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Michael McElroy Year 12 Engineering Studies Engineering HSC Syllabus Summary Civil Structures Engineering mechanics and hydraulics ο· Stress and Strain - Strain βΊ The proportional change in length caused when a specimen is under load - - - ππ‘ππππ (π) = βΊ No units (ratio) Shear stress βΊ A measure of the internal reaction that occurs in response to an externally applied load βΊ - π(ππ₯π‘πππ πππ π) πΏ(ππππππππ πππππ‘β π) βΊ ππ‘πππ π (π ππ) = π (πΏπππ π) π΄(π΄πππ π2 ) Engineering and working stress βΊ Engineering stress β the original C.S.A is used to calculate the stress for every value of the applied force βΊ Working stress β The actual or constantly changing C.S.A value is used to calculate the stress Yield stress, proof stress, toughness, Youngβs Modulus, Hookeβs law, engineering applications βΊ Yield stress- the stress where there is a marked increase in strain without an increase in stress. Yield stress is always greater than the elastic limit, but less that the UTS βΊ Proof stress- used as a measure on materials that do not show a marked yield point. Usually a set amount of strain is given to the material, usually 15 or 2% and the amount of stress can be calculated βΊ Toughness- indicated by the area under the curve in a stress/strain diagram. Ability of a material to absorb energy βΊ Youngβs Modulus- Measure of the stiffness of a material. Applies up to the elastic limit of a material. The gradient of the straight line in section of the graph indicates YM. βΊ Hookeβs law- the amount of elastic deformation that a material can sustain in tension or compression before it undergoes permanent plastic deformation. Factor of safety βΊ For ductile materials β πΉππ = βΊ For brittle materials - πΉππ = πππππ π π‘πππ π πππ₯.πππππ€ππππ π π‘πππ π πππ πππ₯.πππππ€ππππ π π‘πππ π Michael McElroy - ο· Stress/strain diagram Truss analysis - Method of joints Year 12 Engineering Studies Michael McElroy - ο· Year 12 Engineering Studies Methos of sections Bending stress induced by point loads only - Concept of shear force and bending moment βΊ Shear force A shear force causes one part of a material to slide past the adjacent part of the material βΊ Bending moment The bending moment is the amount of bending that occurs in a beam Michael McElroy - Shear force and bending moment diagrams Year 12 Engineering Studies Michael McElroy Year 12 Engineering Studies - Concept of neutral axis and outer fibre stress βΊ As a beam bends, the concave side will compress and set up compressive forces within the beam βΊ The convex side of the beam will stretch, producing tensile forces βΊ In between, there exists a plane where the fibres in the beam are not subjected to tensile or compressive forces. This plane is called the neutral axis βΊ The fibres furthest away from the neutral axis will be subjected to maximum stress - Bending stress calculation (second moment of area given) βΊ To calculate the bending stress (π) at any section of the beam. ππ¦ π= πΌ Where: π = bending stress (Pa) M = max. bending moment (Nmm) Y = distance from neutral axis (mm) I = second moment of area of the cross section of the beam (mm4) Michael McElroy Year 12 Engineering Studies ο· Uniformly distributed loads - Unlike a point load, UDL is a load that is spread across a beam ο· Crack theory - Crack formation and growth βΊ Cracking begins with micro cracks (crack initiation phase) βΊ Cracking then leads to crack propagation, that phase in which the crack grows in size under cyclic loading to ultimate part failure βΊ Crack initiation can be long and develops from repetitive stresses at stress concentrations βΊ Dependant on material and method of component manufacture βΊ Surface cracks can be detected using visual inspection techniques such as magnetic particle add dye penetration tests βΊ Sub surface cracks require ultrasonic or radiographic methods to be detected - Failure due to cracking βΊ Cracking can occur at stresses below yield stress, known as fatigue βΊ Fatigue fracture begins as small crack, that grows in size from repeated stress βΊ As a crack expands, the load carrying cross-section of the component is reduced, with the result that the stress on this section is raised - Repair and/or elimination of failure due to cracking βΊ For metallic materials: welding can repair crack. However doing this will repair crack but micro structural changes will appear around the weld and weaken the material, with weld being a point of stress concentration. Heat treating the material after welding will avoid this βΊ For polymers: adhesives can be used to repair crack. This cannot be done for thermosets, instead it must be replaced βΊ For ceramics: usually canβt be repaired β need to be replaced Michael McElroy βΊ Year 12 Engineering Studies Prevention: designing item without sharp corners will reduce cracks from forming as stress can be concentrated at these points Engineering mechanics and hydraulics ο· Testing of materials - Compressive testing βΊ Used to determine the compressive strength of materials βΊ Test piece is compressed and load deformation is recorded - Transverse beam testing βΊ Many materials used are not only in compression or tension at the same time. They can be exposed to bending stresses βΊ Used to determine bending and shear in materials βΊ Transverse beam testing involves placing a test piece between two βΊ supports and then gradually applying a load - Concrete testing βΊ The water/cement ratio in concrete effects the workability of the mix and also the final strength of the concrete βΊ Slump test β measures the workability of concrete. Wet concrete is placed in a mould. When the mould is removed, the amount of deformation of the shape is measures and is used to describe the workability of the concrete βΊ Compression test β compression testing of concrete is measured after 28 days. This is done to test the strength of the concrete. ο· Ceramics - Structure property relationships, applications βΊ Hard, brittle, chemically inert, electrical/thermal insulation, durable βΊ Compressive strength - Glass βΊ Non-crystalline ceramics βΊ 3 basic ingredients are: silica, limestone, soda ash βΊ Soda-lime glass: accounts for 90% of glass β windows, bottles etc. βΊ Borosilicate glass: used for ovenware, telescopes βΊ Lead glasses: optical components, radiation shielding βΊ Main properties: transparent, brittle, compressive strength βΊ Properties can be improved by: thermal toughening (air quenching), chemical toughening, laminating - Cement βΊ Bonding material βΊ Compressive strength βΊ Low toughness βΊ Easily casted βΊ Excellent workability Michael McElroy ο· Year 12 Engineering Studies Composites - Timber βΊ Organic materials βΊ Structure: cellulose tubes bounded together by glue lignin (wood grain) βΊ Factors affecting strength of timber: loading duration, moisture content, defects (within grain) βΊ Exposure to chemicals - Concrete (reinforced and pre-stressed) βΊ Concrete is a compound of sand, gravel, cement and water βΊ Reinforced concrete: steel bars imbedded in concrete to add tensile strength βΊ Pre-stressed: concrete is poured over steel wires or cables that are placed in tension. After concrete is hardened, tensile stress on cables is released - Asphalt βΊ Consists of aggregate, bitumen and air voids βΊ Aggregate held together with bituminous binder βΊ Adding small amounts of materials, such as rubber, alter asphalt properties βΊ Toughness βΊ Durability βΊ Resistance to moisture, heat etc. (weather resistant) - Laminates βΊ Consists of materials that are sandwiched together βΊ Plywood: layers of timber with adhesive βΊ Laminated glass: two layers of glass with PVB polymer in middle β adds strength βΊ Fibre glass: glass fibres bonded with polymer resin - ο· ο· Geotextiles βΊ Woven polymers or ceramic fibres βΊ Used to stabilise road base, geotextile is placed underneath asphalt β prevents potholes Corrosion - Corrosive environments βΊ Availability of oxygen to enable reactions to proceed βΊ Temperature - Dry corrosion, wet corrosion, stress corrosion βΊ Dry corrosion β occurs through chemical reactions with gases, at high temperatures i.e. in furnaces βΊ Wet corrosion β occurs when material is in contact with fluid or moisture βΊ Stress corrosion β when a material is subjected to stress (i.e. cyclic loads) and cracks begin to form. The material will eventually degrade due to fatigue Recyclability of materials - Steel βΊ B.O.F (basic oxygen furnace) β 25% recycled steel possible βΊ E.A.F (electric arc furnace) β 100% recycled steel possible - Concrete Michael McElroy - - - Year 12 Engineering Studies βΊ Recycled concrete weaker that original product βΊ Usually used as rubble βΊ Concrete is crushed/broken down and re-used Wood βΊ Can be recycled for basic uses i.e. furniture, pallets etc. βΊ Dependant on type of wood βΊ Used as chips for garden mulch, playground covering βΊ Smaller chips to form wood composites βΊ Recycled as paper or cardboard Asphalt βΊ Limited uses for recycled products βΊ Usually crushed and refined with other materials added to reproduce asphalt again Glass βΊ Can be reused to produce glass again Personal and Public Transport Engineering mechanics and hydraulics ο· Static friction - Concept of friction and its use in engineering βΊ Friction is the resistance to motion and efficiency βΊ Friction always acts opposite to the direction in which the body moves βΊ Static friction β frictional force present when two bodies are at rest βΊ Limiting friction - frictional force present when two bodies are at the point of moving βΊ Dynamic friction β frictional force while a body is moving - Coefficient of friction β amount of friction that materials develops between them (ΞΌ) πΉπΉ π π πΉππππ‘ππ πππππ βΊ π= βΊ Normal force ο§ Reaction force ο§ Always perpendicular to supporting surface ο§ Equal to, but opposite direction to weigh force ο§ Balances out forces Friction force ο§ Force that prevents movement ο§ Force that is exerted between contacting surfaces ο§ Always opposes direction of motion ο§ Increases as applied force increases Angle of static friction (Ο) ο§ Resultant force (of friction force and normal force) makes with the normal βΊ βΊ = ππππππ πππππ‘πππ Michael McElroy - Year 12 Engineering Studies ο§ The angle that the resultant force makes with the normal reaction ο§ tan Ο=ΞΌ βΊ Angle of repose ο§ The angle when the angle of static friction will equal the inclination of the plane ο§ When the gravitational force down an inclined slope equals the frictional force, i.e. the system is an equilibrium Energy, power βΊ Potential energy ο§ Stored energy within an object with the ability to do work ο§ The PE is equal to the work done in lifting a bodyβs weight (mg) through a vertical height (h). ο§ PE =mgh ο§ Hydro electricity uses PE βΊ Kinetic energy ο§ Energy a body possess due to its motion ο§ KE = Energy a body possess due to its motion ο§ βΊ βΊ Work ο§ ο§ ο§ Power ο§ ο§ 1 KE = 2 mv2 W=βKE or W=Fs or W=FscosΞΈ When a force causes motion Total work β multiple forces acting on a body The rate at which work is done P=w/t = Fs/t = Fv Engineering materials ο· Testing of materials - Visual testing βΊ Dye penetration ο§ Dye or coloured liquid is placed on the surface of a component and excess is wiped clean ο§ Any cracks or imperfections on surface of component will be highlighted by the dye remaining ο§ Fast, simple, inexpensive ο§ Used for small specimens and various materials ο§ Difficult to detect small cracks ο§ UV light is also used to help show up any imperfections βΊ Magnetic particle testing ο§ Component is placed on a conducting rod, that produces a magnetic field about the component ο§ Fluorescent liquid of charged particles is sprayed over component Michael McElroy ο§ - Year 12 Engineering Studies Fluorescent magnetic particles are drawn to the cracks by the conducting rod, highlighting surface imperfections Radiographic examination βΊ X-Rays ο§ Favourable because a photo film is produced, for close analysis ο§ Detects sub-surface defects ο§ Radiation is used to penetrate the item, with any voids allowing the rays to pass through more easily, resulting in a dark area of film ο§ Used on large objects ο§ Longer/more expensive than visual testing ο§ Exposure to radiation can be harmful to humans βΊ Gamma Rays ο§ Effective when testing thick structures, i.e. steels ο§ Can be used to examine joining methods, i.e. welds ο§ Exposure to radiation can be harmful to humans - Ultrasonic testing βΊ Detects sub-surface defects βΊ A probe transmits high frequency vibrations throughout the component as it passes over the surface of a component βΊ Any imperfections within the component causes the vibration to be reflected without travelling to the bottom βΊ Results are displayed on detection machine ο· - Heat treatment of ferrous metals Heat treatment of steels βΊ Heat treatment used to give steel to alter their properties. Heat treatment involves 3 main processes 1. Heating of metal to pre-determined temp. 2. Soaking (holding) of metal at that temp. until heat becomes uniform throughout 3. Cooling of metal at pre-determined rate such that is will cause formation of, or will maintain desirable structures within the metal Annealing βΊ Process annealing Heat steel to temp. Between 550 and 650ΛC Relieve internal stresses from within material Air cooled Complete recrystallisation of the metal βΊ Full annealing Heated above 923ΛC Soaked Cooled within furnace β slow process - Michael McElroy - - - ο· - - Year 12 Engineering Studies Produces softer steel Normalising βΊ Heating to higher temps. than annealing Soaked Air cooled Produces fine grain structure β stronger material Increase in UTS, hardness Decrease in ductility Hardening and Tempering βΊ Hardening Hardening steel depends on carbon content Heat above 800ΛC Soak Quench (cool very quickly) β water, oil, brine can be used Quenching causes stress to build up in steel β becomes extremely hard Quenching produces martensite β hard + brittle material that needs further treatment to increase toughness βΊ Tempering Remove internal stresses from material that have been quenched (martensite) Retains hardness and replaces brittleness with toughness Heat steel between 200 - 600ΛC Soak Cool in air Tempered martensite produced Structure property relationships βΊ Annealed β coarse grain structure β soft with moderate strength Normalised β fine grain structure β higher strength Hardening β stressed grain structure β hardness + brittleness Tempering β very fine grain structure β toughness + hardness Structure/property relationship in the material forming processes Forging βΊ Shaping a metal through use of force βΊ Can be done above recrystallisation temp. (hot forging) or below (cold forging) βΊ Extrusion βΊ Drop forging β technique that uses hydraulic pressure to operate a hammer that shapes metal o Dimensional accuracy not good o Grain flow/direction is major advantage β grain flow follows profile of part, no points of weakness. Contrasting to machined part where grain flow does not follow profile and provides points of weakness Rolling βΊ Metal pressed into shape between rollers βΊ Can be done as hot or cold rolling βΊ Cold rolling o Compressed grains result in specific directional properties Michael McElroy βΊ - βΊ βΊ βΊ ο· - High strength Grains in material remain stressed Increased harness o o o Unstressed finished product Easily performed than cold rolling Favourable directional grain flow Casting βΊ - o o o Hot rolling βΊ βΊ βΊ βΊ - Year 12 Engineering Studies Pour molten metal into a mould to form a specific shape Good dimensional properties β close to finished product Cheap Can effect physical properties of metal β depending on material mould is made from Sand casting o Mould made form sand o Chill grains form on surface of metal o Larger columnar grains form within metal β present weakness in metal β allows for metal to shear along the grain boundaries Shell moulding o Form of sand moulding o High dimensional accuracy o Fine clean sand with thermosetting binder Die casting o Metal forced into mould cavity under pressure o Excellent surface finish Centrifugal casting o Molten metal injected into spinning mould o Centrifugal force forces molten metal to stick to interior of the mould Extrusion βΊ Metal forced through die so it takes shape of the die it passes Powder forming βΊ Metal power is mixed with other desired materials and put into mould in room temperature βΊ Mixture is then pressed into mould to form desired shape βΊ Pressure compacts particles together βΊ Pressed item is sintered in controlled atmosphere furnace βΊ Heated to temp. Where atoms are allows to diffuse between grains, producing uniform grain structure βΊ Used to form brake pads β materials with different properties mixed together to give superior final product βΊ Difficult to produce certain shapes Non-ferrous metals Aluminium βΊ Non-corrosive βΊ Lightweight Michael McElroy βΊ βΊ βΊ βΊ - - ο· - o Brass βΊ βΊ βΊ βΊ Year 12 Engineering Studies Good strength to weight ratio Easily fabricated Very good electrical conductivity Ductile o Aluminium silicon Good casting properties More corrosive than pure aluminium o Aluminium copper High strength Good electrical conductivity More corrosive than pure aluminium Hard o Aluminium silicon-magnesium Medium strength Weldable Car doors Alloy of copper and zinc Corrosion resistance Cannot spark Low coefficient of friction Bronze βΊ Alloy of copper and tin βΊ Excellent corrosion resistanceβ from oxidization βΊ Hard βΊ Brittle Structure/property relationship Annealing, strengthening Ceramics and glasses Semi-conductors βΊ Operates on the basis of the deficiency/surplus of electrons within a material βΊ Semi conductors are a unique group of materials that, when subjected to a certain type of energy (i.e. thermal, electrical), can act either as conductors or insulators. The properties of these materials make them highly favourable in the electronics industry. Diodes - Form the basis of rectifiers, which allow the conversion of AC power to DC. Allows an electric current to pass in one direction, while blocking current in the opposite direction. Rectifiers are used to function battery charges in most transportation vehicles. - Used to extract modulation from radio signals in radio receivers that can be found in various transportation vehicles such as cars, trains, aeroplanes and even motorcycles. Michael McElroy Year 12 Engineering Studies - o ο· Used to produce light in LEDβs (light emitting diode) that can be found in the instrument panels and consoles of various transportation vehicles. Also used in some modern day headlamps on vehicles such as cars and motorcycles. Transistors - Enables the amplification of current or voltage, or acts as a switch. These qualities are present in practically all electronic devices found in the control systems of transportation vehicles. - Allows for the miniaturization of electronic components which are essential in the electrical equipment found in cars, trains and aeroplanes. - Featured within microprocessors, located in computing systems such as GPS devices, speech recognition technology and various on-board technologies found within modern day cars and aeroplanes. - Overall, transistors are fundamental components to all electronic devices. It can be understood that virtually all electronic devices and equipment found in transportation systems feature transistors. - Justify appropriate choices of ceramics and glasses used in transportation parts and systems βΊ Ceramics β car brake pads β high heat resistance, durability βΊ Glasses β laminated glass β car/train windows for safety reasons Multi-laminated glass β planes - to cope with pressurised cabin, protect against substances such as moisture and salts from occurring between glass layers, safety, strength - Laminating and heat treatment of glass βΊ Heat treatment ο§ Tempered glass - Heat treatment of glass increases resistance to fracture by creating compressive surface layer ο§ Glass heated to around 650ΛC ο§ Subjected to air quench β rapidly cools surface ο§ Cooling surface contracts β placed under compression βΊ Laminated glass ο§ Consists of a sandwich of two layers of glass and a polymer interlayer of PVB joined under heat and pressure Polymers - Structure/property relationships and applications βΊ Basic structure consists of molecules composed of repeating atoms of the same element that are joined together by chains. βΊ The basic unit of any polymer is the carbon atom - forms the backbone of the polymer chain. βΊ Different polymers with different properties can be produced when replacing the hydrogen atom with another element. Michael McElroy Year 12 Engineering Studies βΊ - - Thermoplastics - covalent bonds (atoms sharing same electrons and hence fusing them together) form the polymer chains but only weak secondary bonds between the chains βelastic, malleable βΊ Thermosets - covalent bonds (atoms sharing same electrons and hence fusing them together) form both the polymer chains and secondary bonds between the chains βrigid, strong, less elastic βΊ Applications β thermoplastics - In transportation systems, thermoplastics are mainly used as interior components (dashboards, linings etc.) βΊ Applications β thermosets - In transportation systems, thermosets are mainly used as interior components and textiles, however in some modern transportation systems composite thermosets (more than 2 substances combined) can be used to make exterior parts such as body panels. Exterior components of boats β waterproof, rigid, buoyant, hard, tough Engineering textiles βΊ Thermosets act as binder for textiles β adds tensile and compressive strength and durability to textile Manufacturing processes for polymer component βΊ Injection moulding - plastic is heated from granular form and melted into resin form and then injected through a die by way of a ram into a cavity or cast β usually thermoplastics βΊ Extrusion - plastic is heated from granular form and melted into resin form and then injected through a die by way of a ram onto a conveyer belt to cool βΊ Compression moulding β in granular form, plastic is placed in mould where heat and pressure is applied to melt the plastic allowing it to flow within cavities β used for thermosets Engineering electricity/electronics ο· Power generation/distribution Power generation To produce electricity and power, generators are used to convert the mechanical energy of rotation into electrical energy. In Australia, electricity can be produced in a number of ways that include: - - βΊ Steam Takes place in a coal fire power station This production process includes burning finely crushed coal in a furnace. The heat generated from this heating process is used boil pure fresh water, which produces high pressure steam and turns a series of turbines that are connected to generators. The steam used to rotate the turbines can reach 540ΛC Steam pressure turns the generator shaft at up to 50rps (=50hz β AC frequency in domestic power supply) Advantages: coal is an abundant, relatively cheap and an easily transportable substance. Production process re-uses the steam (or rather the water that is boiled) Michael McElroy - - - - Year 12 Engineering Studies Disadvantages: mining the coal damages the environment greatly, burning the coal produces polluting gases (i.e. sulphur dioxide), releases mass amounts of greenhouse gases which contribute to global warming βΊ Hydro-electric Involves harnessing the potential energy of dammed water by allowing the water to run through a water turbine and generator. Advantages: does not produce atmospheric pollution, renewable energy source (sources that are naturally replenished) Disadvantages: expensive production method (i.e. erecting infrastructure), affects surrounding environment, only possible in particular areas (such as mountainous regions) βΊ Wind Involves using the wind to turn large blades which turn turbines connected to generators. Advantages: does not produce atmospheric pollution, renewable energy source (sources that are naturally replenished) Disadvantages: high initial costs (to erect infrastructure), to produce high amounts of electricity; a lot of wind turbines are needed, consumes large amounts of land. βΊ Nuclear Power Involves using the heat from a nuclear reaction to drive a steam turbine. The turbine connects to a generator that is spun, producing electricity. Advantages: efficient production method, doesnβt contribute to global warming Disadvantages: produces very harmful by-products to the environment, nuclear waste disposal is an issue. Power distribution (generatin g station) 500 kV 132 kV 66 kV 11 kV (district transformer station) (Local transformer station) (substation) (Pole or undeground transformer Industry 20 kV supply Industry From the generation of electricity at a large scale, a number of processes occur during the distribution to the consumer to provide safe and manageable amounts. These processes include: 415/240 kV (Consumer) The transmission lines used for distribution consist of high-tension galvanised steel core aluminium cables. The steel core adds for better support and strength of the wires, with aluminium more favourable over copper for its high conductivity and low density. Michael McElroy ο· - - Year 12 Engineering Studies AC/DC Circuits AC AC (alternating current) is where they electrons oscillate back and forth in the circuit β meaning the current direction alternates The rate of oscillation is called the frequency The current from the power outlet in a domestic home has a frequency of 50hz (changes directions 50 times per second or 3000 times per minute (rpm)) DC DC (direct current) always flows in the same direction (put in AC terms: a current with 0hz) DC power mainly comes from a battery DC is used in applications where we need to control the speed of electric motors e.g. electric trains Single phase and three phase system Single phase - The distribution of AC in which the power supply varies over time (i.e. at some instances power is at max and other instances it is at 0) Has an average output of 50% Does not produce a consistent power output, instead has power lapses (when the current is alternating) Used when loads are most likely heating, and not heavy loads such as machinery and electric motors etc. Ideal for residential consumers and not the industry Common in rural areas Three phase - Makes use of three wires to deliver three independent alternating electrical currents at different time intervals Removes any moment of 0 power and maintains max power average (100%) rather than the average of 50% Produces a stable flow of electricity Desirable for electric motors as a magnetic field can be more effectively produced and maintained as oppose to single-phase Can still power a single phase circuit Michael McElroy - Year 12 Engineering Studies Used in the industry Rectification - - ο· Rectification is the process of converting AC to DC Rectification is achieved with the use diodes, which allows for only certain waveforms to pass through, blocking the others (i.e. allowing positive, but blocking negative) Half wave rectification is achieved with one diode in a circuit and blocks current flowing in one direction, so only the negative waveform direction is blocked. Full wave rectification is achieved with four diodes, which allows the entire wave to pass through but only on the positive side. This produces a varying DC current. The varying DC produced by these rectifiers does not produce true DC current and is not ideal for most DC equipment. However, adding a capacitor to the circuit will get a better waveform. The capacitor stores energy that can be used when the wave form reduces in voltage, resulting in a nearly flat waveform with the capacitor smoothing out the troughs. When the potential (voltage) falls at any time during the propagation, the capacitor releases a charge that causes the voltage to rise again. Electric motors used in transport systems Principles Michael McElroy Year 12 Engineering Studies - DC electric motor A DC electric motor works by passing a DC electric current through the rotor (coils) that produces a magnetic field. When this rotor is placed within an external magnetic field, the magnetic field from the rotor interacts with the external magnetic field, causing either a force of repulsion or a force of attraction on the rotor, causing it to rotate. This rotation is known as torque. To maintain this torque and produce constant rotation, a commutator is used to change the current supply into the rotor after every half turn. This changing current will in turn produce a changing magnetic field from the rotor and alternate its polarity. This reversed magnetic field will ultimately change the forces produced due to the field interactions and cause constant rotation (when this process is repeated). - AC electric motor An AC electric motor works on the same principle as a DC electric motor, in that current through the rotor causes rotation when in an external magnetic field, however in this case an AC electric currant is provided as oppose to DC. The supply of an AC electric current into the rotor means that the current will already be alternating, therefore eliminating the need for a commutator. The input of AC will cause for the current through the rotor to alternate at 50 times per second and therefore causing the magnetic field produced by the rotor to alternate also. This changing current and magnetic field means that constant torque and rotation is produced. βΊ Induction motor Induction motors are the most popular type of AC motors. The principle behind the AC induction motor is in the opposite to the generic AC motor (torque is caused from alternating current passing through rotor in a stationary magnetic field) in that is consists of a rotating magnetic field that exerts a torque on a stationary coil. In an induction motor, electromagnets in the stator produce a changing magnetic field (from the AC current, the oscillation of the current will cause the electromagnets to change polarities) that induces an electric current within the rotor (from electromagnetic induction). This induced current within the rotor will in turn produce its own magnetic field. The interaction between the two magnetic fields will cause the rotor to rotate. When the electromagnets in the stator are connected in single phase, the rotorβs initial torque is low. Three phase on the other hand produces a much better torque due to the timing of the three electromagnets. In three phase, the stator simulates rotation because the rapid switching of current through the magnets one after the other simulates motion of one pair of magnets spinning. This ultimately increases the rate of change of magnetic flux in the motor and increases the induced current within the rotor.
