2019 Cengage CHAPTER 7 Alternating Current Power Distribution

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems INTRODUCTION • Two types of current are used in heating, cooling, and refrigeration today: direct current (DC) and alternating current (AC) • Current is the flow of electrons • Alternating current is an alternating (back and forth) flow of electrons • Direct current will not be discussed in this chapter because it is used in the industry only for special applications © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Basic Concepts of Alternating Current • Overview • Alternating current, or AC, is an electron flow that alternates • Flows in one direction then flows in the opposite direction • Produced by cutting a magnetic field with a conductor • Sine wave is often used as a graphical representation © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Basic Concepts of Alternating Current • Cycle and frequency • Cycle is two alternations (changes in direction) • Frequency is number of complete cycles in a second • Effective voltage • Peak voltage: 90 electrical degrees • Effective voltage: 0. 707 times highest voltage © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Basic Concepts of Alternating Current • Voltage current systems • Several different voltages • Different current characteristics • Phase • Number of currents alternating at different time intervals © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Basic Concepts of Alternating Current • Alternator • Produces alternating current • Inductance and reactance • Inductance is induced voltage • Reactance is resistance alternating current encounters when it changes flow • Impedance is sum of resistance and reactance © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Basic Concepts of Alternating Current • Power • Calculated by using a voltmeter and ammeter • Gives apparent wattage • Power factor • Ratio between true power and apparent power © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Power Distribution • Direct current • Used in the beginning to supply consumers with electrical needs • Disadvantages • Transmission for a long distance is impossible without using generators to boost power • Inability to raise and lower voltages • Requires use of large transmission equipment © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Power Distribution • Power generation

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems 240 -Volt–Single-Phase– 60 -Hertz Systems • Single phase alternating current • Most common voltage system • Exists in most residences • Domestic appliances operating on 120 volts • Most equipment can be operated at plus or minus 10 percent of the rated voltage © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Three-Phase Voltage Systems • Three-phase alternating current • Common in commercial and industrial applications • Three-phase electrical services • Supply three hot legs of power • More versatile than single-phase © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Three-Phase Voltage Systems • Advantages • Require no special starting apparatus • Better starting and running characteristics for motors • Disadvantage • Higher cost of electric panels and distribution equipment © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems 240 -Volt–Three-Phase– 60 -Hertz Delta System • Overview • Also called delta voltage system • Used primarily on systems that have many three-phase circuits of 240 volts • Used on few 120 -volt circuits © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems 240 -Volt–Three-Phase– 60 -Hertz Delta System • Used in structures • Requiring a large supply to motors and other three-phase equipment • Usually supplied to a structure with four wires • Three hot legs and a neutral • Three hot wires only (in rare cases) © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems 208 -Volt-Three-Phase-60 -Hertz Wye System • Overview • Common in structures that require a large number of 120 -volt circuits • Schools • Hospitals • Office buildings © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems 208 -Volt-Three-Phase-60 -Hertz Wye System • Versatile • Possibility of supplying many 120 -volt circuits • Detection system • Important because of the lower voltage requirements • Must be used with the right voltage system for properation and selection © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Higher-Voltage Systems • Overview • Becoming increasingly popular • Several systems available • 240/480 volt-single-phase • 240/416 volt-three-phase • 277/480 -volt-single-phase © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Higher-Voltage Systems • Advantages • Little difference in components used in 208 volt and 480 -volt systems • Saves the consumer money • Service equipment and wiring may be smaller • Disadvantage • Single phase circuits require additional transformers © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems Higher-Voltage Systems • The 277/480 -volt system • Primarily used in industry • Occasionally there are commercial applications © 2019 Cengage

CHAPTER 7 Alternating Current, Power Distribution, and Voltage Systems SUMMARY • AC is used in most heating, cooling, and refrigeration equipment • Properties give it greater flexibility than direct current • Voltage and current are out of phase in an AC circuit • The ration between true power and the effective power is called power factor © 2019 Cengage