Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Ag Eng I Electricity Unit E Electricity Q. Who invented electricity? A. No one, it is based on laws of physics Benjamin Franklin is created with the discovery of electricity. He proved his theory by flying a kite in a thunder storm. Electricity It took almost one hundred years from the time of the discovery of electricity until the first electrical machine or invention was produced. Can you guess what it was? Light Bulb Electricity Where does our electricity come from that we use at our home? In eastern NC its Progress Energy. They produce electricity for a profit just like any other business. Electricity In rural communities, electric membership co-ops were formed to provide electricity to the places that the for profit companies would not go. In our school district that co-op is Four County Electric. Sources of Power Power is produced by a generator. The generator is driven by some type of external power. The common sources of external power are: Fossil Fuels Nuclear Hydro-Electric Solar Sources of Power In our area we have power produced by Coal burning plants in Wilmington Nuclear plants at Shearon Harris and South Port. If you don’t see it on your handout, then write it in. It was added because it was important. Electrical Principles and Wiring Materials Principles of Electricity Electricity is a form of energy that can produce light, heat, magnetism, chemical changes Resistance: tendency of a material to prevent electrical flow Conductor: electricity flows easily Insulator: material that provides great resistance Amps, Volts, Watts Amperes: measure of the rate of flow of electricity in a conductor Volts: measure of electrical pressure Watts: measure of the amount of energy or work that can be done; electrical power Watts = Volts x Amps W =VA West Virginia formula Ohm’s Law Ohm: measure of electrical resistance to flow Resistance = R Volts = E Amps = I Ohm’s Law: E = IR I=E/R R=E/I Electrical Safety Two hazards: Shock and Fire Never disconnect any safety device Don’t touch electrical items with wet hands or feet Don’t remove ground plug prong Use GFI in wet areas Discontinue use of extension cord that feels warm Don’t put extension cords under carpet Electrical Safety Install wiring according to NEC Blown fuse or breaker, determine cause Don’t replace fuse with larger fuse Don’t leave heat producing appliances unattended Keep heaters & lamps away from combustibles Don’t remove back of TV (30,000v when off) Electrical Safety Keep appliances dry Don’t use damaged switches, outlets, fixtures, extension cords Do not remove the long ground prong from 120v three-prong plugs Follow manufacturer’s instructions for installation and use of electrical equipment Service Entrance Power comes from power company Transformer: drops volts from 25,000 volts to 240 volts Service drop: wires that carry electricity from transformer to house Entrance head: weather-proof at house Meter: measure in kw hrs = $$$ Service Entrance Panel (SEP): box with fuses or breakers Electric Meter Kilowatthours: Kilo how electricity is sold = 1000 Watthour = use of 1 watt for one hour 100 watt light bulb for 1 hour - 100 watthours Kilowatthour = 1000 watts for one hour Branch Circuits usually begin at SEP branch out into a variety of places only 1 motor or; series of outlets or; series of lights use correct size wire and fuse or breaker Types of Cable Nonmetallic sheathed cable: copper or alluminum wire covered with paper, rubber, or vinyl for insulation Armored cable: flexible metal sheath with individual wires inside. Wires are insulated Conduit: tubing with individually insulated wires Wire Type and Size Wire size is measured by gauges No 14 (14 gauge) = 15 amp circuits No 12 = 20 amps No 10 = 30 amps Copper is better conductor than aluminum aluminum uses one size larger than copper lower gauge number = larger wire No 8 and larger use bundles of wires current travels on outer surface of wire, so a bundle of smaller wires can carry more Voltage Drop loss of voltage as it travels along a wire Causes lights to dim, motors overheat larger wires have less voltage drop for a given amount of current longer wire = greater problem must increase wire size as distance increases Wire Identification Type of outer covering, individual wire covering, cable construction, and number of wires determines where cable can be used Wire type stamped on outer surface Wire Types Type T - dry locations Type TW - dry or wet RHW – is rubber, heat + moisture resistant THHN - dry, high temps THW and THWN - wet, high temps XHHW - high moisture & heat resistance UF - direct burial in soil but not concrete SE/SEU – unarmored service entrance cabl ASE – armored service entrance cable USE – Underground service entrance Wire Identification Color coded: black, red, & blue = positive or hot wires which carry current to appliances White = neutral wires carry current from appliance back to source Green or Bare = ground all metal boxes and appliances Wire Identification Wire Size: 12-2 has two strands of No. 12 wire (black & white) 12-2 w/g same, with one green or bare 12-3 has three strands of No. 12 (black, red, white) 12-3 w/g same, with green or bare Definitions Breaker – protects circuits from overload of current by tripping to break or open the circuit Fuse – protects circuits from overload by melting a metal strip in the fuse Circuit Def. wire – current carrying conductor under electrical pressure Neutral wire – current carrying conductor not under pressure Ground wire – conducting wire that transmits current to the earth to minimize the danger of electrical shock Hot Computing Electrical Costs Watts = Volts x Amps How many watts of electricity will a 7.5 amp motor use in 5 hrs on a 120 Volt circuit? W = 120 x 7.5 = 900 x 5 hrs W = 4500 How many kilowatts is this? 4500/1000 = 4.5 kw Calculating Volts Volts equal watts divided by amps V = W /A What volt circuit is needed for a 7.5 amp electric motor that uses 900 watts in one hour? V = 900 / 7.5 V = 120 Calculating Amps Amps = Watts / Volts What should the amp rating of an electric motor be if it uses 1500 watts of electricity in 1 hour in a 120v circuit? A = 1500 / 120 A = 12.5 Cost of Electricty $ = cost per kW x # of kw x hours use Note: kilowatt is 1000 watts If electricity costs $.12/kwh, how much would it cost to use 2 kw per hour for 10 hours? = $.12 x 2 kw x 10 hr = $2.40 Switches Single pole- 2 terminals, one location Threeway – 3 terminals, two locations Fourway – four terminals and is used in combination with three way switches to control lights from three or more locations Proper Wire Sizing and Protection of the Circuit Fuse A one time use device that protects the circuit from overloading. If the circuit is overloaded the fuse will “blow” (the filament in the fuse will melt in two) The size of fuse used will depend on the size of wire that is being used in the circuit Circuit Breaker Device that is used to keep the circuit from becoming overloaded. Overloaded circuits become hot and can catch fire. Will “trip” if overloaded The size of breaker depends on the size wire that is being used on the circuit. Wire Size and Breaker Size Wire Size Size 14 12 10 8 6 Breaker 15 20 30 40 55 Wire sizes AWG = American Wire Gauge – a standard for designating the size of electrical wire The larger the number, the smaller the diameter of the wire. The smaller number, the larger the diameter of the wire House Wiring Most circuits in houses are either size 14 or 12. Larger appliances such as dryers and ranges use larger wire (10 or 8) Wire Size Labeling NEC state that all electrical conductors have the following info stamped on the wire AWG Size Voltage Rating Insulation Type Testing Agency Number of Conductors Example of Commmon Wires 12/2 12/2 wg 12/3 wg 14/2 12/2 wg 14/3 wg Single pole switch and light Receptacle is always hot Two threeway switches with light at end of run Three way circuit with light at beginning of run Three way switch with light in middle Four way circuit with light at end Always remember that electricity can kill you. Be certain all power is off before attempting to work with any electrical devices. Safety first.