Emirates Aviation University Module 3: Electrical Fundamentals - DC Sources of Electricity PDF

Module 3: Electrical Fundaments Topic 3.5: DC Sources of Electricity  INTRODUCTION On completion of this topic you should be able to: 3.5.1 Describe the construction & basic chemical action of the following cells:...

Module 3: Electrical Fundaments Topic 3.5: DC Sources of Electricity  INTRODUCTION On completion of this topic you should be able to: 3.5.1 Describe the construction & basic chemical action of the following cells: Primary Secondary Lead acid Nickel cadmium Other alkaline 3.5.2 Describe the purposes of connecting cells in series and parallel. 3.5.3 Describe internal resistance and its effect on a battery. 3.5.4 Describe construction and operation of thermocouples and list materials used in their construction. 3.5.5 Identify photo-cells and describe their operation. 30-12-2024 Slide No. 2  THE CELL Galvanic / voltaic cells Cell – device that transforms chemical energy into electrical energy. Simplest cell – Galvanic or Voltaic cell. Consists of piece of Carbon (C) and piece of Zinc (Zn) suspended in electrolyte. Electrolyte – a solution of Water (H20) and Sulfuric Acid (H2S04). Cell – fundamental unit of the battery. Some cells – container acts as one of the electrodes. 30-12-2024 Slide No. 3  CONTAINER May be constructed from many different materials (hard rubber, plastic etc.). Provides a means of holding (containing) the electrolyte. Also used to mount electrodes. In voltaic cell – container material must NOT be acted upon by electrolyte. 30-12-2024 Slide No. 4  ELECTRODES Electrodes – conductors by which current leaves or returns to electrolyte. In simple cell – Carbon and Zinc strips placed in electrolyte. In dry cell – Carbon rod in centre and Zinc container. 30-12-2024 Slide No. 5  ELECTROLYTE Electrolyte – solution that acts upon the electrodes. Provides a path for electron flow. May be salt, acid, or alkaline solution. In simple galvanic cell – electrolyte is in a liquid form. In dry cell – electrolyte is a paste. 30-12-2024 Slide No. 6  ELECTROCHEMICAL ACTION Cell – device in which chemical energy is converted to electrical energy. Process is called electrochemical action. Load connected – electrons flow from -ve through load to +ve. Voltage across electrodes depends upon electrode materials and electrolyte. Current delivered depends upon resistance of entire circuit – including cell itself. 30-12-2024 Slide No. 7  ELECTROCHEMICAL ACTION Internal resistance of cell depends upon: Size of the electrodes Distance between electrodes in electrolyte Resistance of electrolyte The larger the electrodes and closer they are in electrolyte (without touching): The lower the internal resistance of cell The more current the cell is capable of supplying to load 30-12-2024 Slide No. 8  CELLS Electric cells are classified into 2 categories: Primary Secondary Primary cell – cannot be recharged satisfactorily. Part of electrode deteriorates as cell produces current – cannot be restored. Secondary cell – chemical action can be reversed. 30-12-2024 Slide No. 9  PRIMARY CELL Cell in which chemical action eats away one of the electrodes – usually negative. When this occurs – electrode must be replaced or cell must be discarded. In galvanic-type cell – Zinc electrode and liquid electrolyte are usually replaced. 30-12-2024 Slide No. 10  PRIMARY CELL With dry cell – usually cheaper to buy a new cell. For both electrolytic and galvanic cells: Positive terminal is the electrode at which oxidation occurs – It has a positive charge Negative terminal is the electrode at which reduction occurs – It has a negative charge 30-12-2024 Slide No. 11  ALKALINE CELLS Similar to Carbon-Zinc cells. Electrolyte – alkali solution (Potassium Hydroxide). Enables delivery of sustained high current. More efficient than Carbon-Zinc cell. Much longer shelf life. Perform better under drain and in cold weather. Do not produce any gaseous products. May be rechargeable. Types - Mercury Oxide, Silver Oxide, Zinc air. 30-12-2024 Slide No. 12  SECONDARY CELL Cell in which electrodes and electrolyte are altered when it delivers current. May be restored to original condition by charging. Charging – electric current through cell in opposite direction to that of discharge. Secondary cells sometimes known as wet cells. Automobile battery – common example of secondary cell. 30-12-2024 Slide No. 13  SECONDARY CELLS Some basic types are: Lead-acid Nickel-cadmium Silver-Zinc Silver-cadmium 30-12-2024 Slide No. 14  LEAD-ACID BATTERY CONSTRUCTION Container houses separate cells – 6 cells for 12 volt / 12 cells for 24 volt. Most containers are hard rubber, plastic, or some other insulating material. Resistant to electrolyte and mechanical shock & withstand extreme temperatures. Vent plugs allow gases to escape. 30-12-2024 Slide No. 15  LEAD-ACID BATTERY CONSTRUCTION Separators – hold plates apart whilst allowing free movement of electrolyte. Separator material – wood, perforated glass, rubber or plastic. Space at bottom of cell – collects sediment formed as cell is used. Always one more -ve plate than +ve plates. All cells have uneven number of plates, i.e.. 9 plate cell has 5 -ve & 4 +ve. 30-12-2024 Slide No. 16  LEAD-ACID BATTERY CONSTRUCTION -ve plate group is all -ves of individual cells and +ve plate group is all +ves Plates are interlaced with a terminal attached to each plate group. Terminals of individual cells are connected together by link connectors. Cells are connected in series. +ve terminal of one end cell becomes +ve terminal of battery. -ve terminal of opposite end cell becomes -ve terminal of battery. 30-12-2024 Slide No. 17  LEAD-ACID BATTERY CONSTRUCTION Terminals usually identified from one another by their size and markings. +ve terminal marked (+) – sometimes coloured red. +ve terminal is physically larger than -ve terminal marked (–) (typical). Individual cells of lead-acid battery are not replaceable. In event one cell fails – battery must be replaced.. 30-12-2024 Slide No. 18  LEAD-ACID ELECTROLYTE Specific Gravity (SG) – weight of a given volume of liquid in comparison to eight of same volume of pure water. The higher the SG of a liquid – the denser (thicker) it is. Mixture of 64% distilled water (H20) and 36% sulfuric acid (H2SO4). Typical – SG of 1.270 when fully charged (at 20°C, 68°F). 30-12-2024 Slide No. 19  ADDING WATER When adding water to a battery – use ONLY distilled water. Minerals / chemicals in regular water react with plate material & shorten battery life. Lead-acid – electrolyte level no higher than 1/8 inch below bottom of vent well. To avoid permanent damage, ensure electrolyte level never drops below top of plates. Also, avoid over filling – this may result in electrolyte overflow from battery. 30-12-2024 Slide No. 20  LEAD-ACID Most widely used secondary cell. Car battery – very common example. Positive terminal – lead peroxide Negative terminal – spongy lead Electrolyte – sulfuric acid and water Nominal open-circuit cell voltage – 2 volts. 12 V lead-acid aircraft battery 30-12-2024 Slide No. 21  LEAD-ACID DISCHARGING Discharging Sulphuric acid combines with active material in both plates. Forms ‘Lead Sulphate’ on plates and water (H2O) in electrolyte. Increase in battery internal resistance & decrease in electrolyte specific gravity (SG). (SG - 1.150 – completely discharged). If left discharged for substantial period – sulphation occurs – cells lose capacity. 30-12-2024 Slide No. 22  LEAD ACID CHARGING Charging Chemical action is reversed. Sulphate on +ve and -ve plates disassociates from lead. Sulphate returns to electrolyte to form Sulphuric Acid (H2SO4). +ve plate changes back to lead peroxide & -ve back to spongy lead. Specific gravity of electrolyte rises to charged value (1.270 approx.). 30-12-2024 Slide No. 23  LEAD ACID CHARGING Final Charging Phase Little or no Sulphate left on plates. Charging current decomposes water into its Hydrogen and Oxygen components. Gases released via vent plugs (Hydrogen gas is explosive). Batteries in service need to be periodically topped up with water (distilled water). 30-12-2024 Slide No. 24  LEAD-ACID BATTERY OPERATION REVIEW 30-12-2024 Slide No. 25  LEAD-ACID BATTERY OPERATION REVIEW 30-12-2024 Slide No. 26  LEAD-ACID BATTERY OPERATION REVIEW 30-12-2024 Slide No. 27  LEAD-ACID BATTERY OPERATION REVIEW Discharging a lead acid battery below 10.5 volts will severely damage it. 30-12-2024 Slide No. 28  INTERNAL RESISTANCE Every cell has internal resistance. Opposes current flow through cell – limits battery output. Inversely proportional to area of electrodes (exposed to electro-chemical action). Directly proportional to distance separating electrode surfaces. Influenced by nature and condition of electrolyte – typically increases with age. Becomes part of overall circuit resistance. 30-12-2024 Slide No. 29  NICKEL-CADMIUM CELL Known as Ni-Cad – far superior to Lead-acid cell. Generally require less maintenance in regard to adding of electrolyte or water. Negative terminal – Cadmium. Positive terminal – Nickel Oxide. Electrolyte – Potassium Hydroxide and water (30% by weight). 30-12-2024 Slide No. 30  NICKEL-CADMIUM CELL Electrolyte – SG of 1.300 – does not vary with state of charge. Electrolyte only visible and checked at end of charge cycle. When cell is discharged: Negative terminal becomes Cadmium Hydroxide Positive terminal becomes Nickel Hydroxide 30-12-2024 Slide No. 31  NICKEL-CADMIUM CELL Ni-Cad and Lead-acid cells – comparable capacities at normal discharge rates. At high discharge rates – Ni-Cad cell can deliver a larger amount of power. In addition, Ni-Cad cell can: Be charged in a shorter time Stay idle longer in any state of charge Keep a full charge when stored for a longer period of time Be charged and discharged any number of times without detriment 30-12-2024 Slide No. 32  NICKEL-CADMIUM CELL Nominal open-circuit cell voltage – 1.3 volts Distinct advantage over Lead-acid – internal resistance is very low: Voltage remains constant until almost totally discharged (70-80% of charge) Advantage in recharging – allows high charging rates without damage While high discharge and charging rates are favourable – dangers involved. Dangers begin with high battery temperatures. 30-12-2024 Slide No. 33  NICKEL-CADMIUM CELL THERMAL RUNAWAY High charging cycle produces high temperatures. High temperatures lowers internal resistance – causes higher current draw. Higher current draw increases temperature – lowers internal resistance. Snow-balling effect. 30-12-2024 Slide No. 34  NEUTRALISING SPILLS Acid - Sulphuric Neutralise with Bicarbonate of soda (baking soda) or Ammonia. On skin – flush with copious amounts of cold water Alkaline - Potassium Hydroxide Off aircraft - use a 3% solution of Boric acid or Acetic acid (vinegar). On aircraft - use a chromic acid solution On skin – flush with copious amounts of cold water 30-12-2024 Slide No. 35  AIRCRAFT LEAD-ACID BATTERY Differs only slightly from standard automotive or general purpose lead-acid battery. Quality of manufacturing and maintenance standards are higher. Aircraft battery cell plates slightly thinner – allows more plates per cell. Separators – superior materials allow plates to be placed closer together. Increases efficiency and lowers internal resistance. Aircraft batteries typically have 19 plates per cell and 12 cells for a 24 volt battery. 30-12-2024 Slide No. 36  AIRCRAFT NICKEL-CADMIUM BATTERY From outside – Ni-Cad battery differs very little from lead-acid battery. Cases are about same size – especially large types. Real difference appears when you lift top cover: Lead-acid – 12 cells all formed in one case – large vent caps Ni-Cad – 19 or 20 cells, interconnected individual cells – no large vent caps Ni-Cad – very low internal resistance – introduced into aircraft for high start current. Handle heavy load currents better than lead acid – last longer when used that way. Used on small loads over long period of time – lowers capacity – memory effect. 30-12-2024 Slide No. 37  CHEMICAL STATES IN CELLS ***** CAUTION ***** All aircraft batteries release hydrogen and oxygen during charging HYDROGEN IS HIGHLY EXPLOSIVE!!!!! Do not use naked flames or cause sparks anywhere near charging batteries 30-12-2024 Slide No. 38  TYPES OF CELLS Development of new and different types of cells has been rapid. Virtually impossible to have a complete knowledge of all the various types. A few cell types are: Silver-Zinc Nickel-Zinc Nickel-Cadmium Silver-Cadmium Organic and inorganic Lithium Mercury cells 30-12-2024 Slide No. 39  SILVER-Zinc CELLS Used extensively to power emergency equipment. Relatively expensive. Can be charged and discharged fewer times than other types. Light weight, small size, and good electrical capacity. Same electrolyte as Nickel-Cadmium cell (Potassium Hydroxide and water). Positive terminal – Silver Oxide. Negative terminal – Zinc. 30-12-2024 Slide No. 40  COMBINING CELLS In many cases, a device may require more electrical energy than 1 cell can provide. May require either a higher voltage or more current, and in some cases both. Necessary to combine or interconnect cells to meet higher requirements. Aircraft batteries - nominally rated at 12 or 24 volts. Open circuit voltages of serviceable batteries are a little higher: 12-cell Lead-acid and 20-cell Ni-Cad batteries – 25 to 26.5 volts 19-cell Ni-Cad batteries – 24 to 25 volts Work on basis of 2 volts per lead-acid cell and 1.3 volts per Ni-Cad cell. 30-12-2024 Slide No. 41  CONNECTING CELLS IN SERIES Batteries - 2 or more cells grouped together to form 1 source of supply. Usually housed in one container. Series connection - gives a total emf equal to sum of individual voltages. Purpose – to provide higher output voltage. Higher voltage does not mean more current supply. Current supply capability – same as a single cell. 30-12-2024 Slide No. 42  SERIES-CONNECTED CELLS Assume load requires 6 volts and current of 1/8 amp. Single cell normally supplies only 1.5 volts – connect 4 together in series – 6 volts. CAUTION To connect cells in series – connect alternate terminals together (- to +, - to +, etc.). 2 remaining terminals are used for connection to load only. Do not connect 2 remaining terminals together – will short battery. Would quickly discharge cells but could cause some cell types to explode. 30-12-2024 Slide No. 43  CONNECTING CELLS IN PARALLEL Connecting cells in parallel increases battery’s current supplying capability. Purpose – to provide higher output current. Output voltage is same as that of a single cell – 1.5 volts. Parallel circuit – voltage as per single cell, but total current available increases. 30-12-2024 Slide No. 44  PARALLEL-CONNECTED CELLS Assume an electrical load requires only 1.5 volts, but 1/2 amp of current. Assume that a single cell will supply only 1/8 amp. To meet requirement, cells are connected in parallel. All +ve electrodes together and all - ve electrodes together. Voltage between lines is same as that of one cell, or 1.5 volts. Each cell may contribute 1/8 amp to line – 4 cells x 1/8 Amp = 1/2 Amp. 30-12-2024 Slide No. 45  SERIES-PARALLEL CONNECTED CELLS Example – load requirement of 4.5 volts and ½ amp. To provide required 4.5 volts, 3 x 1.5-volt cells connected in series. To provide required 1/2 ampere of current, 4 series groups connected in parallel. 30-12-2024 Slide No. 46  CONNECTING CELLS A - Lamp will be normal brightness – 1.5 volt supply. 30-12-2024 Slide No. 47  CONNECTING CELLS B – Lamp will be normal brightness – batteries in parallel – 1.5 volts. Batteries will last twice as long. 30-12-2024 Slide No. 48  CONNECTING CELLS C – Batteries in series – 3 volts. Lamp will be very bright but will blow very quickly. 30-12-2024 Slide No. 49  BATTERY QUESTIONS 1. What is the purpose of a cell? A cell is a device that converts chemical energy to electrical energy. 2. What are the 3 parts of a cell? Electrodes, electrolyte, and container. 3. What is the purpose of each of the 3 parts of a cell? Electrodes are the current conductors of the cell. Electrolyte is the solution that acts upon the electrodes. Container holds electrolyte and provides a means of mounting electrodes. 4. What is electrochemical action? The process of converting chemical energy into electrical energy. 5. What serves as the negative terminal of a dry cell? The Zinc container. 30-12-2024 Slide No. 50  BATTERY QUESTIONS 6. Why is a dry cell called a DRY cell? The electrolyte is not a liquid but is in the form of a paste. 7. What are the 2 types of cells? Primary and secondary. 8. What is the main difference between the 2 types of cells? Secondary cell can be restored to its original condition by an electric current. The primary cell cannot. 9. Can a battery be recharged by adding more electrolyte? No, a current must be passed through the battery. 30-12-2024 Slide No. 51  BATTERY QUESTIONS 11. What are the 3 ways of combining cells, and what is each used for? Series – to increase voltage but not current. Parallel – to increase current but not voltage. Series-Parallel – to increase both current and voltage. 12. What are some advantages of a Ni-Cad cell over a lead-acid cell? Can deliver a larger amount of power. Can be charged in a shorter time. Stays idle longer in any state of charge. Be charged and discharged any number of times without detriment. 13. What is used to neutralise (a) acid and (b) alkaline spills? (a) acid – bicarbonate of soda (baking soda) or ammonia. (b) alkaline – Chromic acid solution on aircraft, 3% solution of boric acid or acetic acid (vinegar). 30-12-2024 Slide No. 52  BATTERY SUMMARY ELECTROCHEMICAL ACTION – process of converting chemical energy into electrical energy. CELL – device that transforms chemical energy into electrical energy. 3 parts – electrodes, electrolyte, and container. 2 basic cells – primary and secondary. ELECTRODES – current conductors of cell. ELECTROLYTE – solution that acts upon electrodes. CONTAINER – holds electrolyte and provides means of mounting electrodes. 30-12-2024 Slide No. 53  BATTERY SUMMARY PRIMARY CELL – chemical action destroys one of electrodes (typically –ve) Primary cell cannot be recharged. SECONDARY CELL – chemical action alters electrodes and electrolyte Electrodes and electrolyte restored to original condition by recharging. DRY CELL – type commonly referred to as "flashlight battery". Electrolyte not in liquid form, but paste – term dry cell used. In most dry cells – case is negative terminal. 30-12-2024 Slide No. 54  BATTERY SUMMARY LEAD-ACID CELL – most widely used secondary cell Positive terminal – Lead Peroxide – when charged Negative terminal – sponge lead – when charged Electrolyte – Sulfuric Acid and water – dilutes when discharged NICKEL-CADMIUM CELL – NICAD Positive terminal – Nickel Hydroxide – when discharged Negative terminal – Cadmium Hydroxide – when discharged Electrolyte – Potassium Hydroxide and water – does not change Advantages over lead-acid cell: Charges in a shorter period of time Delivers a larger amount of power Stays idle longer Can be charged and discharged many times 30-12-2024 Slide No. 55  BATTERY SUMMARY BATTERY – voltage source in a single container made from one or more cells Cells can be combined in series, parallel, or series-parallel. SERIES CONNECTED CELLS – provide a higher voltage than a single cell, with no increase in current. PARALLEL CONNECTED CELLS – provide a higher current than a single cell, with no increase in voltage. SERIES-PARALLEL CONNECTED CELLS – provide a higher voltage and a higher current than a single cell. HYDROMETER – provides means to check specific gravity of electrolyte. 30-12-2024 Slide No. 56  BATTERY SUMMARY CAPACITY – an indication of current-supplying capability of battery for a specific period of time; e.g., 400 ampere-hour. BATTERY CHARGE – process of reversing current flow through battery to restore to its original condition. Addition of active ingredient to electrolyte will NOT recharge battery GASSING – production of hydrogen gas caused by charge current breaking down water in electrolyte. Steady gassing is normal during charging process. Violent gassing indicates that charge rate is too high. 30-12-2024 Slide No. 57  THERMOCOUPLES One of a number of devices used to sense temperature. Typical uses in aircraft: Sense temperature of cylinder heads on piston engines Sense temp of gas passing into, through or from turbines (gas turbine engines) Some older aircraft – fire warning detectors – sense abnormally rapid temp increases 30-12-2024 Slide No. 58  THERMOCOUPLE OPERATION Production of EMF by heat – apply heat to a junction formed by 2 dissimilar metals. Joined metal junction – hot junction Unjoined section – cold junction. Application of heat to metal results in freeing electrons. Energy required to free electrons from each metal varies – resultant charge varies. Potential difference between metals can be measured at cold junction. Phenomenon of generating a potential via heat – Seebeck effect. Combination of the junction and the 2 dissimilar metals is known as a thermocouple. 30-12-2024 Slide No. 59  THERMOCOUPLE OPERATION Hot junction is connected to a meter – calibrated in degrees Celsius. Meter effectively forms the cold junction. Leads made from same combination of metals – prevents more junctions. Used in aircraft to measure: Engine turbine inlet temperatures Engine exhaust gas temperatures Engine cylinder head temperatures 30-12-2024 Slide No. 60  THERMOCOUPLE 2 dissimilar metal wires connected at 2 junctions to form a loop. Temperature difference between 2 junctions – small EMF is generated. emf – proportional to temperature difference. Typically 4 to 5 millivolts per 100ºC. Common combinations used in aircraft are: Chromel-Alumel – up to 1200ºC (typical – gas turbine engines) Iron-Constantan – up to 850ºC (typical – piston engines) Copper-Constantan – up to 400ºC (typical – piston engines) 30-12-2024 Slide No. 61  THERMOCOUPLE CONSTRUCTION Constructed of 2 dissimilar metal wires joined at one end. May be constructed of several different combinations of materials. Most important factor – "thermoelectric difference" between 2 materials. A significant difference between 2 materials will result in better performance. 30-12-2024 Slide No. 62  THERMOCOUPLE CONSTRUCTION Some thermocouple types: T type – Copper-Constantan up to 400ºC typical on piston engines. J type – Iron-Constantan up to 850ºC typical on piston engines. K type – Chromel-Alumel up to 1200º typical on turbine engines. 30-12-2024 Slide No. 63  LIGHT Light is electromagnetic radiation – thought to travel in form similar to radio waves. Like radio waves, light is measured in wavelengths. Travels at 186,000 miles / second or 300,000,000 metres / second in a vacuum. Frequency range of light – 300 to 300,000,000 gigahertz (giga = 1,000,000,000). Below 400,000 gigahertz – infrared light 400,00 to 750,000 gigahertz – visible to human eye Above 750,000 gigahertz – ultraviolet light Light waves at upper end of frequency range have more energy than at lower end. 30-12-2024 Slide No. 64  PHOTO CELL Also known as a solar cell or photovoltaic cell. A solar cell is a device that converts sunlight directly into a usable amount of direct current (DC) electricity. Assemblies of cells are used to make solar panels. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials. Electrons are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs. 30-12-2024 Slide No. 65  PHOTO CELL An antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent. Solar modules are made by connecting several cells (usually 36) to achieve useful levels of voltage and current. The most that cells could absorb is around 25 percent, and more likely is 15 percent or less. 30-12-2024 Slide No. 66  CONCLUSION Now that you have completed this topic, you should be able to: 3.5.1 Describe the construction & basic chemical action of the following cells: Primary Secondary Lead acid Nickel cadmium Other alkaline 3.5.2 Describe the purposes of connecting cells in series and parallel. 3.5.3 Describe internal resistance and its effect on a battery. 3.5.4 Describe construction and operation of thermocouples and list materials used in their construction. 3.5.5 Identify photo-cells and describe their operation. 30-12-2024 Slide No. 67  This concludes: Module 3: Electrical Fundaments Topic 3.5: DC Sources of Electricity