1. Topic 9.33.3.1 Generators

2. Generating emf When a coil is rotated in auniform magnetic field an emf isinduced across the coil. When the coil is perpendicular tothe field the induced emf is thegreatest. When the coil is parallel to thefield the induced emf is zero. If the coil is rotated at constantspeed, then the induced emf willvary sinusoidally with the samefrequency as the rotation. 3. AC Generators When a coil is turned in amagnetic field it cuts lines ofmagnetic flux. This causes a current to beinduced in the coil. This current flows around thecoil to the ends where it isattached to a slip ringcommutator. As the coil turns through theperpendicular, the direction ofthe current is reversed. This causes an alternatingcurrent to be produced. 4. Induced EMF(beyond Syllabus) The flux linkage of a coil of N turns and crosssectional area A in a uniform magnetic field B isgiven by:=NBA If this coil is rotating then the flux linkage whenthe normal to the plane of the coil makes anangle to the field becomes:=NBA cos 5. Induced EMF(beyond Syllabus) The induced emf is therefore given by the rateof change of flux linkage: t =NBA cos t= It can be shown that this is the same as:=NBAsin Where is the angular velocity () in rads-1 t Therefore: t=NBA(sin t) 6. Induced EMF(beyond Syllabus) The maximum emf is therefore when sin(t) =1 Thereforemax= NBA Note that this implies that if the frequency ofrotation is doubled then the frequency of theemf will double as will the peak emf. 7. AC Generators The higher the frequency ofrevolution, the greater the rateof change of flux, and thehigher the induced emf. Using an iron core in the coilamplifies the magnetic fieldand increases the induced emf. Using more coils increases theflux linkage increasing theinduced emf. The higher the frequency ofrevolution the higher thefrequency of the alternatingcurrent produced. 8. AC Generator Construction A simple generator is exactly the same inconstruction as a simple motor. It consists of: A magnetic field created by either apermanent magnet or an electromagnet. An armature onto which the coils of wire arewound. A commutator to pass the current out of therotating rotor. Spring-loaded carbon brushes to makecontact with the commutator. 9. AC Generator Construction For practical reasons, most realworld generators are AC and usestator coils and use the magnet asthe rotor. This simplifies the construction ofthe generator and allows for morepowerful generators to be made. The stator windings can be madefrom thicker wires reducing theirresistance and increasing themaximum current carrying ability. The rotor is often a simpleelectromagnet to allow for controlof the output emf. 10. AC vs DC GeneratorsThe primary difference between a simple AC and simple DC generator is thecommutator.In a DC generator a split ring commutator is used so that the current is alwayscoming out in the same direction.An AC generator uses a slip ring commutator so that each end of the coil is alwaysattached to the same brush. This means that the output current changes directionevery 180. 11. AC vs DC GeneratorsGenerator Advantages DisadvantagesAC Simpler constructionMore powerful 3 phasegenerators can be made.AC voltage is more easilydistributed with transformersThe electricity grid needs fineco-ordination to ensure that allgenerators are in phase witheach other.AC current is signifficantly moredangerous than the equivalentDC current in an electric shock.DC A lot of devices rely on DCcurrents for their operation.At a given voltage a DC currentcan be more powerful than theequivalent AC current.More complex construction withsplit-ring commutator.Sparking occurs in the gap of thecommutator wasting energy.DC is more difficult to distributeefficiently. 12. Power Losses in Transmission Lines Electricity is generated at around 10000V, 50Hz. If the electricity was transmitted, at this voltage thenthe currents in the transmission wires would be verylarge. This will cause heating effects in the wires due toP=I2R. For this reason electricity is stepped up to around400kV using a transformer for transmission therebyreducing the current and the power losses. 13. Power Losses As well as the heating effect in the transmissionwires due to the current and the wires'resistance, there are other power losses. Dielectric losses The insulation material acts as acapacitor and causes energy to be lost as heat. Skin effect the alternating E and B fields in thewires causes self-inductance and slows themovement of electrons at the outside of the wire.This effectively makes the wire thinner. Transformer losses 14. Power Transmission Electricity is generated in a power stationat around 10kV by rotating a coil in amagnetic field. All generators in all power stations on thesame grid are synchronised such that theyall produce electricity that oscillatestogether. Three windings are used on the generatorto produce 3-phase electricity. This allowsfor more efficient generation 15. Power Transmission The main substationsteps up the voltage(and the currentdown) to around400kV or higher fortransmission. Each pylon carrieswires for all threephases. 16. Power Transmission The transmissionsubstation reducesthe voltage to around11kV for localisedtransmission. This increases thecurrent in the wiresbut reduces the costsof power poles andincreases safety. 17. Power Transmission A distribution substation stepsthe voltage down to 240V forlocal transmission to houses. Some businesses and housesrequire 2 or three phasepower whilst others needsingle phase. If power is accessed betweenphases then 415V is availablefor use. 18. Transmission Lines Power lines are usually carriedabove ground for economicreasons and for ease of installationand maintenance. However, the power lines, whichare almost always bare, must beinsulated from the metal pylons toprevent the pylons becoming live orthe wires short circuiting. The wires are therefore suspendedfrom porcelain, glass or ceramicdiscs which are strong and verygood insulators even under highvoltages. 19. Transmission Lines In order to try to preventsurges and damage due tolightning strikes, the pylonsusually have another wire attheir highest point. This wire is connecteddirectly to the Earth atregular intervals and providesa very low resistance path forthe lightning surge to traveldown if it is struck, therebysaving the transmission lines. 20. The Battle of the Currents Edison was the first person to set up an electricity company in the1880s. He had selected DC for his system and produced light bulbs for homesand streets and developed DC motors and appliances. Edison built power stations in the individual suburbs of New York andran cables directly to his customers within a 1 mile (1.6km) radius. Edison's DC system was transmitted at a fixed voltage of 100V as thiswas deemed safe enough for general use and because his Edisonappliances were designed to run at this voltage. His cables were limited by length as if they were any longer, theresistance of the wire itself was enough to reduce the voltage too farbelow 100V to be useful. 21. The Battle of the Currents Westinghouse bought patents from Nicola Tesla and built ACmotors and appliances to run on his AC electricity supply. Westinghouse used transformers to increase the voltage up tovery high levels for transmission over long distances. He was convinced that his system of AC with transformers wasfar more efficient. He was also able to use transformers to increase and decreasethe voltage at will to supply different appliances. Westinghouse won a competition to build a power station atNiagara falls in 1886 and his AC system proved to be so efficientthat it eventually marked the downfall of mains DC.