Generation of Electricity PDF

UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only GENERATION OF ELECTRICITY INTRODUCTION Energy cannot be created or destroyed, but it can be converted from...

UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only GENERATION OF ELECTRICITY INTRODUCTION Energy cannot be created or destroyed, but it can be converted from one form to another. Some sources of electricity are listed below: Light Heat Friction Pressure Chemical Action Magnetism Motion B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 1 TRAINING MAN UAL For Traini ng Purpose Only LIGHT When certain photoemissive materials such as selenium are struck by light, light energy is absorbed and electrons are discharged. The electrons are then channelled through a conductor to an electrical circuit. A photoemissive material emits electrons when struck by light One of the applications is in solar powered calculators where the electrical current is produced by light. Although photoelectric devices are limited in use in the modern aircraft, spacecraft and satellites rely heavily on photocells and the sun as a source of electric power. Solar Powered Calculators B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 2 TRAINING MAN UAL For Traini ng Purpose Only HEAT Heat can also be used to produce electricity by subjecting two junctions of dissimilar metals to different temperatures. This is called the thermoelectric effect. A thermocouple is a loop of two wires made of dissimilar metals that are joined in two places. Electrical current flows as there is a temperature difference between the two junctions. Examples of thermocouple are iron/constantan, chromel/alumel and copper/zinc. Electrons flow in a thermocouple Thermocouples are used in many electronic temperature sensors in the aircraft. Some examples are the exhaust gas and cylinder head temperature sensors, electronic equipment temperature monitors and some fire detectors. In a cylinder head temperature measuring sensor, one of its junctions is held tightly against a hot engine cylinder head by a spark plug while the other junction is mounted in an area where the temperature is kept relatively constant. Cylinder Head Temperature (CHT) Thermocouple B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 3 TRAINING MAN UAL For Traini ng Purpose Only FRICTION Friction can produce static electricity by simply rubbing two dissimilar substances together. Static electricity is however, not a typically useful form of power. In fact, most static electricity found on the aircraft creates problems for both communication and navigation systems as well as advanced electronic devices. When an airplane flies through the air it accumulates a static charge, especially on the aircraft control surfaces. This is even more apparent when flying through any kind of precipitation or even worse, volcanic ash. Static wicks attached to the trailing edges of control surfaces are designed to help dissipate the static charge to the surrounding air. They act as a protection to the flight instruments, radio equipment as well as the flight surfaces. Without the static wicks attached, the static charge on the surface would try to “jump” along the un-conductive control hinges to the rest of the aircraft. This “jump” or arc could cause permanent damage to the surface itself if the static charge had the opportunity to build sufficiently. To further protect against this damaging “jump”, manufacturers also attach conductive bonding strips to keep the static build-up to a minimum. Bonding Strap on an aircraft B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 4 TRAINING MAN UAL For Traini ng Purpose Only PRESSURE Pressure is another electricity source. Piezoelectricity means electricity created by applying pressure to certain types of crystals. Since only small amounts of electricity are produced, applications are limited. The piezoelectric effect is used in radio communication microphones to convert sound waves into electrical power. Most piezoelectric devices use crystalline materials such as quartz to produce charge. When a force is applied to certain axis, their molecular structure distorts and electrons are emitted into a conductor. Quartz subjected to pressure An electrical charge builds across the faces of these crystals when they are bent or otherwise subjected to mechanical pressure. The crystal vibrates at its natural frequency and produces alternating voltage with specific frequency when excited by pulses of electric energy. B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 5 TRAINING MAN UAL For Traini ng Purpose Only CHEMICAL ACTION Chemical action is often used to produce electricity for aircraft systems. When materials of opposite charges are connected, immersed in an electrolyte and connected through external load, an electron flow is created. A carbon rod immersed in a paste-like electrolyte enclosed in a zinc container can form an alkaline battery. Chemical reaction occurs between the electrolyte and zinc, which changes the zinc into zinc chloride. During this process, electrons are released by zinc and current flows through a wire connecting a light bulb into the carbon rod. Most aircraft contain a battery used for emergency procedures and other functions like engine starting. Electrons flow between two dissimilar materials when they are connected by a conductor and immersed in an electrolyte Batteries on board the aircraft produced electricity through chemical action for the purposes of engine starting and emergency procedures. B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 6 TRAINING MAN UAL For Traini ng Purpose Only MAGNETISM Magnetism is one of the most effective ways of producing electricity and is used to produce most electrical power. Electromagnetic induction produces voltage when a conductor is moved through a magnetic field. Most aircraft use generators or alternators to produce electricity by this method. Electricity generated by Electromagnetic Induction The amount of electricity induced is dependent on the rate at which the lines of flux are cut. Thus, the rate can be increased through the increase of the number of flux lines with a stronger magnet or by moving the conductor through the lines faster. B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 7 TRAINING MAN UAL For Traini ng Purpose Only MOTION Motion can also be used to generate electricity. By using fossil fuels such as oil, coal and natural gas, mechanical motion is produced to drive generators, which in turn produce electricity. For example, in a gas turbine power plant, fuels are burned to create hot gases that go through a turbine, spinning and turning the copper armature inside the generator and generating an electric current. Gas Turbine Power Plant In the case of a nuclear power plant, nuclear reactions create heat to turn water into steam. The steam goes through a similar turbine, which spins and turns the copper armature inside the generator and generating an electric current. Nuclear Power Plant B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 8 TRAINING MAN UAL For Traini ng Purpose Only In a wind turbine, the wind pushes against the turbine blades, causing the rotor to spin and turn the copper armature inside the generator and generating an electric current. Wind Turbine In a hydroelectric turbine, flowing (or falling) water pushes against the turbine blades, causing the rotor to spin and turn the copper armature inside the generator and generating an electric current. Hydroelectric Turbine B-M3 ELECTRICAL FUNDAMENTALS Generation of Electricity 9 UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only ELECTRICAL TERMINOLOGY The following are some common electrical terms, their units and factors affecting them. WATER ANALOGY The following figure shows an analogy using water to illustrate the electrical terms and their significance. Consider two Tanks A and B with different water levels which are interconnected as shown in the figure. Water will flow from the tank with the higher level to the other tank. The presence of the higher water level in one tank has created a difference in water pressure. Water Analogy Tank A - -ve Charged Body Tank B - +ve Charged Body Water flow- current flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 1 TRAINING MAN UAL For Traini ng Purpose Only POTENTIAL DIFFERENCE As seen from the figure the difference in water levels creates a pressure, which makes water flow. Therefore using the same analogy for an electric circuit, when two points are connected by a conductor, there will be a flow of current from the point with a large number of electrons to the other point with a smaller number of electrons. This results in a potential difference between the two points. When there is a potential difference between the two points, it simply means that a field of force is present which tends to move the electrons from one point to another. The unit for potential difference is Volt (V). A potential difference of 1V exists between 2 points of a conductor when it is carrying a constant current of 1A when the power dissipated between these points is equal to 1W. The following figure illustrates the application of potential difference between two points. B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 2 TRAINING MAN UAL For Traini ng Purpose Only ELECTROMOTIVE FORCE (EMF) Electromotive force (emf) or sometimes called electron-moving force is the driving force that causes the current to flow through a conductor. The unit for emf is Volt (V). It is generated by a battery, or by the magnetic force according to Faraday's Law. Emf is commonly generated by electrochemical reaction (e.g., a battery or a fuel cell), absorption of radiant or thermal energy (e.g., a solar cell or a thermocouple), or electromagnetic induction (e.g., a generator or an alternator). Electromagnetic induction is a means of converting mechanical energy, i.e., energy of motion into electrical energy. The emf generated in this way is often referred to as motional emf. Emf can also be considered electrical potential or pressure. The term voltage, which is measured in volts, is typically used instead of emf. Electromotive force is typically symbolised by the letter E and voltage is symbolised by the letter V. VOLTAGE Potential difference, electrical potential and electromotive force are measured in volts, leading to the commonly used term voltage and the symbol V (sometimes E is used for voltage). B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 3 TRAINING MAN UAL For Traini ng Purpose Only DISTINCTION BETWEEN EMF AND POTENTIAL DIFFERENCE The following example illustrates the distinction between emf and potential difference. The circuit has an emf of 4V. The voltmeter reads the potential drop of 3V between A and B. The potential difference between points B and C is 1 volt. Therefore the potential difference is a voltage difference between two points in a circuit. The emf is the voltage generated by the battery. CHARGE The earlier water analogy has shown that water molecules flow from A to B. By using the same analogy in an electrical circuit the electrons flow. These electrons constitute the charge. The unit for electrical charge is coulomb (C). It is the total charge Q of 6.21 x 1018 electrons. Thus a single electron has a charge of 1.61 x 10-19 C. CURRENT In the water analogy, the rate of flow of water is measured in litres/minute, cm3 /sec etc. Therefore using the same analogy, the current can be defined as the rate of flow of electric charge at a point in a circuit. The unit of current is Ampere (A). B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 4 TRAINING MAN UAL For Traini ng Purpose Only Electric current flow in coulombs/sec = amperes. 1 coulomb of electrical charge flow past any point in a conductor in 1 second constitutes a current of 1 ampere. Current is normally denoted by the letter I. RESISTANCE The resistance of a constriction in a large pipe is so great that essentially all the pressure drop will appear across the resistance. If the Water pipe is constricted (narrowed), the constriction will oppose the flow of water than the remainder of the pipe system. Likewise a resistor in an electric circuit will generally will oppose the flow of current than the wire of the circuit. Resistance is the property of a material to oppose the flow of current and to convert electrical energy into heat. Its magnitude depends on factors such as the nature of conductor material, its physical state, dimensions, temperature and thermal properties. The unit for resistance is Ohms ( ). One ohm is the electrical resistance between two points of a conductor when a constant p.d. of 1V, applied to these points, produces in the conductor a current of 1A, the conductor not being the source of any emf V R I where V is the voltage and I is the current. B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 5 TRAINING MAN UAL For Traini ng Purpose Only CONDUCTANCE Conductance is the ability of a material to conduct electricity. Conductance is the inverse of resistance. A material that has a low value of conductance will not conduct electricity as well as a material that has a high conductance and vice versa. The unit of electrical conductance is Siemens (S). The following table shows the relative conductance of some common metals. Metal Relative Conductance (Copper = 1) Silver 1.06 Copper (annealed) 1.00 Copper (Hard Drawn) 0.97 Aluminium 0.61 Mild Steel 0.12 Lead 0.08 CONVENTIONAL CURRENT FLOW Conventional current flow is from positive to negative. As the early discoverers had no knowledge of electron flow, they based their laws of electricity on the behaviour of electric circuits i.e. they could only consider the effects. Conventional Current Flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 6 TRAINING MAN UAL For Traini ng Purpose Only A discharged to B therefore A is considered to be positively charged with respect to B. In fact B was charge with electrons and more negative than A so electrons flowed from B to A. The convention was too well established to alter when the truth was discovered so it remains the convention today. ELECTRON FLOW Electron current flow is from negative to positive. It shows the “true” direction of current flow. Electron Flow B-M3 ELECTRICAL FUNDAMENTALS Electrical Terminology 7