Chapter 7 Alternating Current, Power Distribution, and Voltage Systems
Objectives Upon completion of this course, you will be able to: Explain the basic difference between direct and alternating current Briefly explain how alternating current is produced Explain the difference between single-phase and three-phase power distribution systems
Objectives (cont d.) Explain inductance, reactance, and impedance Explain a basic power distribution system Explain the common voltage systems Identify the common voltage systems
Key Terms Alternator Capacitive reactance Delta system Effective voltage Frequency Impedance Inductance Inductive reactance Peak voltage Phase Power factor
Key Terms (cont d.) Reactance Sine wave Single phase Three phase Wye system
Introduction Current: flow of electrons Current used in the industry Direct current (DC): flow in one direction Alternating current (AC): alternating (back and forth) flow Most current produced by utilities is AC Used in heating, cooling, and refrigeration equipment
Basic Concepts of Alternating Current Electron flow that alternates Represented by a sine wave Cycle and frequency Cycle: two alternations (changes in direction) Frequency: number of complete cycles in a second
Basic Concepts of Alternating Current (cont d.) Effective voltage Peak voltage: 90 electrical degrees Effective voltage: 0.707 times highest or peak voltage Voltage-current systems Available at several different voltages and with different current characteristics
Basic Concepts of Alternating Phase Current (cont d.) Number of currents alternating at different time intervals Alternator Produces alternating current Made up of a stator and a rotor
Basic Concepts of Alternating Current (cont d.) Inductance and reactance Inductance: induced voltage that counteracts the original voltage Reactance: resistance that alternating current encounters when it changes flow Types: inductive and capacitive reactance Impedance: sum of resistance and reactance
Basic Concepts of Alternating Power Current (cont d.) Calculated by using a voltmeter and ammeter Gives apparent wattage Power factor: ratio between true power and apparent power PF = true power (measured) /apparent power (calculated)
Direct current Power Distribution Used in the beginning to supply consumers with electrical needs Many 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
Figure 7.8 Layout of power distribution. (Delmar/Cengage Learning)
240-Volt-Single-Phase-60-Hertz Systems Single phase alternating current Exists in most residences Domestic appliances operating on 120 volts Older structures Still possible to find a single-phase, two-wire system Most common voltage system
Figure 7.10 Schematic for 240-volt singlephase 60-hertz system using three wires. (Delmar/Cengage Learning) Figure 7.11 Transformer hookup of a 240-volt single-phase 60-hertz system. (Delmar/Cengage Learning)
Three-Phase Voltage Systems Three-phase alternating current Common in commercial and industrial applications Three-phase electrical services Supply three hot legs of power One ground to the distribution equipment and then on to the equipment More versatile than single-phase
Three-Phase Voltage Systems Advantages (cont d.) Require no special starting apparatus Offer better starting and running characteristics for motors Disadvantage Higher cost of electric panels and distribution equipment
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
Figure 7.13 Schematic for the transformer hookup of a 240-volt three-phase 60-hertz system showing the delta transformer secondary hookup. (Delmar/Cengage Learning)
208-Volt-Three-Phase-60-Hertz Wye System Common in structures that require a large number of 120-volt circuits Examples: schools, hospitals, and office buildings Offers versatility of three-phase alternating current and possibility of supplying many 120-volt circuits
Figure 7.15 Schematic for the transformer hookup of a 208 volt threephase 60-hertz system showing the wye transformer secondary hookup. (Delmar/Cengage Learning)
Higher-Voltage Systems Becoming increasingly popular Many advantages Mostly used in industrial structures Used in commercial structures in some cases Several systems available 240/480 volt-single-phase 240/416 volt-three-phase 277/480-volt-single-phase
Higher-Voltage Systems (cont d.) Advantages Little difference in switches, relays, and electric panels used in 208-volt and 480-volt systems Service equipment and wiring may be smaller for 480-volt systems than 208-volt systems Disadvantage Common high-voltage system implementation
Figure 7.16 Schematic diagram for a 277/480-volt three-phase 60-hertz system. (Delmar/Cengage Learning) Figure 7.17 Schematic for the transformer hookup of a 277/480-volt three-phase 60-hertz wye system. (Delmar/Cengage Learning)