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2-Pole Turbine Generators Rotor Construction WEG Group The rotor is designed to withstand cyclic duty and rapid loading. Radial ventilation provides even cooling. Low vibration levels are achieved by dynamic balancing and machine design. s Field Coils s coils are made of silver-bearing s Rotor strap copper for greater creep resistance. The coil is accurately formed to precisely fit in the rotor slot with minimum distortion. Ventilating slots are punched in the straight portion of the coils to
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  The rotor is designed to withstand cyclic duty and rapid loading.Radial ventilation provides even cooling. Low vibration levels areachieved by dynamic balancing and machine design. WEG Group 2-Pole Turbine Generators Rotor Construction Rotor Forging and Machining The rotor forging is a special alloy steelcontaining nickel, chromium, molybdenum,and vanadium. It has the physicalproperties required to withstand thestresses encountered over a wide rangeof temperature and load conditions. Theforging material is produced from anelectric furnace vacuum degassed ingot. All forgings are heat treated after theforging operation and stress relieved afterrough machining. To control quality, achemical analysis is made of each heatcycle, and radial test samples are takenfrom the rotor body to indicate the physicalproperties. Longitudinal tensile and impacttest specimens are taken from the extramaterial at the end of the forging. Theholes from which the radial samples weretaken are tapped and plugged with steel,and are positioned to be at the centers ofthe poles. The forging manufacturerperforms an impact test to determine thelow temperature ductility of the steel. Inaddition, ultrasonic tests and magneticparticle tests are made to assure that theforging is sound. Field Coils    Rotor coils are made of silver-bearing strap copper for greater creep resistance. The coil is accurately formed to precisely fit in the rotor slot with minimum distortion. Ventilating slots are punched in the straight portion of the coils to allow for radial discharge of cooling air.    The rotor body coil slot is lined with a Nomex insulating cell formed to the exact shape of the slot. The slot cell ground insulation level is checked before the coils are installed.    Each field coil is suspended over a pole and inserted, turn-by-turn, into the slots. Individual turns are insulated with Nomex which is coated with a B-state resin. The coils are pressed to a specified size, then heated to cure the resin and bond the turns. Aluminum alloy slot wedges are inserted to hold the winding firmly in place.     A teflon coated strip is placed on top of the coils to help in providing a controlled slip plane for coil expansion and contraction that occurs during start/stop and loading cycles. Rotor Slots Slots for field windings are step milled overthe entire length of the rotor body. Theyare milled in a pattern to give the 2-poleconfiguration and also to provide liberalarea for field copper. Step millingminimizes stress level at the roots of theteeth, one of the most critical parts of therotor. Grooves for slot wedges areaccurately machined into the slot walls.Slots for rotor ventilation are positioneddirectly under the rotor coil slots and arereduced width extensions of the regularmilled coil slots. Rotor slots for field winding.achining radial slots in rotor forging.  Retaining Rings Retaining rings support the rotor coil endturns against the centrifugal forcesencountered during high-speed operation.They are made from forgings that meetrigid specifications and are resistant tostress corrosion cracking. Compliance withthese specifications is assured by tests forphysical properties made on prolongationsallowed at the end of the forging.Ultrasonic tests are conducted to assuresoundness of the forging. Any forgingfound to be defective is rejected. Circularsteel plates are shrunk into the outer endof the retaining ring to add section and toreduce distortion. The retaining rings areshrunk on a machined fit on the rotorbody. This fit is calculated to provide firmcentering of the ring on the rotor bodyeven under overspeed conditions.Circumferential keys at the rotor body lock the rings in place. In effect, the retainingrings nest the end turns inside a shelf orextension of the rotor body and preventdisplacement and consequentialunbalance. Since the retainers do notdepend on the shaft for support, shaftdeflections cannot transmit forces thatcause such displacements and unbalance.The field coil end turns are cooled byair passing circumferentially and axiallybetween the coils and dischargingthrough radial slots in the rotor windingsand wedges.Electric Machinery800 Central Avenue NEMinneapolis, Minnesota 55413United StatesTel: +1 612 378 8000Fax: +1 612 378 WEG Group © Electric Machinery 2011. Publication Electric Machinery, the Electric Machinery logo and any version thereof are trademarks and service marks of Electric Machinery.The other names mentioned, registered or not, are the property of their respective companies. Rotor Fans Steel axial flow fans are fabricated, testedand balanced. The fans are keyed andshrunk on the rotor body outboard of theretainer rings. Balance And Vibration Precautions are taken throughout thedesign and manufacturing operations toensure precise balance and minimizevibration. For example, account is takenof the fact that the stiffness of the usual2-pole rotor varies as the rotor turns.When the poles are positioned vertically,the rotor is more resistant to bendingand deflection than when the poles arehorizontal. As the rotor turns, the positionof these planes of stiffness changes twiceeach revolution, tending to cause 7,200vibrations per minute at 3,600 rpm (120per second). On larger ratings, to reducethe magnitude of the double frequencyvibrations, transverse slots are milled intoeach pole face at intervals along the rotorbody to equalize the rigidity of the rotor inthese two planes.Similarly, the critical speed of the rotoris controlled to avoid possible excitationof bearing oil whip and consequentialvibration. This is accomplished by graduallystepping down the shaft diameter fromthe rotor body so as to provide optimumshaft stiffness.The rotor is balanced in three places in ahigh speed balancing facility. An overspeedrun is made with the rotor assembled inthe generator. Brushless Excitation  An axial hole is drilled in to the shaft at theexciter end and is used to carry DC powerfrom the exciter, under the bearing journal,to the main field winding. Rotor end turn area.Rotor winding process.Rotor retaining rings.
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