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Generators [compatibility mode]

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  • 1.GENERATORS BY Lt NS Wickramasinghe BSc(E & E Eng)

2. INTRODUCTION Electrostatic generators are used for scientific experiments requiring high voltages. Because of the difficulty of insulating machines producing very high voltages, electrostatic generators are made only with low power ratings and are never used for generation of commercially-significant quantities of electric power. Before the connection between magnetism and electricity was discovered, generators used electrostatic principles. An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy. 3. BASIC CONSTRUCTION Two ends of the coil are joined to slip-rings are insulated from each other and from the central shaft. Two collecting brushes press against the slip rings. Their function is to collect the current induced in the coil and to convey it to the external load resistance R.Brushes Rings RThe rotating coil may be called ARMATURE and magnet as FIELD MAGNETS. 4. WORKINGemf0a90b180c360270de 5. WORKING cont aWhen the plane of the coil is at the right angles to lines of flux. Flux linking with the coil is maximum but rate of change of flux linkages is minimum. It is because the coil sides RED and VILOT do not cut the flux. There is no induced e.m.f in the coil.bWhen the coil is rotating further rate of change of flux linkage increase till coil plane is parallel to the lines of flux. The flux linkage with coil is minimum but rate of change of flux is maximum. Hence maximum e.m.f induced.cFrom 90 to 180, the flux linkage with the coil is increases but the rate of change of flux linkage decreases. Gradually the induced e.m.f decreases till the angle 180.d eThis is same as positionb. This is same as position a. 6. WORKING contNS segmentsbrushes 7. WORKING cont emf0a90b180c360270de 8. BASIC PARTS OF A GENERATOR 1. 2. 3. 4. 5. 6. 7.Magnetic frame or York Pole cores and pole shoes Pole coils or field coils Armature core Armature winding or conductors Commutator Brushes and bearing 9. MAGNETIC FRAME OR YORK York York serves double purpose 1. It provides mechanical support for the pole and acting as protecting cover for the whole machine. 2. It carries the magnetic flux produced by the poles.feetYork is made of cast iron ,cast steel or rolled steel. It must have sufficient strength and have high permeability. 10. POLE CORES AND POLE SHOES Laminated pole coreSolid pole coreRivetsScrews The pole core itself may be a solid piece made out of cast iron or steel but pole shoe is laminated and fastened to the pole.Pole shoe YorkPoleComplete pole cores and shoes are build of lamination of annealed steel which are riveted together under hydraulic pressure. The thickness is (0.25-1)mm . 11. POLE COILS OR FIELD COILS FrameLaminated pole core Exciting coilWireLaminated Pole shoePole coils which consist of copper wire or strip, are former wound for correct dimensions. When current is passed through these coils ,they electromagnetise the poles which produce the necessary flux tat is cut by revolving armature conductors. 12. ELECTROMAGNETIC POLES 13. ARMATURE CORE Lamination SlotKey wayA complete circular lamination is made up of four or six or even eight segmental laminations. The purpose of using laminations is to reduce the loss due to eddy current.Air holes Teeth 14. ARMATURE WINDING OR CONDUCTORS Armature winding basically divided in to Two. 1. Lap winding 2. Wave winding Wave windingLap winding ConductorsYrYb YrYb YfYcCommutatorYcYf 15. ARMATURE WINDING OR CONDUCTORS cont Pole pitch -: The distance in between two adjacent poles equal to the number of armature conductors or armature slots per pole. EX : if there are 36 conductors and 4 poles, the pole pitch is 36/4 = 9. Pitch of a winding (Y) -: It is the distance between the beginning of two consecutive turns. Y= Yb -Yf ----------------- Lap winding Y = Yb +Yf -------------------Wave winding Back pitch (Yb) -: The distance, measured in terms of the armature conductors which a coil advances on the back of the armature is called back pitch. Front pitch (Yf) -: The distance between the second conductor of one coil first conductor of the next coil which are connected together at the front. 16. ARMATURE WINDING OR CONDUCTORS cont Resultant pitch (Yr)-: It is the distance between the beginning of the next coil to which it is connected. Commutator pitch (Yc)-: It is the distance between the segments to which the two ends of the coil are connected. 17. COMMUTATOREnd clampInsulated copper segments Commutator lugsThe function of the commutator is to facilitate collection of current from the armature conductors. 18. EQUIVALENT CIRCUIT G RG VGRLVLG = generator VG=generator open-circuit voltage RG=generator internal resistance VL=generator on-load voltage RL=load resistance 19. GENERATORSSynchronousAsynchronous 20. SYNCHRONOUS GENERATORS All 3-phase generators use a rotating magnetic field. In the picture to the left we have installed three electromagnets around a circle. Each of the three magnets is connected to its own phase in the three phase electrical grid. As you can see, each of the three electromagnets alternate between producing a South pole and a North pole towards the centre. The letters are shown in black when the magnetism is strong, and in light grey when the magnetism is weak.