Assignment 1


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Assignment 1

  2. 2. QUESTION NO 1:Synchronous Generator Construction RotorTypes?The rotor of a synchronous machine is a large electromagnet. The magnetic poles can beeither salient (sticking out of rotor surface) or non-salient construction.Salient Pole type rotor: The term salient means”Protruding”or”Sticking out”and a salient pole is a magnetic polethat sticks out from the surface of the rotor. Salient pole rotor is normally used for four ormore pole rotors because rotor is subjected to changing magnetic field and Poles arelaminated to reduce eddy current lossesThis type of rotor is used for slow speed machines which have large diameters and smallaxial lengths.The pole are made of thick steel laminations riveted together and attached to a rotor by adovetail joint. The overhang of the pole gives mechanical support to the field coil.In most of the alternators, where the oscillation or the hunting effect is very high, the damperwinding in the pole faces is provided. The copper bars short circuited at both ends are placedin the specially holes. The relative velocity of the damping winding with respect to main fieldwill be zero when the speed is steady but as soon as it departs from the synchronous speedthere will be relative motion between the damper winding and the main field. This willinduce current in them. This induced current will exert a torque in such a way as to bring thealternator to operate at synchronous speed.The Pole face is so shaped that the radial gap length increases from the pole centre to poletips. This makes the flux distribution over the armature uniform uniform to generatesinusoidal waveform of e.m.f.
  3. 3. Salient pole rotors are used in application with speeds from 100 to 1500rpm. They arealternative known as "projected pole" type of rotors. The poles mounted on the rotor aremade of laminations made of steel. The poles are connected to the rotor shaft by means ofdovetail joints. Each pole has a pole shoe around which the winding is wound. The salientpole rotor is generally used in applications where the prime mover is a hydel turbine or acombustion engine which have low or medium speeds. Salient pole rotors usually containdamper windings to prevent rotor oscillations during operation.
  4. 4. Advantages and Disadvantages:  They have large diameter and short axial length.  The pole shoe covers about 2/3rd of pole pitch.  Poles are laminated to reduce eddy current losses.  These employed with hydraulic turbines or diesel engines. The speed is 100 to 375 r.p.m.  large in diameter small in axial length  Where flux density is high so lows speed alternators are used for generating electrical energy.  Arrangement of large pole such to max 64  These type pole rotors are used in hydro-generatorNon-salient pole rotor:Non-salient pole rotor is a magnetic pole constructed flush with the surface of rotor. Non-salient pole rotor consists of two or four pole rotor.
  5. 5. It is used for stream turbine-driven alternator i.e. turbo-alternators, which run at very highspeeds. The rotor consists of a smooth solid forged steel cylinder, having a number of slotsmilled out at intervals along the outer periphery (and parallel to the shaft) foraccommodating field’s coils. Such rotors are designed mostly for 2-pole (or 4-pole) turbogenerators running at 3600 r.p.m.The central polar area is surrounded by the field windings placed in slots. The field coils areso arranged around these polar areas that flux density is maximum on the polar central lineand gradually falls away on either side. It should be noted that in this case, poles are non-salient i.e. they do not project out from the surface of the rotor. To avoid excessiveperipheral velocity, such rotors have very small diameters (about 1 meter or so). Hence,turbo-generators are characterized by small diameters and very long axial (or rotor) length.The cylindrical construction of the rotor gives better balance and quieter-operation and alsoless winding losses.Advantages and Disadvantages:  The winding loss is reduced.  The noise produced is also less.  These rotors have higher axial length.  These rotors do not need damper windings.  small in diameter large in axial length  Flux density appx. Low high speed alternator is used generating electrical energy.  No pole field are less appx.=4  so these type pole rotors are used in turbo-generator
  6. 6. QUESTION NO 2:EXCITION METHOD FOR SYNCHRONOUSGENERATOR:Two common approaches are used to supply a DC current to the field circuits on therotating rotor:1. Supply the DC power from an external DC source to the rotor by means of slip rings andbrushes. 2. Supply the DC power from a special DC power source mounted directly on the shaft ofthe machine.Slip rings are metal rings completely encircling the shaft of a machine but insulated from it.Graphite-like carbon brushes connected to DC terminals ride on each slip ring supplying DCvoltage to field windings.On large generators and motors, brushless exciters are used. A brushless exciter is a smallAC generator whose field circuits are mounted on the stator and armature circuits aremounted on the rotor shaft. The exciter generator’s 3-phase output is rectified to DC by a 3-phase rectifier (mounted on the shaft) and fed into the main DC field circuit. It is possible toadjust the field current on the main machine by controlling the small DC field current of theexciter generator (located on the stator).A rotor of large synchronous machine with a brushless exciter mounted on the same Shaft.Many synchronous generators having brushless exciters also include slip rings and brushesto provide emergency source of the field DC current.
  7. 7. A brushless exciter: a low 3-phase current is rectified and used to supply the field circuit ofthe exciter (located on the stator). The output of the exciter’s armature circuit (on the rotor)is rectified and used as the field current of the main machine.
  8. 8. To make the excitation of a generator completely independent of any external powersource, a small pilot exciter is often added to the circuit. The pilot exciter is an AC generatorwith a permanent magnet mounted on the rotor shaft and a 3-phase winding on the statorproducing the power for the field circuit of the exciter. THE END