마스터제목스타일편집 A Quick Guide to Vibration Diagnostics of Rotating Machinery
마스터제목스타일편집 Belt Problems
Worn, Loose or Mismatched Belts Belt frequencies are below the RPM of either the motor or the driven machine. When they are worn, loose or mismatched, they normally cause 3 to 4 multiples of belt frequency. Often 2 x belt frequency is the dominant peak. Amplitudes are normally unsteady, sometimes pulsing with either driver or driven RPM. On timing belt drives, wear or pulley misalignment is indicated by high amplitudes at the timing belt frequency. Belt frequency = D RPM / L D: Sheave diameter L: Belt length RPM: Sheave speed 3
Belt / Sheave Misalignment Misalignment of sheaves produces high vibration at 1x RPM predominantly in the axial direction and axial harmonics of the fundamental belt frequency The ratio of amplitudes of driver to driven RPM depends on where the data is taken as well as on relative mass and frame stiffness Often with sheave misalignment, the highest axial vibration will be at the fan RPM 4
Eccentric Sheaves, Sheave Run-out Eccentric/ unbalanced sheaves cause high vibration at 1 x RPM of this sheave. The amplitude is normally highest in line with the belts, and should show up on both driver and driven bearings This can be checked by removing the belts and measuring again It is sometimes possible to balance eccentric sheaves by attaching washers to taper lock bolts However, even if balanced, the eccentricity will still induce vibration and reversible fatigue stresses in the belt 5
Eccentric Sheaves Center of rotation different from geometrical center The eccentric rotor will produce high vibration at the rotation speed. The phase will be the same in both horizontal and vertical direction. If you try to balance an eccentric rotor, you may reduce the vibration readings in one direction, but the readings will increase in the other. 10 3.1 1 0.31 Fan RPM Motor RPM 6
Belt Resonance Belt resonance can cause high amplitudes if the belt natural frequency should happen to approach or coincide with either the motor or the driven machine RPM Belt natural frequency can be altered by either changing the belt tension or the belt length Can be detected by tensioning and the releasing belt while measuring response on sheaves or bearings 7
마스터제목스타일편집 Pump Problems
Hydraulic Forces : Blade Pass & Vane Pass Blade pass frequency (BPF) = number of blades (or vanes) x RPM. This frequency is inherent in pumps, fans and compressors and normally does not present a problem. However, large amplitude BPF and harmonics can be generated in the pump if the gap between the rotating vanes and the stationary diffusers is not kept equal all the way round. Also, BPF(or harmonics) sometimes coincide with with a system natural frequency causing high vibration. High BPF can be generated if the wear ring seizes on the shaft or if welds fastening diffusers fail. Also, high BPF can be caused by abrupt bends in line work (or duct), obstructions which disturb the flow path, or if the pump or fan rotor is positioned eccentrically within the housing. 9
Hydraulic & Aerodynamic Forces: Flow Turbulence Flow turbulence often occurs in blowers due to variations in pressure or velocity of the air passing through the fan or connected line work. This flow disruption causes turbulence which will generate random, low frequency vibration, typically in the range of 20 to 2000 CPM. 10
Cavitation Cavitation is caused by the collapse of small bubbles that occurs during local boiling at certain condition of the fluid (low dynamic pressure) The Collapses are short in time and thus wide in frequency. The resonances are exited throughout the spectrum Specially high frequencies are exited In envelope spectra an increase of the background level with no distinct lines are seen. CPB Spectrum Envelope Spectrum 11
Cavitation The faster a fluid travels by an object the lower the pressure will be, this phenomenon is well known as Bernoullis law, and it is the reason that aero planes can fly and turbo machines are working. The lower the pressure, the lower the boiling temperature of water. In some instances the water of a pump may start boiling locally as a result of the local fluid speed will decrease local dynamic pressure and hence decreased the boiling point below the fluid temperature. When the local pressure increases again the small bubbles formed in the boiling process collapses very rapidly. The rapid collapse causes shock pulses which may be strong enough to break apart fragments of metal on the location it occurs - cavitation wear. The collapsing bubbles also induce shock waves which are transferred through the structure. Since the pulses are very short, they have a very high frequency content, and they will excite resonances throughout the spectrum range. 12
Cavitation Cavitation normally generates random, higher frequency broadband energy which is sometimes superimposed with blade pass frequency harmonics. Normally indicates insufficient suction pressure (starvation). Cavitation can be quite destructive to pump internals if left uncorrected. It can particularly erode impeller vanes. When present, it often sounds as if "gravel" is passing through the pump Broadband high-frequency noise, indicates cavitation in a centrifugal pump due to low inlet pressure. 13
Unsuitable Pump Assembly Excessive vibration at Ash Sluice pump-motor (3600RPM) Vane passing frequency component (5X) : 16.764 mm/s Rotating frequency component (1X) : 2.54 mm/s Cause: Casing distortion during assembly after overhaul Motor vibration Sluice pump vibration 14
마스터제목스타일편집 Fan Problems
Fan Most fans are either axial flow propeller-type fans, or are centrifugal type Fans, especially when they are handling particle-laden air or gas, are prone to uneven buildup of detritus on the blades. This causes imbalance, and should be corrected as soon as it is diagnosed. If any of the blades become deformed, cracked, or broken, the vibration peak of blade pass frequency will increase in level, and if there are many blades, sometimes 1X sidebands will appear around the blade pass frequency. 16
마스터제목스타일편집 Gear Problems
Normal Gear Spectrum Normal spectrum shows 1x and 2x RPM, along with gear mesh frequency (GMF) GMF commonly will have running speed sidebands around it relative to the shaft speed which the gear is attached to. All peaks are of low amplitude and no natural gear frequencies are excited. 18
Gear Tooth Wear A key indicator of gear tooth wear is excitation of the Gear Natural Frequency, along with sidebands around it spaced at the running speed of the bad gear Gear mesh frequency (GMF) may or may not change in amplitude, although high amplitude sidebands surrounding GMF usually occur when wear is noticeable Sidebands may be a better wear indicator than GMFs themselves. 19
Tooth Load Gear mesh frequencies are often very sensitive to load. High GMF amplitudes do not necessarily indicate a problem, particularly if sideband frequencies remain low and no gear natural frequencies are excited Each analysis should be performed with the system at maximum operating load 20
Gear Eccentricity & Backlash Fairly high amplitude sidebands around GMF often suggest gear eccentricity, backlash or non-parallel shafts which allow the rotation of one gear to "modulate" the running speed of the other. The gear with the problem is indicated by the spacing of the sideband frequencies Improper backlash normally excites GMF and gear natural frequencies, both of which will be sidebanded at 1x RPM. GMF amplitudes will often decrease with increasing load if backlash is the problem 21
Gear Misalignment Gear Misalignment almost always excites second order or higher GMF harmonics which are sidebanded at running speed. Often will show only small amplitude 1x GMF, but much higher levels at 2x or 3x GMF Important to set the F max high enough to capture at least 2 GMF harmonics if the transducer has the capability. 22
Cracked or Broken Gear Tooth A cracked or broken tooth will generate a high amplitude 1x RPM of this gear, plus it will excite the gear natural frequency (f n ) sidebanded at its running speed It is best detected in time waveform which will show a pronounced spike every time the problem tooth tries to mesh with teeth on the mating gear Time between impacts ( ) will correspond to 1/speed of gear with the problem Amplitudes of impact spike in time waveform will often be much higher than that of 1x gear RPM in FFT. 23
Hunting Tooth Problems Hunting Tooth Frequency (HTF) is particularly effective for detecting faults on both the gear and the pinion that might have occurred during the manufacturing process or due to mishandling. It can cause quite a high vibration, but since it occurs at low frequencies, predominantly less than 600 CPM, it is often missed. A gear set with this tooth repeat problem normally emits a "growling" sound from the drive. The maximum effect occurs when the faulty pinion and gear teeth both enter mesh at the same time (on some drives, this may occur once every 10 or 20 revolutions, depending on the f HT formula). f HT = GMF N a / (T Gear T Pinion ) T Gear, T Pinion = number of teeth on the gear and pinion, respectively. N a = number of unique assembly phases for a given tooth combination which equals the product of prime factors common to the number of teeth on each gear 24
Hunting Tooth Problems 25
Case History : Tooth Wear Vibration due to gear wear in reduction gear box 26
마스터제목스타일편집 Bearing Problems
What s Bearing? A part which supports a journal and in which the journal revolves (ISO 1925) A mechanical element which inserted rolling elements or lubrication between two bodies with relative motion for reducing the friction Without rolling elements or lubrication With rolling elements or lubrication 28
Bearing Types Bearings ROLLING ELEMENT FLUID FILM ACTIVE CONTROL Ball Roller Spherical Hydrodynamic Hydrostatic Hybrid Magnetic Piezoelectric Electrorheological 29
Alternating Stress(MPa) Rolling Element Bearings Merit Standardization, compatibility Simple structure, easy repair & check Small starting friction torque support radial/axial loads simultaneously Demerit Limited life span due to fatigue failure of rolling elements Poor damping capacity Limited load support capacity For small/ low speed machinery 900 800 B Polynomial Fit of A_B 700 600 500 400 300 200 Fatigue failure of rolling bearing 100 1 10 100 1000 10000 100000 1000000 1E7 1E8 Cycle(N) S-N curve 30
Fluid Film Bearings Merit High load capacity Large/high speed M/C Strong impact Good damping Low noise Long life Demerit Oil whip/ whirl Complex structure (oil supplying system) Week temperature Expensive Effect of damping Journal bearing Thrust bearing 31
Use Limits : Load, Speed 32
Rolling 마스터제목 Element 스타일 Bearings 편집
Bearing Frequencies D1 D2 D 1 D 2 PD 2 n = number of balls BD f r = rotation frequency n BD BPFO = f outer ( Hz ) f r 1 cos 2 PD (Ball Pass Frequency of Outer race) n BD BPFI = f inner ( Hz ) f r 1 cos 2 PD (Ball Pass Frequency of Inner race) PD BD f ( Hz ) f 2 ball r 1 cos BSF = BD PD (Ball Spin Frequency) 1 BD FTF = f cage ( Hz ) f r 1 cos 2 PD (Fundamental Train Frequency) 34
Faults in Rolling Element Bearing Fault Frequency Time signal/ spectrum Outer race BPFO, harmonics Harmonics of BPFO Inner race BPFI, harmonics Initial fault : Harmonics Ball/ roller Unsuitable lubrication 느 슨 함 Shaft & bearing Housing & bearing Excessive internal clearance BSF or FTF, harmonics Natural frequency, BPFI 1X, harmonics (2X, 3X, ) Natural frequency Progress : harmonics ± rotating frequency Modulated natural frequency with FTF Modulated natural frequency with BPFI 3X or higher harmonics dominant 1X, 4X components dominant Modulated natural frequency with RPS 35
Fault Frequency of Rolling Element Bearing (KOWACO) 880rpm (8P) Wondong, Isacheon Pumping Stations Point Part. No MFG. BPFO BPFI FTF BSF No. of Ball Pump DE Pump NDE NU324 6324 SKF 77.90 112.77 77.53 5.99 12 FAG 83.86 121.44 77.44 5.99 13 SKF 45.92 71.41 64.34 5.74 8 FAG 45.67 71.28 63.01 5.71 8 Motor DE Motor NDE Pump DE Pump NDE Motor DE/NDE NU326 6330 NU320 6320 6326 SKF 71.33 104.67 74.63 5.94 13 FAG 83.86 121.44 77.44 5.99 14 SKF 52.18 80.08 67.23 5.80 9 FAG 52.62 79.38 69.39 5.85 9 SKF 78.39 112.28 79.89 6.03 13 FAG 78.50 111.76 80.61 6.04 13 SKF 45.07 72.27 59.87 5.63 8 FAG 52.27 80.08 67.41 5.81 9 SKF 45.95 71.39 64.46 5.74 8 FAG 45.94 71.28 64.42 5.74 8 36
4 Failure Phases Phase 1 : Earliest indications of bearing problems appear in ultrasonic frequencies ranging from approximately 20 ~ 60kHz Appear operation frequency and lower harmonics (2x, 3x) 37
4 Failure Phases Phase 2 : Slight bearing defects begin to "ring" bearing component natural frequencies (fn) which predominantly occur in the 30K ~ 120K CPM range. Sideband frequencies appear above and below natural frequency peak at end of phase 2 38
4 Failure Phases Phase 3 : Bearing defect frequencies and harmonics appear when wear progresses. More defect frequency harmonics appear and a number of sidebands grow, both around these and around bearing natural frequencies. Wear is now usually visible and may extend throughout periphery of bearing, particularly when well formed sidebands accompany any bearing defect frequency harmonics, replace the bearings now. 39
4 Failure Phases Phase 4 : Towards the end, the amplitude of the 1x RPM is even effected. It grows, and normally causes growth of many running speed harmonics. Discrete bearing defect and component natural frequencies actually begin to "disappear" and are replaced by random, broadband high frequency "noise floor". In addition, amplitudes of both high frequency noise floor may in fact decrease 40
Case History : Outer Race Defects Specification : Roller Bearing(NU319), N = 29.6Hz, Z = 14, BD = 26mm, PD = 147.5mm, = 0 º Fault frequency : BPFO = 29.6(14/2)(1-1.024/5.807 1) = 170.67 Hz, FTF = 12.5Hz Frequency analysis 12.5Hz(FTF) : Looseness 30Hz(N) : Unbalance 170Hz(BPFO) : Outer race 140Hz = 170-30: Sidebands ( 운전속도차 ), 축의운동을허용할정도로결함이큼을표시 340Hz : 2nd BPFO 852.5Hz : 5th BPFO BPFO 2 BPFO Outer race defect of roller bearing (NU319) for electric motor 41
Case History : Inner Race Defects Specification : Ball bearing (#6313), N = 19.6Hz, Z = 8, BD = 23.8mm, PD = 102.5mm Fault frequency : BPFI = 19.6(8/2) (1 + 23.8/102.5 1) = 96.9Hz Frequency analysis 19.5Hz : 1X 39Hz, 59Hz : Harmonics, Looseness symptom 96Hz : BPFI, 변조 ( 측대역성분 ) 되지않음. 결함이미소함을표시 193Hz : 2nd BPFI BPFI Inner race fault for ball bearing 2 BPFI 42
Case History : Inner Race Defects Inner race fault of ball bearing 1) BPFI : 37Hz 2) Harmonics : 74 Hz, 116 Hz, 155 Hz 얇은파편 (shallow flaking) 이내륜에발생 Spectrum after 2 weeks 1) 베어링상태악화에따라측대역성분발생 2) Sideband : BPFI ± RPS 43
Case History : Ball Defects 2 배볼자전주파수 (2BSF) : 구름요소 ( 볼, 롤러 ) 에결함시, 구름요소가 1 회전당 2 개의레이스 ( 내, 외륜 ) 에충격을가하며회전할때발생. 접촉면을통과할경우이고, 볼의자전방향이일정하지않으므로측정불가능한경우도있음. 기본열주파수 (FTF) : 케이지를타격하거나긁는경우발생. 독립된주파수로발생하지않고, 다른주파수를변조. 이성분이크면, 여러개의구름요소에결함을표시 Frequency analysis 11.4Hz : FTF 243Hz : Natural freq. 고유진동수영역에서 FTF 로변조된주파수 (210, 221.25, 232.5, 255, 266.25 Hz) 발생, 이에따라넓은범위의노이즈생성 Vibration spectrum with 3 ball faults 44
Selective Envelope Detection (SED) Amplitude-modulated time signal, waveform Bandpass filter 1/3 octave or broader 1. Filtering (with bandpass filter) < 10dB 2. Rectifying (amplitude demodulation) time 3. FFT-analysis Envelope (energy vs. time) 10 khz Envelope spectrum (modulating frequencies) 500 Hz (typical for 50Hz machine) 45
Selective Envelope Detection Effective tool for detection and analysis Rolling element bearing faults Repeated shock wave Modulated random noise Filtered Amplitude demodulated FFT analysis Envelope filter ranges in 1/3 octave bands from 709Hz to 44.7kHz 10Hz - 20kHz of FFT display span Dynamic range ~90dB Tracking & gear exchange triggering Averaging 1-1000 (spectrum or enhanced) 46
Envelope Analysis Vibration time signal time Spectrum frequency 10 khz Filtered time singal time Envelope Spectrum 50 Hz frequency 47
Bearing defects are easily seen in Envelope spectrum In normal spectrum the bearing frequencies are not always visible when there is a bearing fault 50 Hz Envelope Spectrum In the Envelope Spectrum early developements of bearing faults can easilybe identified 50 Hz Envelope spectrum with no fault show no peaks! 50 Hz 48
Random Noise When a rolling element bearing is rotating random noise is emitted caused by the metal contact between the rollers and the bearing. The random noise: has a flat spectrum. contains energy in very high frequency range. 40 khz 49
Beginning Wear The maximum metal stress of a rolling element bearing is In the load zone of outer race Few millimeters below the bearing surface. Most bearing wear starts as a spall or crack. The crack will produce Impacts Energy in high frequencies Exciting of bearing resonances. Load Direction Crack Spall 1X 10 khz 50
Progressing Wear As the wear progresses: Defects tend to smooth out The signal is not so impactive The random noise of the good bearing becomes modulated As the defect becomes deeper, the balls will jump and erode the inner race.. 10 khz 51
Bearing Faults 1. Outer Race Faults Lead Time Month s Ball Pass Frequency Outer Race ( BPFO) and Harmonics BPFO 2. Inner Race Faults Lead Time Days - Weeks Ball Pass Frequency Inner Race (BPFI) with sidebands of rotational speed RPM BPFI 3. Ball Defects Requires Immediate action Ball spin frequency BSF with harmonics. Often in combinations with above with various harmonics. BSF 52
Mounting Rotor and Lubrication Defects Rotor Misalignment Rotor Unbalance Force Revolution around outer race Radial Tension of Bearing Misalignment of outer Race 1 RPM 2 RPM 2 BPFO RPM 2*RPM 2*BPFO Slip of Race in the Mounting Seat Harmonics of RPM RPM Lubrication Defect Increase of Background level 53
마스터제목스타일편집 Fluid Film Bearings
Types of Journal Bearings 55
Elliptical Journal Bearing High load carrying capacity, simple, cheap RING CASING OVERSHOT GROOVE FEED HOLE DOWEL END LEAKAGE GROOVE 자료제공 : 터보링크 56
Tilting Pad Journal Bearing High stability, self-aligning, complex, expensive RING ANNULUS GROOVE LOCKING PIN PAD ADJUST PLATE COVER HALF FEED HOLE SEAL TOOTH ALIGNMENT PAD 자료제공 : 터보링크 57
Taper Land Thrust Bearing 58
Thrust Bearing Pivot shoe type self-equalizing thrust bearing Self-equalizing: Misalignment 가있더라도 leveling pad 에거의모든패드의하중이거의일정하게작용되도록함 자료제공 : 터보링크 59
Principle of Lubrication Hydraulic Pressure Shaft rotation (velocity) Oil suction (shearing force) Wedge effect (eccentricity) Important Design Variables Size, rotating speed Clearance, oil viscosity Load, shape etc. 동압 : 점성유체가채워져있는두평판의상대운동에의해스스로발생되는압력 60
Common Hydrodynamic Journal Bearings 61
Principle of Lubrication Plain (cylindrical) journal bearing 62
Principle of Lubrication Elliptical journal bearing Three-pad tilting pad journal bearing 63
Theoretical Analysis Performance Analysis of Journal Bearing Static characteristics analysis Distribution of oil film pressure Distribution of oil film thickness Temperature distribution Load capacity Attitude angle Friction loss & temperature rise Minimum required oil flow rate Dynamic characteristics analysis Stiffness and damping coefficient Critical mass, threshold speed (stability criterion) 64
Instability Instability phenomena Self-excited vibration due to stiffness and damping coefficient of oil film Oil whirl or oil whip phenomena Oil whip: resonance phenomena of system natural frequency & oil whirl frequency Stability characteristics will be changed bearing geometry, operating Remedial condition action Increase unit load Decrease oil viscosity Use tilting pad bearing Tilting pad bearing is best choice, stable always Circular Pressure dam Elliptical 3-lobe Tilting pad type 65
Effects of Bearing Geometry on Stability (b) 4 GROOVE 66
Oil Whip Instability A spectral map showing oil whirl becoming oil whip Instability as shaft speed reaches twice critical. Oil whip may occur if a machine is operated at or above 2 x rotor critical frequency. When the rotor is brought up to twice critical speed, whirl will be very close to rotor critical and may cause excessive vibration that the oil film may no longer be capable of supporting. Whirl speed will actually "lock onto" rotor critical and this peak will not pass through it even if the machine is brought up to higher and higher speeds. 67
Oil Whirl Instability Oil whirl instability occurs at 0.42 ~ 0.48 RPM and is often quite severe. Considered excessive when amplitude exceeds 50% of bearing clearances. Oil whirl is an oil film excited vibration where deviations in normal operating conditions (attitude angle and eccentricity ratio) cause oil wedge to "push" the shaft around within the bearing. Destabilizing force in the direction of rotation results in a whirl (or precession). Whirl is unstable since it increases centrifugal forces which increase whirl forces. Can cause oil to no longer support the shaft, or can become unstable when whirl frequency coincides with a rotor natural frequency. Changes in oil viscosity, lube pressure and external pre-loads can affect oil whirl. 68
Journal Bearing Faults ωo= 0 ωo= ωs Oil Instability Normally 42%~ 47% of running speed May appear from 0.3~0.7X in some occasions Non synchronous 10 3.1 1 0.31 ω o ~ 0.3-0.5 ω s 0.43X 1X 2X mm/ Wear Clearance Problems Harmonic series of rotational speed 10 3.1 1 0.31 1X 2X 3X 4X 5X 6X 7X 8X 9X 10X... 69
Bearing Failure Failure Causes Design Improper type Excessive load Light load Excessive clearance Small clearance Operation Manufacturing Misunderstand operational manual Improper Flushing 이물질혼입 Frequent start & stop Lubrication breakdown Improper material Poor bonding Under/over cutting Misalignment Unbalance Load capacity diagram for plain journal bearings 70
Wear / Clearance Problems Latter stages of sleeve bearing wear are normally evidenced by the presence of whole series of running speed harmonics (up to 10 or 20). Wiped sleeve bearings often allow high vertical amplitudes compared to horizontal. Sleeve bearings with excessive clearance may allow a minor unbalance and/or misalignment to cause high vibration which would be much lower if bearing clearances were to specification. 71
마스터제목스타일편집 Electric Motors
Induction Motors 73
Rotor and Stator Stator Rotor 74
Speed vs. Torque Stator Rotor 75
Characteristic Frequency 76
Electric Motor : Cracked Rotor Bars Stator Bars Rotors Bars Broken Rotor Bars Cracked Rotor Bar Loose Rotor Bar Shorted Rotor Laminations Poor End Ring Joints Side bands of Slip Freq around 1X, 2X 3X etc. < - 35 db = Serious > - 45 db = OK. 1X 2X Lin freq. spacing RBPF Loose Rotor Bars may also cause Sidebands of Line frequency around Rotor bar passing frequency and 2*RBPF Pole Pass Freq. = Slip Freq.* No. of Poles Slip Freq. = Synch Speed - RPM Rotor Bar Freq. = No. of rotor Bars * RPM 35 db 45 db (1X- n*slip Freq) 1X (1X+n*Slip Freq) Zoom Spectrum 77
Analysis Method for Electric Faults 전기적인원인에의한결함이의심되면, 전기적인상태를평가하기위해전부하 (full load) 또는그부근의상태에서조사가필요. 이는특히전자기력이고정자전류의제곱에비례하여변화하기때문. 종종전기적인문제는단독시험 (solo test) 이나심지어무부하 (no load) 상태로운전하여도증상을나타내는진동신호가발생하지않는경우가있음. 이경우는 100% 부하를받을때명확한증상이나타남 대부분의전기적인문제는 2X 전원주파수 (2F L, 120Hz 또는 100Hz) 에서정상상태보다높은진동진폭이발생하고, 이를이용하여감지 불평형, 정렬불량등의기계적인문제가회전자와고정자사이의공극변화와같은전기적인문제를야기시킴. 따라서기계적인문제를먼저검토하고, 그원인을제거한후전기적인문제를검토하는것이바람직. 결함을분석할때에는전동기의전류분석을함께실시하는것이바람직. 전동기내의자기장 (magnetic field) 은전자기력을일으키는자속 (flux) 을발생. 이힘은기계적인가진력과함께모두베어링에의해지지됨. 양호한주파수분해능과확대스펙트럼 (zoom spectrum) 분석이중요. 이는기계적인문제 ( 운전주파수, 조화성분 ) 와전기적인문제 ( 전원주파수, 조화성분 ) 의분리에필요 78
Fault Type Mechanical Fault Elements Faults Bearing Shaft Coupling Rolling element bearing defect (ball, Inner & outer race, cage) Journal bearing defect (oil whip/whirl, wear etc.) Mass unbalance, bent shaft, looseness, rubbing, resonance Misalignment (parallel, angular) Electrical Fault Elements Air-gap Rotor, end ring Stator Faults Air-gap unbalance (stator eccentricity, rotor eccentricity) Rotor bar breakage / crack Rotor bar looseness / open Rotor lamination short Rotor bar thermal bow due to local overheating End ring breakage / crack Flexible stator Stator lamination short Stator lamination looseness 79
Vibration in Electric Motors Fault Frequency Spectrum Action Air-gap variation 120Hz 120Hz 성분, 운전속도의배수성분과 120Hz 와의 beat - 프레임의변형제거, - 아마추어와고정자의중심조정 Stator short 120Hz harmonics 120Hz, harmonics - 고정자교체 Flexible stator 120Hz 2x 와 120Hz 와의 beat - 고정자강도를키움 Rotor bar breakage 1x 1x 와 sidebands (slip freq. No. of pole) - 파손된바의교체 Rotor eccentricity 1x 120Hz 와 fp 의측대역성분 1x, 2x 와 120Hz 와의 beat - 공극변동이발생할수있음 Magnetic center variation 1x, 2x Impact in axial direction - 베어링추력과커플링등에의한축 - 방향제약조건을제거 80
Air-gap Problems Static eccentricity Stator core ovality Incorrect positioning of the rotor or stator Dynamic eccentricity Bent shaft Mechanical resonance at C. S. Bearing wear Characteristics 2 x line frequency (2F L ) Same symptom of stator fault (2F L, F s 2kF L ) Different classification between these faults Causes Unequal center line of stator and rotor 81
Stator Eccentricity, Shorted Laminations & Loose Iron Stator problems generate high vibration at 2x line frequency (2F L ). Stator eccentricity produces uneven stationary air gap between the rotor and the stator which produces very directional vibration. Differential air gap should not exceed 5% for induction motors and 10% for synchronous motors. Soft foot and warped bases can produce an eccentric stator. Loose iron is due to stator support weakness or looseness. Shorted stator laminations cause uneven, localized heating which can significantly grow with operating time. 82
Eccentric Air-Gap (Variable Air-Gap) Eccentric Rotors produce a rotating variable air gap between rotor and stator which induces pulsating vibration (normally between (2F L ) and closest running speed harmonic). Often requires "zoom" spectrum to separate the (2F L ) and the running speed harmonic. Eccentric rotors generate (2F L ) surrounded by Pole Pass frequency sidebands (F P ) as well as F P sidebands around running speed F P appears itself at low frequency (Pole Pass Frequency = Slip Frequency x # Poles). Common values of F P range from approximately 20 to 120 CPM (0.30 ~ 2.0 Hz) 83
Case History : Air-gap Problem Cause : Motor unbalance, armature eccentricity, flexible stator Symptom : 2 times of line frequency (120Hz) Vibration signal of induction motor (4,000 HP) 84
Rotor Problems Causes : Broken/ cracked Rotor Bar Broken/ cracked end ring, bad coupling between rotor bar and end ring Rotor lamination short, loosed/ opened rotor bar Electric arcing between loosed rotor bar and end ring Symptoms : 원인 회전자봉과단락링의파손 / 크랙 회전자봉과단락링사이의불량결합 회전자적층의단락 운전속도 (1X) 성분의높은진동과 극통과주파수 (F P ) 의측대역성분발생 증상 크랙이발생한회전자봉 헐겁거나개방된회전자봉 회전자봉과 End ring 사이의전기적아크 운전속도 (1X) 의높은진동과극통과주파수 (F P ) 의측대역성분발생. 종종조화성분 (2X, 3X, 4X, 5X) 의주위에 F P 의측대역성분발생 회전자봉통과주파수 (RBPF) 와이의조화성분및주위에 2F L 의측대역성분 2F L 과 2 RBPF 성분의높은레벨 Vibration spectrum 85
Rotor Bar Problems Broken or cracked rotor bars or shorting rings, bad joints between rotor bars and shorting rings, or shorted rotor laminations will produce high 1x running speed vibration with pole pass frequency sidebands Fp. In addition, cracked rotor bars will often generate Fp sidebands around the 3rd, 4th and 5th running speed harmonics. Loose rotor bars are indicated by 2x line frequency (2F L ) sidebands surrounding the rotor bar pass frequency (RBPF) and/or its harmonics (RBPF = Number of rotor bars x RPM). Often will cause high levels at 2x RBPF with only small amplitude at 1x RBPF. 86
Case History 1 : Broken Rotor Bar Broken or loosed rotor bar is occurred when motor is connected with load 운전주파수 Fr 부근에극수와슬립주파수를곱한극통과주파수 (pole passing frequency) Fp 와같은간격의측대역 (sideband) 성분 Fr ± Fp 발생 Fr Fr ± Fp Vibration in broken rotor bar (2,000 HP) 87
Case History 2 : Broken Rotor Bar Symptom frequency for broken or cracked rotor bar and end ring, shorted rotor lamination : 1X ± Fp (2 pole motor : Fp = 2 Fs, 4 pole motor : Fp = 4 Fs) Broadband spectrum (Fig. a) : 1X, harmonics, symptom of mechanical looseness Zoomed spectrum (Fig. b) : 1X ± Fp, 2X ± Fp a) Broadband spectrum b) Zoomed spectrum(1x) Vibration spectrum for rotor bar fault 88
Case History 2 : Broken Rotor Bar Zoomed spectrum (Fig. c - d) : 1X, 2X, 3X, 4X 등의주변에잘형성된 PPF 성분의측대역성분발생 Pole Passing Frequency (PPF) : Fp = No. of poles (P) slip frequency (Fs) Rotating speed : 1176 rpm, Fs = 24 cpm (0.4 Hz), Fp = 6 24 = 144 cpm (2.4 Hz) 4 cracked rotor bars. 고차조화성분의발생은 1개이상의봉에결함이발생함을의미 c : 2X 확대스펙트럼 그림 4-11 회전자봉결함진동스펙트럼 d : 3X 확대스펙트럼 89
Loose or Open Rotor Bar Cause : Looseness or open of rotor bar Symptom Frequency : 회전자봉통과주파수 (RBPF) 의매우높은주파수의진동과이의조화성분들이발생 Rotor Bar Passing Frequency (RBPF) = No. of rotor bar RPM RBPF, 2 RBPF 또는 3 RBPF 에서진폭이대략 1.5 mm/s 를초과할때문제가됨 RBPF 와이의조화성분주위에는정확하게 2 배전원주파수 (2F L ) 의측대역 (sideband) 성분이존재 RBPF ± 2nF L 2 RBPF ± 2nF L 90
Case History : Opened Rotor Bar Symptom frequency for loosed or opened rotor bar Rotor Bar Passing Frequency (RBPF), harmonics (2RBPF etc) Sidebands : RBPF ± 2F L,, 2RBPF ± 2F L RBPF : F r p = No. of rotor bar Fr = 57 1,793 rpm = 102,201 cpm (1,703 Hz) Zoomed spectrum (Fig. a, b) RBPF : 0.203 mm/s (0.008 in/s), very small amplitude 2 RBPF : 8.636 mm/s (42 times of RBPF), very large amplitude 2 RBPF ± 2F L (7,200 cpm) : sidebands Confirmed over two opened rotor bars by overhaul Vibration spectrum for fan motor 91
Stator Problems Detectable Stator problems by Vibration Analysis Stator Eccentricity Cause : 연약지반 (soft foot), 뒤틀린기초 (warped base) 회전자와고정자사이의불균일한정적공극 (stationary air-gap) 의미소한차를발생시키는편심 Shorted Stator Lamination Cause : 고정자적층의절연문제 (insulation problem of stator lamination) 고정자자체를뒤틀리게할수있는불균일하고국부적인열을야기 운전시간과함께크게성장할수있는 thermal induced vibration 을발생 Loose Iron Cause : Weakness and looseness of stator supports Symptom frequency : 2F L 92
Vibration due to Stator Fault Characteristics 진동수는전원주파수의 2 배 (2F L ) 전동기전원을차단하면, 이진동성분은일순간에소멸 진동은고정자프레임이나베어링등에발생 극통과주파수 (Pole Passing Frequency) 의측대역성분이발생하지않음 고정자내에서발생, 운전속도나슬립주파수에의해변조되지않기때문 고정자문제파악의중요한지침 Causes 일반적으로는전동기의기초나공통베드와케이싱사이의볼트가느슨해져전동기의 공진주파수가낮아지고, 2F L 에 10% 이내로접근하여공진을일으키는경우가많음 고정자권선의각상사이 ( 단상, 3 상전원 ) 에전기적불평형이있어도전자진동을발생 철심이나고정자권선이느슨해져고정자진동이증대할때는더큰진동을수반하므로 2 n F L (n = 1, 2, 3, ) 성분도발생 93
Vibration due to Stator Fault Discrimination Guideline for Normal/Abnormal Peak value of 2F L component (120Hz) : New or reassembled induction motor (50 ~ 1,000HP) : < 1.25 mm/s Operating motor (50 ~ 1,000 HP) : < 2.54 mm/s Precision machine tool spindle : < 0.625 mm/s Criterion for Trend Monitoring 예지정비 ( 상태감시기반정비 ) 프로그램을가지는경우, 2F L 성분의피크치가 2.54 mm/s 를초과할때는향후조사를위해경향감시가필요 피크치가크게증가하거나, 측대역성분이 2 F L 성분의좌우에나타나면, 집중감시가필요 피크치의크기가 4.45 mm/s 이하의안정한상태를유지하면, 경향감시만실시 Allowable Air-gap Eccentricity Induction motor 5%, synchronous motor 10% 이내 94
Stator Eccentricity Cause : 연약지반 (soft foot) 과뒤틀린기초 (warped base) 는편심고정자를발생시킴 Symptom frequency : 고정자문제는전원주파수 2배수 (2F L ) 에서높은진동을발생 고정자편심은매우지향성의진동을발생시키는회전자와고정자사이의불균일한정지된공극을발생시킴 Vibration spectrum 95
Case History : Stator Eccentricity Analysis results of Broad band spectrum (Fig. a) : 정기적인상태감시정비기간중에취득한자료 회전속도 (3,560 rpm) 성분속도진폭치 : 0.381 mm/s (0.015 in/s) 2F L (7,200 cpm) 성분피크치 : 5.842 mm/s (0.230 in/s) 한계치 ( 영역 3) 를초과하는과대한진동발생 Analysis results of zooming spectrum (Fig. b) : 회전속도의 2배성분 (7,173 cpm) 의진폭 : 0.111mm/s (0.0044 in/s) 2F L 성분 (7,200cpm) 의진폭 : 5.791mm/s (0.228 in/s), 가장탁월극통과주파수성분이발생하지않음 그림 4-14 급수펌프구동용전동기의고정자편심진동 96
Shorted Stator Laminations Causes : poor core construction of stator 예 ) 큰펀치버어 (heavy punction burr), 불량적층정렬, 불량철심판절연 (core plate insulation), 너무무겁거나혹은너무가벼운철심압력, 마찰혹은철심에대한외부금속물질같은물리적손상등 Symptom frequency : 2X (rotor), 2F L (stator) 고정자와회전자모두진동의두드러진증가는철심단락 (core shorting) 의존재를나타냄 단락된고정자적층은고정자자체를뒤틀리게할수있는불균일하고, 국부적인열을야기시킬수있고, 운전시간과함께중대하게성장할수있는열여기진동 (thermally-induced vibration) 을발생 불행히도진동 pick-up 에의해일단감지되면단락은급속도로진전되므로, 이때는너무늦음 보다빠른지시계로는매우낮은크기의단락된철심전류를감지기 (detector) 로이용 Vibration spectrum 97
Phasing Problems Phasing problems due to loose or broken connectors can cause excessive vibration at 2 x line frequency (2F L ) which will have sidebands around it at 1/3rd line frequency (1/3 F L ). Levels at 2F L can exceed 25 mm/s if left uncorrected. This is particularly a problem if the defective connector is only sporadically making contact and periodically not. 98
Recommended Maximum Measurement Frequency Fault frequency identification In low frequency region : Fmax = 200 Hz (12,000 cpm), 3200 FFT line, 2 Average 2F L 성분과전동기운전속도및이의조화성분의참진폭을분리가능 In high frequency region : Over 4 pole motor : F max = 6 khz (360,000 cpm), 1600 FFT lines, 8 average 2 pole motor : Fmax = 4 khz (240,000 cpm), 1600 FFT lines, 8 average Detection of RBPF and its harmonics 회전자봉의수를모르는경우는높은주파수영역에서 2F L 성분의측대역성분발생에주목 99