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1
40th
TURBOMACHINERY SYMPOSIUM
CASE STUDY
‘BALANCE INSTABILITY AND VIBRATION ON A 6 MW
INDUCTION MOTOR ROTOR’Authors
Gunnar Porsby (ABB, Sweden) Sameer S. Patwardhan (Bechtel OG&C Corp., Houston, Texas)
Siddharth (Sid) Shinde (Chevron, Houston, Texas)Barry Wood (Chevron, Richmond, California)
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Induction Motor at Test Stand
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Stabilizer Overhead Compressor• Centrifugal compressor (with ‘side stream’)• Two stage compression used for mixing
vapor/gas from ‘stabilizer’ column into feed gas stream
• Speed increasing gearbox• 6 MW induction motor driver with variable
frequency drive• Large variation in inlet flow• Complicated control system• No spare compressor
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Induction Motor Driver• 6 MW induction motor
• 6.6 kV 3 Ph 60 Hz
• Coupled with variable frequency drive (VFD)
• Motor designed per API 541 and IEC (hybrid standard)
• Routine and complete run tests including heat run test, unbalance response test, mechanical run test, over-speed test etc.
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Motor Design: Squirrel Cage Induction Rotor
Rotor core(Laminated steel)
Spider shaft(Homogenous forged steel)
Squirrel cage(Copper)
Unbalance weights
Complete rotor
The sheetlaminations are
shrunk on tothe rotor shaft
and compressedin axial direction
Rotor assembly AND
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Motor Design: Sleeve Bearings and Stiffness Chain
Stiff shaft design with unbalance force pathway
Sleeve bearings with oil film for damping and stiffness
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Separation Margin and Balancing
Normally 4 pole motors are run below first critical speed with sufficient separation margin(sub-critical operation)
Motor speed range:1350-1890 rpm
2-plane balancing at reducedspeed is often sufficientfor sub-critical operation
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API 541 Vibration Requirement
Shaft vibration limits(Relative bearing housing using non-contact probes)
Bearing housing vibration limits(Using bearing-mounted velocimeters)
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Factory Acceptance Test (FAT)
• High vibration levels during over-speedtest
• Vibrations above normal for the machine type at running speed (1800 rpm)
• High vibrations during over-speed test led to bearing failure
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Factory Acceptance Test Results
API Criteria is 1.5 mills = 38µm
Bearing Vibrations during Overspeed Test
Failed NDE Bearing during 4 hour Mechanical Run Test
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Investigation After FAT• Assumption:
– The vibrations are caused by unbalance in the rotor due to initial settlement during the first heat run and/or over speed test
• Decisions– Residual Unbalance Check � Re-Balance at
Low Speed (1000 rpm) � New Test
• Result– Vibrations still not meeting the requirements
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FAT Results After Re-Balancing
Bearing Vibration Results After Re-Balancing
Balancing p lane
Re sidual unbala nce
[kg mm ]
Residual unbalance ma ss [g ]
Re sidual unbala nc e
[kg mm ]
Residual unbalance ma ss [g ]
AP I 541 requirem ent re sidual unbalance
m ass [g ]DE 33 117 2 6 .2 7.8
ND E 47 169 1 2 .1 7.8
After R e-balancingFAT
Residual Un-Balance After Re-Balancing
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More Investigations…• Assumptions:
– Other effects than only settling.
• Decisions:
– Check balancing state again, this time at different speeds
• Findings
– The balancing state at 1000 rpm had changed again
– Balancing state was also changing with speed
• Conclusions
– Unbalance of the rotor was not caused by only settlings in the rotor
Speed [rpm]
Residual unbalance[kg mm]
Residual unbalance mass [g]
Residual unbalance[kg mm]
Residual unbalance mass [g]
249 7.4 26.3 18.8 67.1500 3.8 13.7 20.2 72751 1.9 6.7 23.2 82.7
NDE DE
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A Theory Was Formulated…• Measurements showed that individual sheets in the
lamination were skewed/buckled• Centrifugal force acts to ‘straighten’ the sheet metal plates
which could lead to changed balancing state
Rotor Core
Centrifugal Force
Sheet metal plateat ‘low’ speed
CentrifugalForce
Axis of rotation
Sheet metal plateat ‘high’ speed
Rotor core measurements
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Corrective Actions• Attempt to recompress the rotor core lamination to make it
perpendicular to the rotor centre line
• Result:
– Improved vibration but still not meeting requirement, balancing state still changes with speed
• Conclusion:
– Recompression not working due to high friction betweenrotor core and spider shaft
Rotor Core Compression
‘Press-rings’ ‘Pull-rods’Speed [rpm]
Residual unbalance[kg mm]
Residual unbalance mass [g]
Residual unbalance[kg mm]
Residual unbalance mass [g]
249 20.2 72.1 17.1 61.2500 19.7 70.5 20.1 71.7750 17.4 62.1 22.3 79.8
1000 14.4 51.3 25.1 89.7
NDE DE
Balancing plane
Residual unbalance[kg mm]
Residual unbalance mass [g]
API 541requirement residual unbalance
mass [g]DE 1 4.8 7.8
NDE 2 7.6 7.8
Balancing @ 1000 rpm
After recompression:
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Test After Operating SpeedBalancing
• Solution for the problem:
– Balancing of rotor at full speed (settling effects within balancing)
• Results
– After full speed balancing vibration levels are within required limits
• Forced unbalance tests for final verification
NDE Y-direction
API Criteria is 1.5 mills = 38µm
NDE X-direction
API Criteria is 1.5 mills = 38µm
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FAT: Residual Unbalance TestPer API 541
• The residual unbalance in the rotor was found to be above maximum allowable residual unbalance
• It was now decided that a new rotor should be manufactured
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Root Cause Analysis Findings
High vibrationsbearing failure
Balancing statechanges with speed
Skewed/buckledsheet metal plates
Insufficient compression of rotor core due to malfunction in thecooling process of rotor core during manufacturing process
Problem:
Analysis:
Manufacturing of new rotor with changed manufacturing process for the core shrinking
Resolution:
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Resolution: Manufacture New Rotor• New process for shrinking the core onto the
shaft:
– Cooling from the top down to get axial pressure on the entire length of the rotor core
Fans to cool ‘TopPart’ of rotor core
‘Insulation’ around bottom part of rotor core
Rotor core cooling
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New Rotor: Residual Unbalance& Vibration Tests
• Residual unbalance test was successfully passed
• Vibration levels were lowercompared to old rotor, normal for the machine type and within API 541 prescribed limits
Old rotor
New rotor
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CONCLUSION• It is very difficult to correct a rotor after a
distorted cooling or skewed lamination fit on the rotor core
• Trial and error attempts to diagnose and repair this type of rotor problem can be very time consuming and without guarantee of success.
• In a schedule oriented environment, it is important to have all the necessary resources involved to quickly determine if the problem can be corrected and a quality machine assured. Sometimes it may be necessary to move in parallel in attempting to repair the rotor and preparing to manufacture a new rotor.
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T40LTCDSLecturesT40LECT1LECT2LECT3LECT4LECT5LECT6LECT7LECT8LECT9/
LECT10LECT11LECT12
Turbo40TutorialsTUTT1TUTT2TUTT3TUTT4TUTT5TUTT6TUTT7TUTT8CONSEQUENCES OF POOR INLET FILTRATIONErosionFoulingCorrosion
FILTRATION CHARACTERISTICSFiltration MechanismsFilter Efficiency and ClassificationFilter Pressure LossFilter Loading (Surface or Depth)Face VelocityHigh Velocity SystemsLow Velocity Systems
Water and Salt Effects
COMPONENTS OF A FILTRATION SYSTEMWeather Protection and Trash ScreensAnti-icing ProtectionInertial SeparatorsMoisture CoalescersPrefiltersHigh Efficiency FiltersSelf-Cleaning FiltersStaged Filtration
OPERATING ENVIRONMENTCoastal, Marine, or OffshoreLand Based EnvironmentDesertArcticTropicalRuralLarge CityIndustrial Area
Temporary and Seasonal Contaminant SourcesSite LayoutSite Evaluation
LIFE CYCLE COST ANALYSISLife Cycle Cost BasicsConsiderations for an Inlet Filtration SystemPurchase Price/Initial CostMaintenance CostAvailability/Reliability of Gas TurbineGas Turbine Degradation and Compressor WashingPressure LossFailure/Event Cost
SUMMARY
TUTT9
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