Synchronous Motors

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TORQUE200Pull-out TorqueNEMA defines pull-out torque of asynchronous motor as the maximum sustainedtorque which the motor will develop at synchronousspeed for one minute at rated frequency and normalexcitation. Normal pull-out torque is usually 150%of full load torque for unity power factor motorsand 200% to 225% for 0.8 leading power factormotors. It can be increased by increasing the fieldstrength, which generally means increasing the airgapand flux density. This usually means increasingthe physical size of the motor.As shown in Figure 29, any sudden increase inload is accompanied by an increased angle positionof the rotor and stator and a corresponding increasein line current. Due to transformer action, there is atransient increase in excitation and pull-out torque.The This ABC’s increased of value disappears in a few cycles so itSynchronous can be effective only on instantaneous peaks.MotorsHowever, the change in excitation can be used to“trigger” an increase in excitation voltage, thustending to maintain the value of excitationestablished by transformer action.The reactance of the motor windings willdetermine the amount of increase in excitationcurrent and pull-out torque for a typical motor.The effect is shown in Figure 30.TRANSIENT TORQUEPulsating TorqueSince the rotor of a salient pole synchronousmotor has poles and interpole spaces, its magneticcircuit varies from point to point around the rotor.The instantaneous starting or accelerating torqueis greater when the flux set up by the stator passesa rotor position where the poles provide a goodflux path. Less torque is developed when passingbetween poles. It is common to show a torquecurve as a smooth curve from zero to 95% speed;this is really the average torque. At a given speed,the torque varies from maximum to minimum attwice slip frequency. At standstill, the torquepulses at 120 Hz. As shown in Figure 31, thetorque at 95% speed has a pulsating component at2 x 60 x (1.00-0.95) = 6 Hz; one cycle every 0.167second. At 80% speed this is 2 x 60 x (1.00-0.80) =24 Hz, or 0.042 second per cycle. Figure 32demonstrates torque pulsation at 80% speed.PULL-OUT TORQUE18175150125100STEADY STATE PULL-OUT TORQUE O.8 P.F. MOTORTRANSIENT TORQUESTEADY STATE PULL-OUT TORQUE I.O. P.F. MOTOR00 2 4 6 8 10 12 16TIME IN CYCLESFigure 30 Transient pull-out torque of a synchronousmotor. This torque is influenced by reactance of the motorwindings.Synchronous motor 1750 HP at 240 RPM with Paper MillDripproof enclosure and double shaft extensions drivingvacuum pumps in a paper mill.The pulsating component of torque needs to becarefully evaluated as to its effect on the combinedmotor driven equipment. A typical pulsatingtorque is approximately 20% of rated torque.Typically, we have the mass of the motor, the gearand the load (i.e. compressor) all connected by theshaft. Each individual part has a natural frequencyas well as the combinations. The responsibility toavoid a torsional natural frequency that is nearrunning speed is that of the system designer.Further, special consideration has to be given to thepulsating torque in certain applications. Forexample, large 6 pole motors driving compressorsfrequently have a resonance between 60 and 80%speed. Fortunately, it is only present duringstarting. If the start-up is fast, the resonance haslittle time to build up. The motor manufacturerprovides curves of the average and the pulsatingtorque so that an overall system analysis can bemade. Figure 33 is a representation of a torque vs.speed curve. Shaft sizes may be adjusted or thecoupling changed, but the pulsating torque willstill be there.
~20%Figure 31Pulsations in total torquedue to reluctancevariations in magneticcircuit at 95% speed; 6 Hz.or .167 sec./cycle..167 sec.3 cycles0.0 0.1 0.2 0.3 0.4 0.5TIME IN SECONDS~20%Figure 32Pulsations in totaltorque due to reluctancevariations in magneticcircuit at 80% speed;24 Hz. or .042 sec./cycle..042 sec.12 cycles0.0 0.1 0.0 0.3 0.4 0.5TIME IN SECONDS100Figure 33Typical speed-torque curveshowing average torque.MOTOR %TORQUE950025 50 75 100% SYNCHRONOUS SPEED19
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 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 28 and 29: APPLICATIONSCentrifugal fans, blowe
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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