 Opposition to Alternating Current 23 Phase Angle 23 Phase Angle in Resistive AC Circuits 25 Phase Angle in Reactive AC Circuits 26 Power in Resistive AC Circuits 27 Power in Purely Reactive AC Circuits 30 Power in Partially Reactive AC Circuits 32 Apparent Power in an AC Circuit 34 Real Power in AC Circuits 37 Reactive Power in AC Circuits 38 Application of Power Formula 38 Power Factors in Industry 42 SECTION 4: POLYPHASE ALTERNATING CURRENT 45 Polyphase Systems 45 Single-Phase System 46 Three-Phase Circuits 46 Star- or Y-Connected Three-Phase Systems 47 Delta-Connected Three-Phase Systems 48 Power in Three-Phase Systems 49 SELF-CHECK ANSWERS 51 © PENN FOSTER, INC. 2017 ALTERNATING CURRENT PAGE III Contents INTRODUCTION This study unit covers the most common form of electric power used in homes, busi- nesses, and industry: AC current. AC current is used in industry to power computers, control systems, ovens, motors, and there are many more applications. We’ll begin with a study of the basic characteristics of alternating current and the values used to describe AC cycles. You’ll then be presented with information on single-, split-, and three-phase AC current. When you complete this study unit, you’ll be able to QQ Draw a graph of an AC voltage and describe how AC voltage is created QQ Explain what an AC cycle is using the terms alternation, peak, positive, and negative QQ Express the time period of an AC cycle in degrees QQ List the characteristic values of an AC cycle and describe the relationship between the values QQ Define phase angle and describe how it relates to reactive circuits QQ Calculate power for single-phase and three-phase circuits QQ Describe how a 220 VAC, single-phase circuit operates QQ Calculate the phase and line voltages of multiphase wave forms QQ Determine real power by reading a power factor meter QQ Describe delta-connected and wye-connected three-phase circuit connections © PENN FOSTER, INC. 2017 ALTERNATING CURRENT PAGE 1 Introduction SECTION 1: INTRODUCTION TO ALTERNATING CURRENT USES OF ALTERNATING CURRENT Our modern way of life depends on electricity: we use electrically operated machines and devices in our work and recreation, and nearly all manufactured products are produced with the aid of electricity. Most electric devices and machines are powered by alternating current (AC). Alternating current is very different from direct current (DC). In a DC circuit, there’s a steady flow of electrons from the negative terminal of the generator, power supply, or battery. These electrons flowing through the circuit or load are attracted to the positive terminal of the supply. In an AC circuit, the flow of electrons reverses periodically. You may have heard of 60 cycle current delivered by your local utility to your plant or home. 60 cycles means the current begins as a positive cycle and then reverses to a negative cycle 60 times per second. AC is widely used to provide electricity for lighting, for the majority of household appli- ances, and for most industrial motors, controllers, ovens, and process systems. Because of the widespread use of AC electric energy, it’s something you’ll want to know about and be able to work with. Before you study the applications of alternating current, though, you should be familiar with its general characteristics. ALTERNATING CURRENT VERSUS DIRECT CURRENT AC power can be produced and transmitted at a lower cost than DC power. Also, AC can be distributed more conveniently than DC. For these reasons, AC is a widely used choice. An alternator, or AC generator, is a machine that produces or generates AC voltage. It can be designed and built to produce higher voltages and to have higher power capacity than any DC generator of the same size. Also, the larger an alternator is, the more effi- cient it is. It’s relatively simple to generate a high alternating voltage and, using transformers, step it up to a still higher voltage and, finally, transmit it through power lines over long distances to the place where it will be used. For instance, electric power systems can generate voltage at about 1300 volts (V); step this power up to 500,000 V or more for transmission; © PENN FOSTER, INC. 2017 ALTERNATING CURRENT PAGE 2 Section 1 and, finally, step it down to 480, 240, or 120 volts alternating current (VAC) at the points of industrial use. Very high voltage is desirable in a transmission line because it makes it possible to reduce the power loss in that line. A low current at a high voltage and a high current at a low voltage transmit the same amount of power. During any power transmission, losses occur mostly due to the heating of conductors due to the conductor’s resistance. Power lost due to the heating of conduc- tors can be determined by using the following formula: P 5 I 2R In this formula, the letter P stands for the power loss in watts, the letter I stands for the current in amperes, and the letter R stands for the resistance of the conductors in ohms. Resistance in a transmission line is constant, but we can change the current. As you can see in the formula, decreasing the current will substantially lower the power loss. Therefore, the current in transmission lines should be kept as low as possible. Sending the desired electric power through the line at very high voltage will lower the current and reduce the overall power level. Because step-up transformers can easily raise the voltage in AC circuits to high values, and in this way lower the power loss, AC is much more economical than DC for transmis- sion of electric power. Transformers allow for the convenient distribution of alternating current within an indus- trial plant. The high transmission voltage is first stepped down by transformers. Then, the transformed electric power is applied wherever it’s needed, at a safer and more conve- nient low voltage such as the common 480, 240, or 120 VAC used for general-purpose industrial circuits. In the early days of electric power generation and distribution, such an aggressive com- petition raged over how our electric power was to be distributed that it was referred to as a “war.” The Edison Electric Light Company wanted to distribute low voltage DC current to homes and businesses. On the other side of the distribution competition, the Westinghouse Electric Company wanted to distribute electric power as alternating cur- rent. At that time, alternating current was widely believed to be dangerous. This idea came from the highly publicized electrocution of relatively few people by the high trans- mission voltages present on the electricity distribution poles found in cities with AC power systems. The competition went so far that the Edison company publicized the fact that AC power was used in the electric chair. Thankfully, the advantages of AC power generation and transmission gradually won out and today we rely on an AC power distribution system. An AC transformer, an extraor- dinarily simple and efficient device, makes it easy to increase or decrease voltages to any required value. Meanwhile, changing voltage level in a DC system usually requires a number of electronic devices connected in a circuit to convert DC to AC. Once the DC is converted to AC, it’s regulated to the desired voltage level then rectified, or converted back to DC. © PENN FOSTER, INC. 2017 ALTERNATING CURRENT PAGE 3 Section 1 GENERATION OF ALTERNATING CURRENT Most of the alternating current used in home and industry is generated by alternators. Alternators operate on the principle of electromagnetic induction. The generating action of electromagnetic induction occurs whenever there’s a relative motion between a conductor and a magnetic field. As the result of electromagnetic induction, a voltage is induced in the conductor and, if the conductor is a part of a closed circuit, a current will flow through the conductor. The direction of the conventional current (current flow from positive to negative) induced in a conductor by electromagnetic induction can be found by Fleming’s right-hand, or generator, rule. Do you know that rule? It’s illustrated in Figure 1. Here’s how you can apply it. First, using your right hand, aim your index, or pointing, finger in the direction of the magnetic field. At the same time, aim your thumb in the direction in which the con- ductor is moving through the magnetic field. Your thumb and index finger will now be at a right angle to each other. Now, point your middle finger so that it’s at a right angle to both your thumb and your index finger. Look again at Figure 1 to see how to hold your fingers. If you follow directions, your middle finger will point in the direction of the voltage induced in the conductor. That’s the same direction as that of conventional current flow through the conductor. Conductor Direction Current Magnetic Field S N Magnetic Field Current FIGURE 1—The right-hand rule is illustrated here.
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