Electrical Principles, Terminology, and Safety. Instructional Materials Service Texas A&M University

Transcription

1 Electrical Principles, Terminology, and Safety Instructional Materials Service Texas A&M University

2 Definition of Electricity Layman s Definition: a form of energy that can be converted to light, heat, sound & motion. Electrical Engineer s Definition: the flow of electrons from one atom to another. This flow of electrons is controlled in an electric circuit. The amount of energy produced depends on the number of electrons in motion.

3 Nucleus: Center of an atom Contains positively charged protons Electrically neutral ( Balanced State ): Equal number of electrons & protons Electrons: Arranged in orbital layers around the nucleus Negative charge Basics of an Atom Can be forced to leave their outer orbital layer & attach to the outer ring of an adjacent atom. Atom that receives extra electron becomes negatively charged. Atom that gives up electron becomes positively charged.

4 Hydrogen: One proton One electron Good insulator Copper: 29 protons 29 electrons Examples of Atoms Electrons in outer orbit are loosely held; allows for exchange of electrons Good conductor of electricity

5 How Electricity is Generated Produced by generators run by water, steam or other energy forms. Hydroelectric Generator: Water is used as a source of power to turn generators Less than 5% of electricity is produced this way in Texas Water power uses principle of ancient water wheel Water wheel is called a turbine wheel Turbine wheel is constructed of metal, has an electric generator attached to turbine shaft Water flows against turbine blades, turning turbine & generator Coils of wire inside the generator turn through a magnetic field allowing electrons to flow along a conductor from the generating site.

6 Steam Power: Produces approximately 85% of all electricity used in the U.S. Water is heated in a boiler converting liquid to steam. Steam at high temperatures & pressure causes the blades to rotate the turbine. The turbine operates a generator producing electricity. Also referred to as thermal-powered generators Heat supplied from fossil fuels, nuclear fission, biomass, wood, wind & geothermal

7 How Electricity is Distributed Electric energy is transported from the plant to the consumer in wires known as transmission or distribution lines. Step-Up Transformers: necessary to move large quantities over long distances through transmission lines from the step-up substation to the distribution area. Step-Down Transformers: allows for distribution to customers or consumers through distribution lines.

8 15,000 volts 69, ,000 volts 120 or 240 volts 7,200-14,000 volts

9 How Transformers Work Transformer: used to increase or decrease the amount of voltage. Voltage & Amperage are dependent on each other. When voltage is stepped-up (increased), amperage is reduced proportionately. When voltage is stepped-down (decreased), amperage is must be increased. Basic Transformer: Consists of two coils (windings): primary coil & secondary coil Both are made of wire The coils are wound around an iron core. One coil has more windings than the other Coils & iron core are used to transform electrical energy into magnetic energy & then back into electrical energy.

10 Step-Up Transformer: Power source (power plant) is connected to the coil with the fewest number of turns (primary coil). Electricity is increased through induction in the secondary coil. Increase in voltage due to greater number of turns in the secondary coil Step-Down Transformer: Power source (transmission lines) is connected to primary coil Power from secondary coil is lower in voltage

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12 How Electrical Energy Moves Through a Circuit Circuit: path through which an electrical current flows. Current: movement of electrons through an electrical conductor. Generators produce & release current the moment it is needed. Closed Circuit: complete electrical pathway from the power source through the load & back to the source. Continuous flow of current as long as equipment is turned on and the circuit is closed Open Circuit: an incomplete circuit. When electrical equipment is turned off

13 Electrical circuits are made up of a hot and a neutral wire. Hot wire: A conductor that carries electrical pressure to the load Will show voltage when measured with a voltmeter Usually black or red Neutral wire: A conductor that carries current from the load back to the source Not under electrical pressure (zero voltage) Usually white Ground wire: Conducting wire that transmits stray electrical current to the earth to minimize the danger from electrical shock Hot, neutral & ground wires are usually made of copper because copper is a good conductor.

14 Load: any device that converts electrical energy into another form of useable energy. Ex: a light bulb converts electrical energy into light and heat. Insulator: a material that will not conduct electricity because it will not release its own electrons. Materials used as insulators: Glass Paraffin Porcelain Rubber Silk Cotton Dry wood Most plastics

15 Conductor: any material that allows electrons to move readily offering low resistance. Best Conductors: 1. Gold (expensive) 2. Silver (expensive) 3. Copper (commonly used) 4. Aluminum 5. Bronze DO NOT splice copper & aluminum together because it will result in deterioration and oxidation.

16 Measuring Electricity Electric Current: Amperes: unit of measure for the flow of electrical current. Abbreviated as A or I (Intensity of Current) May be referred to as amperage (or amp) Amperage: the rate at which electric current flows through a conductor per second. Ammeter: instrument used to measure this electrical current flow.

17 Electric Pressure: Volt: unit of measure of electrical pressure or the pressure being applied to force electrons through a circuit. Abbreviated as V or E (Electromotive Force) Voltage: pressure available in energized circuits all the time whether electrical equipment is being used or not. Common service voltages are 120 & 240 volts.

18 120 volts: One hot wire & one neutral wire Circuits used for lighting & small appliances are typically supplied with 115 to 120 volts.

19 240 Volts: Two hot wires & one neutral wire Each hot wire is one half of the total voltage Used by larger equipment such as heating & air conditioning appliances, welders & motors ½ horsepower or larger. If three incoming wires are present, both 120V & 240V service is available.

20 Volts can be measured with a voltmeter Some voltage may be used trying to overcome the resistance in the conductors. Happens when an appliance that requires a lot of current is started Voltage Drop: the reduction of voltage. Influencing factors: Size of wire Length of wire # of amps flowing May cause a loss of heat, light, or power output of a motor Could cause motor burnout unless the motor is properly protected with items such as a time-delay fuse

21 Electric Power Measured in watts (W) Is the rate that electrical energy is transformed into some other form of energy such as light May be referred to as wattage Volts and amperes by themselves do not give a measure of the amount of power produced Important Relationships: Watts = Volts X Amperes Volts = Watts/Amperes Amperes = Watts/Volts

22 Kilowatt (KW) = 1,000 watts Used in computing electrical energy consumed Determined by dividing the # of watts by 1,000 Horsepower (hp): the unit of mechanical power equal to 746 watts of electrical power (assuming 74.6% electric motor efficiency) 1 hp & above motors are rated at 1,000 watts per hp Motors below 1 hp are rated at 1,200 watts per hp

23 Resistance: Any material that opposes the flow of electricity Conductors have very low resistances Insulators have very high resistances Ohm: unit of measure of electrical resistance Abbreviated R for resistance Amount of resistance (ohm) is determined by: Material conductor is made of Size of the conductor Length of the conductor Resistance is proportional to the length & size of the conductor. If length of wire is doubled, then resistance is doubled If diameter of wire is reduced by half, then resistance is doubled.

24 Ohm s Law: The relationship between amps, volts & resistance The Law Volts are equal to amperes times resistance. The Equation E = I x R E = Volts I = Amperes R = Resistances Equation can be rearranged to solve for any of the three values as long as the other two values are known R = E/I (must know E & I) I = E/R (must know E & R)

25 Multimeters are capable of measuring voltage, resistance & current flow in milliamperes. Commonly called a voltohmmilliammeters (VOM)

26 Computing Electrical Energy Use & Cost Watt-hour = use of 1 watt for 1 hour Kilowatt-hour = use of 1,000 watts for 1 hour This is the unit that electrical energy is measured and purchased in. Formulas: Watt-hours = Watts x hours of operation Kilowatt-hours = Watt-hours/1,000 Cost = kwh x local rate per kwh

27 Example: Cost problem using the Watts and Time Estimate method The nameplate data from an appliance indicates that it properly operates at 120 volts and 5 amps. The motor s monthly hours of operation are 10 hours and the local rate per kwh used is 8 cents. The estimated cost of operation per month would be calculated as follows:

28 Step 1) Watts = Volts x Amperes = 120 x 5 = 600 Step 2) Watt-hours = Watts x hours = 600 x 10 = 6000 Step 3) kwh = Watt-hours/1,000 = 6000/1000 = 6 Step 4) Cost = kwh x rate = 6 x 8 = 48 cents

29 Types of Circuits Basic Types: Series Parallel Series-Parallel Series-parallel circuit is a network of circuits that incorporates both series and parallel circuits.

30 Series Circuit: All current must flow through each device in the circuit Current has only one path to flow Removing or opening any one device will stop the flow of current for the whole circuit Same current or amperage flows through each load Voltage is equal to the individual voltage drops from each load Switches, fuses, and circuit breakers are always connected in series

31 Parallel Circuit: Separate paths for the current to flow through Provide the same voltage across each load (lights or appliances) Removing one device has no effect on the other devices Current flow is divided through each load on the circuit & is equal to the total current from the source

32 Types of Electric Current Direct Current (DC): electrons always flow in one direction. Examples: Dry cell batteries Batteries used in automobiles & tractors Alternating Current (AC): electrons move back and forth, reversing their direction regularly. Change direction of flow several times per second, depending on the direction the voltage forces it Electric utilities produce alternating current Examples: Lights Refrigerators Other home equipment

33 Cycle: flow of electricity with voltage fluctuation. Hertz (Hz): term for cycles per second Power suppliers in the US control the power to 60 hertz. This causes electric clocks to keep accurate time because most operate on 60 cycles. European power is on a different cycle.

34 Single-phase: Most common type of electrical service or power available to consumers One transformer is used between the distribution line and the meter Usually three wires, two hot and one neutral, are installed to provide 120V & 240V singlephase service. May also be supplied with threephase service Type of Electric Power

35 Three-phase: Designed especially for large electrical loads More expensive due to installation of four wires & three transformers Three wires are hot & one is neutral Total electrical load can be divided among the three phases, requiring smaller wires and transformers

36 Circuit Failures Many types of conditions can cause a circuit to fail. Open Circuit: Break or interruption in the electrical pathway that stops the current flow. Example: turning off the switch for a light Many times they are unintentional resulting from damage to the equipment or conductor Example: cutting the wire

37 Overload: Occurs whenever excess current flows through a circuit Can cause shock or a fire Examples: Plugging too many products into a circuit Using an extension cord that is rated 15 amps to supply electricity to a 20-amp load The load(s) require the use of more amps than the circuit can safely supply

38 Short Circuit: Occurs when current flows in an unintended path. Unintended path usually offers a lower resistance path for the current to follow, bypassing the normal circuit load(s) Can result from an insulation breakdown or a direct connection between conductors Extremely dangerous Example: A drill fails to operate because wires in the cord are in direct contact with one another.

39 Circuit Protection Devices Fuses and Circuit Breakers Used to protect electrical equipment and wiring from overloads When an overload occurs the circuit breaker trips or the fuse melts inside to prevent the wiring from over heating and causing a fire.

40 Fuses: Contain a link made from a low-melting alloy designed to carry current up to the rating of the fuse. Link melts when current exceeds amperage rating of fuse This opens the circuit & stops the flow of electricity Must be replaced once the link melts or blows inside the fuse Common Fuses: Fusetrons (time-delay) Fustats (two-part time-delay) Time-delay fuse has the ability to carry a temporary overload of current for a short duration without disengaging the fuse links Types of Fuses: Plug Cartridge

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42 Circuit Breakers: Eliminate the replacement of fuses Commonly used Circuit breaker box cost more than a fuse box Two Types: Thermal: Has two contacts held together by a bi-metal strip and latch Overload causes bi-metal strip to heat & expand Latch releases & the points spring open After bi-metal strip cools, switch is reset & service is restored

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44 Magnetic: Has contacts that are held together by a latch, which is released by the action of an electromagnet Amount of current flowing through the circuit determines size of electromagnet Moving toggle switch to on position resets this type

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46 Ground Fault Circuit Interrupters (GFCI): Designed to protect humans, equipment, and/or electrical systems from injury or damage if there is a short circuit. Very sensitive device Compares current flowing out from source through the hot wire of a circuit with that returning back to the source through the neutral wire Ground-fault: one wire is carrying less current than the other Some of the current may take an alternate path back to the source

47 A difference in current flow as little as 5 milliamperes (ma) or greater will cause the GFCI to open the circuit, shutting off the power and eliminating any shock hazard. Current flow as little as 10mA through the human body may be fatal under certain circumstances. Once source of load imbalance has been identified and corrected, GFCI may be reset and tested Current flow from a ground-fault may or may not trip a circuit breaker or blow a fuse, unless the current exceeds the amperage rating.

48 Some GFCI breakers will provide protection for everything on the circuit. National Electric Code (NEC) requires GFCI s for all 120V, single-phase, 15 and 20 amp receptacles installed outdoors, in bathrooms & in garages for residential buildings. A GFCI is required at construction sites. A variety of GFCI equipment is made for 120V & 240V circuits.

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50 Organizations National Fire Protection Association (NFPA) Electrical Safety Devoted to promoting and improving the science & methods of fire protection Publishes the NEC which serves as a national standard for safe installation of electrical wiring and devices and is revised every three years NEC is not law, but all wiring should meet its requirements as well as local building codes. Underwriter s Laboratory (UL) Provides voluntary product testing of all types of wiring materials and electrical devices to determine if they operate safely and prevent electrical shock and /or fire

51 Safety Precautions Wear appropriate clothing. Wear boots or shoes with rubber heels or soles to insulate against shock. Ground all outlets, service entrances, light fixtures, etc. Inspect all electrical equipment and devices to ensure there is no damage or exposed wires that may cause a fire or shock. Avoid using electric equipment near wet, damp areas. Always disconnect the power at the service entrance before you begin work by turning the circuit breaker to the OFF position.

52 Always be sure electric motors are rated at an amperage that the power source can carry. If someone is undergoing electrical shock, do not attempt to grab them and pull them free; by doing so, you will become part of the circuit. Locate switch, circuit breaker, or electrical outlet & disconnect as soon as possible. It is possible to use a dry board or a thick dry piece of rubber or plastic to push or pull the hot conductor from the victim. In the case of an electrical fire, use only extinguishers that are appropriate for electrical fires. Type C fire extinguisher can be used

53 Acknowledgements Developed by Kristie Weller, Undergraduate Technician, Instructional Materials Service, developed this PowerPoint presentation. Kirk Edney, Curriculum Specialist, Instructional Materials Service, reviewed this PowerPoint presentation. Vickie Marriott, Office Software Associate, Instructional Materials Service, assisted in the development of this PowerPoint presentation.

54 ALL RIGHTS RESERVED Reproduction or redistribution of all, or part, of this presentation without written permission is prohibited. Instructional Materials Service Texas A&M University 2588 TAMU College Station, Texas