Synchronous Motors

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APPLICATIONSCentrifugal fans, blowers, compressors,and exhaustersCentrifugal fans, blowers and compressorsdiffer from reciprocating units in that the pressuredeveloped is a function of the density of the air orgas handled, the speed of the impeller, and ofrestriction to flow. The gas is accelerated inpassing through the rapidly rotating impeller, thisvelocity being converted into pressure in thevolute casing surrounding the impeller.Units operating at pressures above 30 psi arecalled compressors. For pressures above 10 psi,multi-stage compression is commonly used. Theseunits are especially suited for high volume at lowpressure. NEMA recommended torques areshown in Figure 47.Many of these applications have substantiallymore than normal load WK 2 . The actual valueshould be taken into account in applying anddesigning motors for such loads. Care should alsobe taken in applying motors to fans normallyhandling hot gases. The load, when handling coldair, as in starting, will be substantially higher.NEMA Recommended Torques Torques in % of Full-load TorqueStarting Pull-in Pull-outBlower, Centrifugal –Inlet or discharge valve closed 30 40-60 150Inlet and discharge valve open 30 100 150Compressor, CentrifugalInlet or discharge valve closed 40 40-60 150Inlet and discharge valve open 40 100 150Fans, Centrifugal (Except Sintering)Inlet and discharge valve open 30 40-60 150Inlet and discharge valve open 30 100 150Fan, Sintering –Inlet gates open or closed 40 100 150Fan Propeller –Discharge open 30 100 150Figure 47Type Specific Speed % Torque at Shut-offRadial-flow 500 451000 502000 603000 70Mixed-flow 5000 120Axial-flow 10000 220Figure 48Synchronous motors for centrifugal pumpsThe term “centrifugal” may be applied toradial-flow, mixed-flow or axial-flow pumps. Inall cases, energy is added to the flowing liquid bymeans of pressure differences created by vanes ofa rotating impeller or propeller. The radial-flowpump discharges the liquid at right angles to theshaft, the mixed-flow discharges it at an angle, andthe axial-flow propels the liquid in an axialdirection.Hydraulically, the difference is fundamentallyone of the “specific speed” used in the design.Mathematically, it is shown by this formula:NQNs = H 3 ⁄4Ns = Specific speedN = rpmQ = gpmH = Total head in feet where Q and Hcorrespond to conditions at whichmaximum efficiency is obtainedThese differences are important in theapplication of synchronous motors, as can be seenin Figure 48.The important factor is the torque required atfull speed and closed discharge. In the radial-flowcentrifugal pump this is usually about 55% of fullload torque. In the axial-flow it may reach 220%.Obviously, no attempt should be made tosynchronize a motor driving a mixed-flow oraxial-flow pump under shut-off conditions.However, the inertia of the water column alonesimulates a partially closed discharge in case ofattempted rapid acceleration, as in the case ofpulling into step. See Figure 49.The pull-in torques given above areinadequate where mixed-flow or axial-flowpumps are concerned. In some cases it is possibleto start these pumps with partially empty casings,thus reducing pull-in requirements.On vertical pump motors, additional thrustcapacity is usually required where rigid couplings orhollow shafts are used, and the motor thrust bearingmust support all rotating parts plus hydraulic load.Where thrust loads are unusually high, the breakawayor starting torque required may beconsiderably in excess of values in Figure 49. In suchcases hydraulic lift bearings should be considered.28
Synchronous motors for crushers,grinders, and millsCrushing, grinding or pulverizing is anecessary step in separation of metal from ore,preparation of crushed rock for the constructionindustry, preparation of agricultural limestone,and manufacture of portland cement.Primary crushers, usually jaw, gyratory orhammermill type, are fed the rock or ore directlyafter blasting. The material next goes to thesecondary crusher where reduction to about 1/2"may be accomplished. Secondary mills may begyratory, cone, hammermill, or roll type.Rod mills may also be used as secondarygrinders. A rod mill consists of a steel shell linedwith wear-resistant material and rotated about ahorizontal cylinder axis. The mill uses steel rods 2to 5 inches in diameter running the length of themill. As the mill rotates, the tumbling action of therods causes grinding.Final grinding, possibly down to a materialwhich will pass through a 200 mesh screen orfiner, is usually done in a ball mill. This is similarto a rod mill except that steel balls tumble over thematerial to be crushed. Autogenous mills use thecrushed rock to grind itself to powder. See Figure50 for NEMA recommended torques for thevarious types of mills.Synchronous motors for pulp and paper millsOne of the largest power consumingindustries is that of processing wood and woodproducts in the manufacture of paper and pulp.Although alternative fibers may be used inrelatively small quantities in the manufacture ofpulp and paper, by far the largest source is wood.Paper may be made either by the groundwood orchemical process. The application table andrecommended NEMA torques shown in Figure 51cover the usual synchronous motor applications inpaper mills, most of which are common to bothprocesses.Paper mills are large users of synchronousmotors for pumps, refiners, chippers and otherequipment.NEMA Recommended Torques Torques in % of Full-load Torquefor Pumps Starting Pull-in Pull-outPumps, axial flow, adjustable blade– starting with:a. Casing dry 5-40 15 150b. Casing filled, blades feathered 5-40 40 150Pumps, axial flow, fixed blade– starting with:a. Casing dry 5-40 15 150b. Casing filled, discharge closed 5-40 175-250 150c. Casing filled, discharge open 5-40 100 150Pumps, centrifugal, Francis impeller– starting with:a. Casing dry 5-40 15 150b. Casing filled, discharge closed 5-40 60-80 150c. Casing filled, discharge open 5-40 100 150Pumps, centrifugal, radial impeller– starting with:a. Casing dry 5-40 15 150b. Casing filled, discharge closed 5-40 40-60 150c. Casing filled, discharge open 5-40 100 150Pumps, mixed flow – starting with:a. Casing dry 5-40 15 150b. Casing filled, discharge closed 5-40 82-125 150c. Casing filled, discharge open 5-40 100 150Pumps, reciprocating – starting with:a. Cylinders dry 40 30 150b. By-pass open 40 40 150c. No by-pass (three cylinder) 150 100 150Figure 49Torques in % of Full-load TorqueApplication Starting Pull-in Pull-outGyratory (unloaded) 100 100 250Cone (unloaded) 100 100 250Hammer Mill (unloaded) 100 80 250Roll Crusher (unloaded) 150 100 250Rod Mill – Ore 160 120 175Ball Mill – Ore 150 110 175Ball Mill – Rock or Coal 150 110 175Figure 50NEMA RecommendedTorques in % ofTorquesFull-load TorqueStarting Pull-in Pull-outRefiners (unloaded) 50 50-100 150Conical (Jordan) disc 50 50 150Chippers – Empty (1) 60 50 250Grinders (unloaded)MagazinePocket (unloaded)50404030150150Vacuum Pumps (Hytor) 60 100 150(1) These are high-inertia loads and motor torque requirements can not be determined fromload values alone. WK 2 values must be known to permit proper motor application.Figure 5129
Page 2 and 3: SYNCHRONOUS2The ABC’s ofSynchrono
Page 4 and 5: GEAREDSynchronous motors are“gear
Page 6 and 7: POWER FACTOR6How synchronous motors
Page 8 and 9: POWER FACTOR8Figure 10For any given
Page 10 and 11: TORQUETorque makes the wheelsgo ’
Page 12 and 13: TORQUEField WindingsThe amortisseur
Page 14 and 15: TORQUESince it was previously deter
Page 16 and 17: TORQUEFREQUENCY OF FIELD-DISCHARGE
Page 18 and 19: TORQUE200Pull-out TorqueNEMA define
Page 20 and 21: STARTING & EXCITATION20Starting & e
Page 22 and 23: STARTING & EXCITATION22Brushless ex
Page 24 and 25: APPLICATIONSWhere synchronous motor
Page 26 and 27: APPLICATIONSFigure 42Oscillogram of
Page 30 and 31: SPEEDSystem resistance or fan head1
Page 32 and 33: SPEEDAs with any thyristor circuit,
Page 34 and 35: SPEED34(a)6-Pulse Converter With De
Page 36: AUTHORSAbout the authorsGerry Oscar

Synchronous Motors

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