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Three axis pneumatic modern trailer
 

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    Three axis pneumatic modern trailer Three axis pneumatic modern trailer Document Transcript

    • CHAPTER-1 1. INTRODUCTION Automation can be achieved through computers, hydraulics, hydraulics, robotics,etc., of these sources, hydraulics form an attractive medium. Automation plays animportant role in automobile. Nowadays almost all the automobile vehicle is beingatomized in order to product the human being. The automobile vehicle is being atomizedfor the following reasons:  To achieve high safety  To reduce man power  To increase the efficiency of the vehicle  To reduce the work load  To reduce the fatigue of workers  To high responsibility  Less Maintenance cost 1
    • CHAPTER-2 2. LITERATURE SURVEY2.1 PNEUMATICS: The word ‘pneuma’ comes from Greek and means breather wind. The wordpneumatics is the study of air movement and its phenomena is derived from the wordpneuma. Today pneumatics is mainly understood to means the application of air as aworking medium in industry especially the driving and controlling of machines andequipment. Pneumatics has for some considerable time between used for carrying out thesimplest mechanical tasks in more recent times has played a more important role in thedevelopment of pneumatic technology for automation. Pneumatic systems operate on a supply of compressed air which must be madeavailable in sufficient quantity and at a pressure to suit the capacity of the system. Whenthe pneumatic system is being adopted for the first time, however it wills indeed thenecessary to deal with the question of compressed air supply. The key part of any facility for supply of compressed air is by means usingreciprocating compressor. A compressor is a machine that takes in air, gas at a certainpressure and delivered the air at a high pressure. Compressor capacity is the actual quantity of air compressed and delivered and thevolume expressed is that of the air at intake conditions namely at atmosphere pressureand normal ambient temperature. The compressibility of the air was first investigated by Robert Boyle in 1962 andthat found that the product of pressure and volume of a particular quantity of gas. The usual written as PV = C (or) PıVı = P2V2 2
    • In this equation the pressure is the absolute pressured which for free is about 14.7Psi and is of courage capable of maintaining a column of mercury, nearly 30 inches highin an ordinary barometer. Any gas can be used in pneumatic system but air is the mostlyused system now a days.2.2 SELECTION OF PNEUMATICS: Mechanization is broadly defined as the replacement of manual effort bymechanical power. Pneumatic is an attractive medium for low cost mechanizationparticularly for sequential (or) repetitive operations. Many factories and plants alreadyhave a compressed air system, which is capable of providing the power (or) energyrequirements and the control system (although equally pneumatic control systems may beeconomic and can be advantageously applied to other forms of power). The main advantage of an all pneumatic system are usually economic andsimplicity the latter reducing maintenance to a low level. It can also have out standingadvantages in terms of safety.2.3 PRODUCTION OF COMPRESSED AIR: Pneumatic systems operate on a supply of compressed air, which must be madeavailable. In sufficient quantity and at a pressure to suit the capacity of the system. Whenpneumatic system is being adopted for the first time, however it wills indeed thenecessary to deal with the question of compressed air supply. The key part of any facility for supply of compressed air is by means usingreciprocating compressor. A compressor is a machine that takes in air, gas at a certainpressure and delivered the air at a high pressure. Compressor capacity is the actual quantity of air compressed and delivered and thevolume expressed is that of the air at intake conditions namely at atmosphere pressureand normal ambient temperature. Clean condition of the suction air is one of the factors,which decides the life of a compressor. Warm and moist suction air will result in 3
    • increased precipitation of condense from the compressed air. Compressor may beclassified in two general types: 1. Positive displacement compressor. 2. Turbo compressor Positive displacement compressors are most frequently employed forcompressed air plant and have proved highly successful and supply air for pneumaticcontrol application. The types of positive compressor 1. Reciprocating type compressor 2. Rotary type compressor Turbo compressors are employed where large capacity of air required at lowdischarge pressures. They cannot attain pressure necessary for pneumatic controlapplication unless built in multistage designs and are seldom encountered in pneumaticservice.2.4 RECIPROCATING COMPRESSORS: Built for either stationary (or) portable service the reciprocating compressor is byfar the most common type. Reciprocating compressors lap be had is sizes from thesmallest capacities to deliver more than 500 m³/min. In single stage compressor, the airpressure may be of 6 bar machines discharge of pressure is up to 15 bars. Dischargepressure in the range of 250 bars can be obtained with high pressure reciprocatingcompressors that of three & four stages. Single stage and 1200 stage models are particularly suitable for pneumaticapplications , with preference going to the two stage design as soon as the dischargepressure exceeds 6 bar , because it in capable of matching the performance of single stagemachine at lower costs per driving powers in the range . 4
    • CHAPTER-3 3. FACTORS DETERMINING THE CHOICE OF MATERIALS The various factors which determine the choice of material are discussed below.3.1 Properties: The material selected must posses the necessary properties for the proposedapplication. The various requirements to be satisfied can be weight, surface finish,rigidity, ability to withstand environmental attack from chemicals, service life, reliabilityetc. The following four types of principle properties of materials decisively affect theirselection a. Physical b. Mechanical c. From manufacturing point of view d. Chemical The various physical properties concerned are melting point, ThermalConductivity, Specific heat, coefficient of thermal expansion, specific gravity, electricalConductivity, Magnetic purposes etc. The various Mechanical properties Concerned are strength in tensile, compressiveshear, bending, torsional and buckling load, fatigue resistance, impact resistance, elasticlimit, endurance limit, and modulus of elasticity, hardness, wear resistance and slidingproperties. 5
    • 3.2 Manufacturing Case: Sometimes the demand for lowest possible manufacturing cost or surface qualitiesobtainable by the application of suitable coating substances may demand the use ofspecial materials.3.3 Quality Required: This generally affects the manufacturing process and ultimately the material. Forexample, it would never be desirable to go for casting of a less number of componentswhich can be fabricated much more economically by welding or hand forging the steel.3.4 Availability of Material: Some materials may be scarce or in short supply. It then becomes obligatory forthe designer to use some other material which though may not be a perfect substitute forthe material designed. The delivery of materials and the delivery date of product should also be kept inmind.3.5 Space Consideration: Sometimes high strength materials have to be selected because the forces involvedare high and the space limitations are there.3.6 Cost: As in any other problem, in selection of material the cost of material plays animportant part and should not be ignored. Some times factors like scrap utilization, appearance, and non-maintenance of thedesigned part are involved in the selection of proper materials. 6
    • CHAPTER-4 4. COMPONENTS AND DESCRIPTION4.1 MAJOR PARTS: The major parts “PNEUMATIC THREE AXIS MODERN TIPPER” are describedbelow:  Air compressor  Direction Control Valve  Cylinder  Connecting hoses  Flow control valve  Bearing with bearing cap  Wheel arrangement  Vehicle model frame  Rotating Plates4.1.1AIR COMPRESSOR: The main function of the air compressor is to compress the air up to the requiredpressure. The maximum capacity of the compressor is 103105 to 12 3105 N/m2. This is atwo stages or two-cylinder reciprocating air compressor. The two cylinders are for lowand high compression. The air pressure is measured at various places by the use ofpressure gauges. V-belt and pulley are used to drive the compressor. Compressors can be broadly classifieds into two groups. They are:  Positive Displacement Compressor  Dynamic Compressors 7
    • 4.1.1.1Positive Displacement Compressor: Successive volumes of air isolated and then compressed to a higher pressure.There are essential two forms of positive displacement compressor, reciprocating androtary.4.1.1.2Dynamic Compressors: These are rotary continuous machines in which a high speed rotating elementaccelerates the air and converts the resulting velocity head into pressure. Positive displacement compressors work on the principle of increasing the pressure ofa definite volume in an enclosed chamber. Dynamic (turbo) compressor employs rotatingvanes or impellers to impart velocity and pressure to the flow of the air being handled.The pressure comes from the dynamic effects such as centrifugal force.4.1.2 PRESSURE GAUGE: Pressure gauge is used for measuring the outlet pressure of air from thecompressor. The gauge used is Bourdon type pressure gauge. The maximum capacity ofthis gauge is 10 3105 to 12 3105 N/m2. The gauge is fitted at the outlet of the aircompressor.4.1.3 DIRECTIONAL CONTROL VALVES:4.1.3.1 Pneumatic valves: The pneumatic cylinder is regulated and controlled by pneumatic valves. Thesevalves are actuated manually, mechanically, electrically, pneumatically, and by variouscombined mode of actuation. 8
    • 4.1.3.2Need of Valves: DIRECTIONAL CONTROL VALVES To control the to and fro motion ofcylinder, the fluid energy has to be regulated, controlled and reversed with apredetermined sequence in a pneumatic system. Similarly one may have to control the quantity of pressure and flow rate togenerate the desired level of force and speed of actuators. To achieve these functions,valves are used. Valves are fluid power elements used for controlling and regulating theworking medium. The main functions of the valves are: • Start and stop the fluid energy • Control the direction of flow of compressed air • Control the flow rate of the fluid • Control the pressure rating of the fluidAlthough various types of valves are available, they are mainly classified as below: • Direction control valves • Direction control check valves • Flow control valves • Pressure control valves The main purpose of a valve in a pneumatic circuit is to control outputs. Valvescan be divided into a number of groups according to what they control.4.1.3.3 Directional control valves: Directional control valve on the receipt of some external signal, which might bemechanical, electrical or a fluid pressure pilot signal, charges the direction of or stops, orstarts the flow of fluid in some part of the pneumatic/hydraulic circuit. 9
    • 4.1.3.4 Pressure Control Valves: These are used to control the pressure in part of the pneumatic/hydraulic circuit.4.1.3.5 Flow Control Valves: These are used to control the rate of flow of a fluid through the valve. A directional control valve on the receipt of some, external signal, which might bemechanical, electrical or a fluid pilot signal, changes the direction of stops, or starts theflow of fluid in some part of the pneumatic/hydraulic circuit. They can be used to carryout such functions as: 1. Controlling the direction of motion of an actuator 2. Selecting alternative flow paths for a fluid. 3. Stopping and starting the flow of fluid.Carrying out logic functions such as AND, OR, NAND4.1.3.6 Actuators: An actuator is a device that is used to apply a force to an objectFluid power actuators can be classified into two groups:  Linear actuators are used to move an object or apply a force in a straight line.  Linear actuators can be divided into two types.They are: 1. Single acting cylinders 2. Double acting cylinders A single acting cylinder is powered by fluid for the movement of the piston in onedirection with it being returned in the other direction by an internal spring or an externalforce, a double acting cylinder is powered by fluid in both directions.  Rotary actuators are used to move an object in a circular path. Rotary actuators are the fluid power equivalent of an electric motor. 10
    • 4.1.4 PNEUMATIC CYLINDERS: Cylinders are the one, which offers the rectilinear motion to mechanical elements.Cylinders are classified as light, medium, and heavy duty with respect to theirapplication.4.1.4.1Single Acting Cylinders: In this type, the cylinder can produce work only in one direction. The returnmovement of the piston is effected by a built in spring or by application of an externalforce. The spring is designed to return the piston to its initial position with a sufficientlyhigh speed. Types of single acting cylinders: • Diaphragm cylinder • Rolling diaphragm cylinder4.1.4.2Double Acting Cylinder: The force exerted by the compressed air moves the piston in two directions ina double acting cylinder. They are used particularly when the piston is required toperform work not only on the advance movement but also on the return. In principle, thestroke length is unlimited, although buckling and bending must be considered before weselect a particular size of piston diameter, rod length and stroke length. We use cylinders that are double acting type (i.e.) the compressed air can bepassed to either end of the cylinder. These cylinders are made up of cast iron. 11
    • 4.1.5 SEALS:4.1.5.1 Air Seal: Air seal is used to prevent the leakage of air pressure from the cylinder. Normallyit is made up of neoprene rubber. If there are any air leakages in the system, it will reducethe efficiency.4.1.5.2Wiper Seal: Wiper seal is provided at the entrance of the cylinder to avoid dust materials fromthe environment. It is made up of neoprene rubber.4.1.5.3“O” Ring: The “O” rings are fitted into the grooves of piston to maintain perfect seal betweenthe piston and the cylinder wall. They are mostly made up of neoprene rubber.4.1.6 CYLINDER TECHNICAL DATA:Barrel: It is made of cold drawn aluminimum honed to 25mm.Piston Rod: M.S. hard Chrome platedSeals: Nitrile (Buna – N) ElastomerEnd Covers: Cast iron graded fine grained from 25mm to 300mmPiston: Aluminium.Media: Air.Temperature Range: 0^c to 85^cCushions: Adjustable standard on 400mm bore and above. 12
    • 4.1.7 CONNECTORS: In our system there are two types of connectors used; one is the hose connector andthe other is the reducer. Hose connectors normally comprise an adapter (connector) hosenipple and cap nut. These types of connectors are made up of brass or Aliminium orhardened steel. Reducers are used to provide inter connection between two pipes or hoses ofdifferent sizes. They may be fitted straight, tee, “V” or other configurations. Thesereducers are made up of gunmetal or other materials like hardened steel etc.4.1.8 FLOW CONTROL VALVE: In any fluid power circuit, flow control valve is used to control the speed of theactuator. The floe control can be achieved By varying the area of flow through which theair in passing. When area is increased, more quantity of air will be sent to actuator as a result itsspeed will increase. If the quantity of air entering into the actuator is reduced, the speedof the actuator is reduced. 13
    • 4.1.9BEARING WITH BEARING CAP: The bearings are pressed smoothly to fit into the shafts because if hammered thebearing may develop cracks. Bearing is made upof steel material and bearing cap is mildsteel. Ball and roller bearings are used widely in instruments and machines inorder to minimize friction and power loss. While the concept of the ball bearingdates back at least to Leonardo da Vinci, their design and manufacture has becomeremarkably sophisticated. This technology was brought to its p resent state o f perfection onlyafter a long period of research and development. The benefits of such specializedresearch can be obtained when it is possible to use a standardized bearing of theproper size and type. However, such bearings cannot be used indiscriminately without a carefulstudy of the loads and operating conditions. In addition, the bearing must beprovided with adequate mounting, lubrication and sealing.4.1.10 WHEEL ARRANGEMENT: The wheels are fitted to the body of the vehicle with the help of end bearing and bearing caps. The wheels are made up of fiber material.4.1.11 TIPPER BODY: The tipper body is made up of mild steel sheet metal. This frame is look like a small model trailer.4.1.12 ROTATING PLATES: The rotating plates are fixed in the bottom the trailer body, so that the cylinder willrotates in the required side. The plates are made up of mild steel materials. 14
    • CHAPTER-5 5. BATTERY5.1 INTRODUCTION: In isolated systems away from the grid, batteries are used for storage of excesssolar energy converted into electrical energy. The only exceptions are isolated sunshineload such as irrigation pumps or drinking water supplies for storage. In fact for smallunits with output less than one kilowatt. Batteries seem to be the only technically andeconomically available storage means. Since both the photo-voltaic system and batteriesare high in capital costs. It is necessary that the overall system be optimized with respectto available energy and local demand pattern. To be economically attractive the storageof solar electricity requires a battery with a particular combination of properties: (1) Low cost (2) Long life (3) High reliability (4) High overall efficiency (5) Low discharge (6) Minimum maintenance (A) Ampere hour efficiency (B) Watt hour efficiency We use lead acid battery for storing the electrical energy from the solar panel forlighting the street and so about the lead acid cells are explained below. 15
    • 5.2 LEAD-ACID WET CELL: Where high values of load current are necessary, the lead-acid cell is the type mostcommonly used. The electrolyte is a dilute solution of sulfuric acid (H₂SO₄). In theapplication of battery power to start the engine in an auto mobile, for example, the loadcurrent to the starter motor is typically 200 to 400A. One cell has a nominal output of2.1V, but lead-acid cells are often used in a series combination of three for a 6-V batteryand six for a 12-V battery. The lead acid cell type is a secondary cell or storage cell, which can be recharged.The charge and discharge cycle can be repeated many times to restore the output voltage,as long as the cell is in good physical condition. However, heat with excessive chargeand discharge currents shortened the useful life to about 3 to 5 years for an automobilebattery. Of the different types of secondary cells, the lead-acid type has the highestoutput voltage, which allows fewer cells for a specified battery voltage.5.3 CONSTRUCTION: Inside a lead-acid battery, the positive and negative electrodes consist of a groupof plates welded to a connecting strap. The plates are immersed in the electrolyte,consisting of 8 parts of water to 3 parts of concentrated sulfuric acid. Each plate is a gridor framework, made of a lead-antimony alloy. This construction enables the activematerial, which is lead oxide, to be pasted into the grid. In manufacture of the cell, aforming charge produces the positive and negative electrodes. In the forming process,the active material in the positive plate is changed to lead peroxide (pbo₂). The negativeelectrode is spongy lead (pb). 16
    • 17
    • Automobile batteries are usually shipped dry from the manufacturer. Theelectrolyte is put in at the time of installation, and then the battery is charged to from theplates. With maintenance-free batteries, little or no water need be added in normalservice. Some types are sealed, except for a pressure vent, without provision for addingwater. The construction parts of battery are shown in figure (6).5.4 CHEMICAL ACTION: Sulfuric acid is a combination of hydrogen and sulfate ions. When the celldischarges, lead peroxide from the positive electrode combines with hydrogen ions toform water and with sulfate ions to form lead sulfate. Combining lead on the negativeplate with sulfate ions also produces he sulfate. There fore, the net result of discharge isto produce more water, which dilutes the electrolyte, and to form lead sulfate on theplates. As the discharge continues, the sulfate fills the pores of the grids, retardingcirculation of acid in the active material. Lead sulfate is the powder often seen on theoutside terminals of old batteries. When the combination of weak electrolyte andsulfating on the plate lowers the output of the battery, charging is necessary. On charge, the external D.C. source reverses the current in the battery. Thereversed direction of ions flows in the electrolyte result in a reversal of the chemicalreactions. Now the lead sulfates on the positive plate reactive with the water and sulfateions to produce lead peroxide and sulfuric acid. This action re-forms the positive platesand makes the electrolyte stronger by adding sulfuric acid. At the same time, charging enables the lead sulfate on the negative plate to reactwith hydrogen ions; this also forms sulfuric acid while reforming lead on the negativeplate to react with hydrogen ions; this also forms currents can restore the cell to fulloutput, with lead peroxide on the positive plates, spongy lead on the negative plate, andthe required concentration of sulfuric acid in the electrolyte. 18
    • The chemical equation for the lead-acid cell is ChargePb + pbO₂ + 2H₂SO₄ 2pbSO₄ + 2H₂O Discharge 19
    • 20
    • On discharge, the pb and pbo₂ combine with the SO₄ ions at the left side of theequation to form lead sulfate (pbSO₄) and water (H₂O) at the right side of the equation.One battery consists of 6 cell, each have an output voltage of 2.1V, which are connectedin series to get an voltage of 12V and the same 12V battery is connected in series, to getan 24 V battery. They are placed in the water proof iron casing box.5.5 CARING FOR LEAD-ACID BATTERIES: Always use extreme caution when handling batteries and electrolyte. Weargloves, goggles and old clothes. “Battery acid” will burn skin and eyes and destroycotton and wool clothing. The quickest way of ruin lead-acid batteries is to discharge them deeply and leavethem stand “dead” for an extended period of time. When they discharge, there is achemical change in the positive plates of the battery. They change from lead oxide whencharge out lead sulfate when discharged. If they remain in the lead Sulfate State for afew days, some part of the plate dose not returns to lead oxide when the battery isrecharged. If the battery remains discharge longer, a greater amount of the positive platewill remain lead sulfate. The parts of the plates that become “sulfate” no longer storeenergy. Batteries that are deeply discharged, and then charged partially on a regular basiscan fail in less then one year. Check your batteries on a regular basis to be sure they aregetting charged. Use a hydrometer to check the specific gravity of your lead acidbatteries. If batteries are cycled very deeply and then recharged quickly, the specificgravity reading will be lower than it should because the electrolyte at the top of thebattery may not have mixed with the “charged” electrolyte. Check the electrolyte level in the wet-cell batteries at the least four times a yearand top each cell of with distilled water. Do not add water to discharged batteries.Electrolyte is absorbed when batteries are very discharged. If you add water at this time,and then recharge the battery, electrolyte will overflow and make a mess. 21
    • Keep the top of your batteries clean and check that cables are tight. Do not tightenor remove cables while charging or discharging. Any spark around batteries can cause ahydrogen explosion inside, and ruin one of the cells, and you. On charge, with reverse current through the electrolyte, the chemical action isreversed. Then the pb ions from the lead sulfate on the right side of the equation re-formthe lead and lead peroxide electrodes. Also the SO₄ ions combine with H₂ ions from thewater to produce more sulfuric acid at the left side of the equation.5.6 CURRENT RATINGS: Lead-acid batteries are generally rated in terms of how much discharge currentsthey can supply for a specified period of time; the output voltage must be maintainedabove a minimum level, which is 1.5 to 1.8V per cell. A common rating is ampere-hours(A.h.) based on a specific discharge time, which is often 8h. Typical values forautomobile batteries are 100 to 300 A.h. As an example, a 200 A.h battery can supply a load current of 200/8 or 25A, usedon 8h discharge. The battery can supply less current for a longer time or more current fora shorter time. Automobile batteries may be rated for “cold cranking power”, which isrelated to the job of starting the engine. A typical rating is 450A for 30s at a temperatureof 0 degree F. Note that the ampere-hour unit specifies coulombs of charge. For instance, 200A.h. corresponds to 200A*3600s (1h=3600s). the equals 720,000 A.S, or coulombs.One ampere-second is equal to one coulomb. Then the charge equals 720,000 or7.2*10^5ºC. To put this much charge back into the battery would require 20 hours with acharging current of 10A. The ratings for lead-acid batteries are given for a temperature range of 77 to 80ºF.Higher temperature increase the chemical reaction, but operation above 110ºF shortensthe battery life. 22
    • Low temperatures reduce the current capacity and voltage output. The ampere-hour capacity is reduced approximately 0.75% for each decreases of 1º F below normaltemperature rating. At 0ºF the available output is only 60 % of the ampere-hour batteryrating. In cold weather, therefore, it is very important to have an automobile battery untofull charge. In addition, the electrolyte freezes more easily when diluted by water in thedischarged condition.5.8 CHARGING THE LEAD-ACID BATERY: The requirements are illustrated in figure. An external D.C. voltage source isnecessary to produce current in one direction. Also, the charging voltage must be morethan the battery e.m.f. Approximately 2.5 per cell are enough to over the cell e.m.f. so that the chargingvoltage can produce current opposite to the direction of discharge current. Note that thereversal of current is obtained just by connecting the battery VB and charging source VGwith + to + and –to-, as shown in figure. The charging current is reversed because thebattery effectively becomes a load resistance for VG when it higher than VB. In thisexample, the net voltage available to produce charging currents is 15-12=3V. Acommercial charger for automobile batteries is essentially a D.C. power supply,rectifying input from the AC power line to provide D.C. output for charging batteries. Float charging refers to a method in which the charger and the battery are alwaysconnected to each other for supplying current to the load. In figure the charger providescurrent for the load and the current necessary to keep the battery fully charged. Thebattery here is an auxiliary source for D.C. power. It may be of interest to note that an automobile battery is in a floating-chargecircuit. The battery charger is an AC generator or alternator with rectifier diodes, driverby a belt from the engine. When you start the car, the battery supplies the crankingpower. Once the engine is running, the alternator charges he battery. It is not necessaryfor the car to be moving. A voltage regulator is used in this system to maintain the outputat approximately 13 to 15 V. 23
    • The constant voltage of 24V comes from the solar panel controlled by the chargecontroller so for storing this energy we need a 24V battery so two 12V battery areconnected in series. It is a good idea to do an equalizing charge when some cells show avariation of 0.05 specific gravity from each other. This is a long steady overcharge,bringing the battery to a gassing or bubbling state. Do not equalize sealed or gel typebatteries. With proper care, lead-acid batteries will have a long service life and work verywell in almost any power system. Unfortunately, with poor treatment lead-acid batterylife will be very short. 24
    • CHAPTER-6 6. D.C MOTOR6.1 INTRODUCTION: The electrical motor is an instrument, which converts electrical energy intomechanical energy. According to faraday’s law of Electro magnetic induction, when acurrent carrying conductor is placed in a magnetic field, it experiences a mechanical forcewhose direction is given by Fleming’s left hand rule. Constructional a dc generator and a dc motor are identical. The same dc machinecan be used as a generator or as a motor. When a generator is in operation, it is drivenmechanically and develops a voltage. The voltage is capable of sending current throughthe load resistance. While motor action a torque is developed. The torque can produce mechanical rotation. Motors are classified as serieswound, shunt wound motors.6.2 Principles of operation: The basic principle of Motor action lies in a sample sketch. Movement of Conductor N S Magnetic flux current carrying Conductor 25
    • The motor run’s according to the principle of Fleming’s left hand rule. When acurrent carrying conductor is placed in a magnetic field is produced to move theconductor away from the magnetic field. The conductor carrying current to North and South poles is being removed. In theabove stated two conditions there is no movement of the conductors. Whenever a currentcarrying conductor is placed in a magnetic field. The field due to the current in theconductor but opposes the main field below the conductor. As a result the flux densitybelow the conductor. It is found that a force acts on the conductor to push the conductordownwards. If the current in the conductor is reversed, the strengthening of the flux linesoccurs below the conductor, and the conductor will be pushed upwards. As stated above the coil side A will be forced to move downwards, where as thecoil side B will be forced to move upwards. The forces acting on the coil sides A and Bwill be the same coil magnitudes, but their directions will be opposite to one another. InDC machines coils are wound on the armature core, which is supported by the bearings,enhances rotation of the armature. The commutator periodically reverses the direction ofcurrent flow through the armature. Thus the armature rotates continuously. 26
    • An electric motor is all about magnets and magnetism: a motor uses magnets tocreate motion. If you have ever played with magnets you know about the fundamentallaw of all magnets: Opposites attract and likes repel. So if you have 2 bar magnets with their ends marked north and south, then theNorth end of one magnet will attract the South end of the other. On the other hand, theNorth end of one magnet will repel the North end of the other (and similarly south willrepel south). Inside an electric motor these attracting and repelling forces create rotationalmotion. In the diagram above and below you can see two magnets in the motor, thearmature (or rotor) is an electromagnet, while the field magnet is a permanent magnet(the field magnet could be an electromagnet as well, but in most small motors it is not tosave power).6.3 Electromagnets and Motors: To understand how an electric motor works, the key is to understand how theelectromagnet works. An electromagnet is the basis of an electric motor. You canunderstand how things work in the motor by imagining the following scenario. Say that you created a simple electromagnet by wrapping 100 loops of wirearound a nail and connecting it to a battery. The nail would become a magnet and have aNorth and South pole while the battery is connected. Now say that you take your nailelectromagnet, run an axle through the middle of it, and you suspended it in the middle ofa horseshoe magnet as shown in the figure below. If you were to attach a battery to the electromagnet so that the North end of thenail appeared as shown, the basic law of magnetism tells you what would happen: 27
    • The North end of the electromagnet would be repelled from the north end of thehorseshoe magnet and attracted to the south end of the horseshoe magnet. The South end of the electromagnet would be repelled in a similar way. The nailwould move about half a turn and then stop in the position shown. You can see that this half-turn of motion is simple and obvious because ofthe way magnets naturally attract and repel one another. The key to an electric motor is tothen go one step further so that, at the moment that this half-turn of motion completes, thefield of the electromagnet flips. The flip causes the electromagnet to complete another half-turn of motion. Youflip the magnetic field simply by changing the direction of the electrons flowing in thewire (you do that by flipping the battery over). If the field of the electromagnet flipped atjust the right moment at the end of each half-turn of motion, the electric motor wouldspin freely.The Armature: The armature takes the place of the nail in an electric motor. The armature is anelectromagnet made by coiling thin wire around two or more poles of a metal core. Thearmature has an axle, and the commutator is attached to the axle. In the diagram aboveyou can see three different views of the same armature: front, side and end-on. In the end-on view the winding is eliminated to make the commutator more obvious. You can seethat the commutator is simply a pair of plates attached to the axle. These plates providethe two connections for the coil of the electromagnet. 28
    • 6.4 The Commutator and brushes: The "flipping the electric field" part of an electric motor is accomplished by twoparts: the commutator and the brushes. The diagram at the right shows how the commutator and brushes work together tolet current flow to the electromagnet, and also to flip the direction that the electrons areflowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, sothey spin with the magnet. The brushes are just two pieces of springy metal or carbon thatmake contact with the contacts of the commutator.Putting It All Together:When you put all of these parts together, what you have is a complete electric motor: In this figure, the armature winding has been left out so that it is easier to see thecommutator in action. The key thing to notice is that as the armature passes through thehorizontal position, the poles of the electromagnet flip. Because of the flip, the North pole of the electromagnet is always above the axleso it can repel the field magnets North pole and attract the field magnets South pole. If you ever take apart an electric motor you will find that it contains the samepieces described above: two small permanent magnets, a commutator, two brushes and an 29