Electrical Motor Current Signal Analysis using a Modulation Signal Bispectrum for the Fault Diagnosis of a Gearbox Downstream

Transcription

1 Journal o Phyic: Conerence Serie Electrical Motor Current Signal Analyi uing a Modulation Signal Bipectrum or the ault Diagnoi o a Gearbox Downtream To cite thi article: M Haram et al J. Phy.: Con. Ser View the article online or update and enhancement. Related content - A Comparion o Dierent Technique or Induction Motor Rotor ault Diagnoi A Alwodai, Gu and A D Ball - Two Stage Helical Gearbox ault Detection and Diagnoi baed on Continuou Wavelet Tranormation o Time Synchronou Averaged Vibration Signal Elbarghathi, T Wang, D Zhen et al. - ault Detection o Gearbox rom Inverter Signal Uing Advanced Signal Proceing Technique C Pilaru, M Lane, A D Ball et al. Recent citation - A Spare Modulation Signal Bipectrum Analyi Method or Rolling Element Bearing Diagnoi Guangbin Wang et al Thi content wa downloaded rom IP addre on 3/4/8 at :48

2 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 Electrical Motor Current Signal Analyi uing a Modulation Signal Bipectrum or the ault Diagnoi o a Gearbox Downtream M Haram, T Wang, and Gu and A D Ball Univerity o Hudderield, Queengate, Hudderield HD 3DH, UK; Department o Vehicle Engineering Taiyuan Univerity o Technology, Taiyuan, 34, China .gu@hud.ac.uk Abtract. Motor current ignal analyi ha been an eective way or many year o monitoring electrical machine themelve. However, little work ha been carried out in uing thi technique or monitoring their downtream equipment becaue o diicultie in extracting mall ault component in the meaured current ignal. Thi paper invetigate the characteritic o electrical current ignal or monitoring the ault rom a downtream gearbox uing a modulation ignal bipectrum (MSB), including phae eect in extracting mall modulating component in a noiy meaurement. An analytical tudy i irtly perormed to undertand amplitude, requency and phae characteritic o current ignal due to ault. It then explore the perormance o MSB analyi in detecting weak modulating component in current ignal. Experimental tudy baed on a kw two tage gearbox, driven by a three phae induction motor, how that MSB peak at dierent rotational requencie can be baed to quantiy the everity o gear tooth breakage and the degree o hat mialignment. In addition, the type and location o a ault can be recognized baed on the requency at which the change o MSB peak i the highet among dierent requencie. Key word: Gearbox, Motor current ignature analyi, Modulation ignal bipectrum.. Introduction In the lat three decade a number o paper have been publihed on condition monitoring (CM) uing MCSA or detection and diagnoi o mechanical deect. Benbouzid [] reviewed MCSA (pectral analyi o the tator current) a a mechanim or ault detection and howed how MCSA could be ued or the detection and localiation o abnormal electrical and mechanical condition that indicate, or may lead to, a ailure o an induction motor. Kar and Mohanty [] ued MCSA to detect gearbox ault intead o conventional vibration technique. Baed on a 7.5kw tet rig he ound that the MCSA can detect low requencie uing T analyi, but that high requencie were diicult to detect. They alo ound that the ue o MCSA combined with dicrete wavelet tranorm DWT can be a good replacement or conventional vibration mentoring, but that continuou wavelet tranorm CWT i not uitable or detecting ault or load luctuation. In addition, Kia et al [3] developed a ignal model or monitoring gearboxe uing phae current and demontrated the eectivene o the pectrum method in extracting the requency component relating to a ignal tage gearbox driven by a 5.5kw. Publihed under licence by IOP Publihing Ltd

3 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 To expand the capability o the current ignature analyi, Rgeai [4] ued the pectrum and conventional bipectrum to analye phae current ignal and demontrated that thee can be ued to detect dierent degree o tooth breakage o a two tage helical gearbox driven by an kw motor. More recently, Naid and Gu et al [ 5, 6, 7] propoed a new modulation ignal bipectrum (MSB) and howed reliable ault diagnoi baed on the current ignal rom a reciprocating compreor in which the motor wa ubject to a high ocillatory load. To improve the perormance o monitoring gearboxe, thi tudy ocue on applying MSB to current ignal rom a two tage helical gearbox o kw under dierent incipient ault.. Motor Current Repone Due to ault.. Phae current repone under healthy condition When a motor drive uch a the gearbox ytem hown in igure i operating under healthy condition, the ideal electromagnetic relationhip o the driving motor can be examined in phae A or a imple undertanding o the ault eect. By neglecting the higher order harmonic and mechanical error and reerring to upply voltage ignal, the current ignal in phae A or a healthy motor drive can be expreed [8, 9] a Correpondingly, the magnetic lux in the motor tator i i A I I co( t ) A co( t ) () The electrical torque produced by the interaction between the current and lux can be expreed a T 3 I PI in( ) where I and denote the root mean quared (RMS) amplitude o the upply current and linkage lux repectively, I and are the phae o the current and lux reerring to upply voltage, the undamental requency o electrical upply and P i the number o pole pair. () (3) i.. Phae current repone under aulty condition I there i a ault, uch a a broken rotor bar, a damaged gear and a mialigned hat occurring in the gear drive ytem, there will be an additional mechanical torque component induced by the ault. To balance thi torque component, a correponding electric torque will be generated by the motor. Suppoing that the additional torque T i a inuoidal wave with a requency and an aociated current wave with an amplitude I and phae, the ocillatory torque can be expreed T which caue a correponding angular ocillation: 3 I P I in[ t ( ) ] 3P I dt in[ 4 J t ( )] where J i the inertia o the rotor ytem o motor. Thi angular ocillation modulate the phae o linkage lux in equation. () and eventually lead to additional motor current ignal in conjunction with aociated phae to be approximated a [8]: (4) (5)

4 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 i A I co( t ) I I l r co[ ( co[ ( I ) t I ] ) t I ] where i the angular diplacement o the motor equivalent circuit impedance at the upply requency, I l and I r are the RMS value o the lower ideband component and the upper ideband component, repectively, which are the current induced by the back-em voltage produced by the lux variation at requencie o and. Thi impliied expreion o current ignal i employed widely or motor ault condition monitoring. By checking the amplitude o the ideband through pectrum analyi, variou ault uch a rotor bar breakage and eccentricity can be diagnoed with a high degree o accuracy. It i noted in equation. (5) that the amplitude o angular ocillation depend inverely on quared requency and the inertia and hence that ideband amplitude alo correlate to the requency and inertia in the ame way. However, conventional pectrum analyi ue amplitude inormation only and overlook the phae eect which alo contain ault inormation, a hown in equation. (6). The conequence o ignoring phae inormation may degrade diagnoi perormance in the cae o incipient ault when the ideband amplitude i very mall and i maked by random noie. Thi i particularly true or diagnoing ault rom the downtream mechanical ytem. ortunately, bipectrum analyi allow the retention o both the amplitude and the phae inormation. In addition, it ha good noie uppreion and identiication o nonlinear eect. Thereore, bipectrum can be a more powerul tool to proce the current ignal or reolving the weak ault ignal and or better ault diagnoi. 3. Sideband Extraction uing Modulation Signal Bipectrum 3.. Modulation ignal bipectrum or a dicrete time current ignal x(t) it Dicrete ourier Tranorm X ( ) i deined a: (6) X ( ) t x( t) e j t and the econd-order meaure (power pectrum) o x (t), can be computed by the ormula (7) P( ) E[ X ( ) X * ( )] * where X ( ) i the complex conjugate o ( ) E. i the tatitical expectation. The power pectrum i a linear tranorm and i a unction o the requency. Extending thi deinition to the meaure o order 3 give rie to the conventional bipectrum: X and B(, ) E X ( ) X ( ) X ( ) Conventional bipectrum o equation.(8) examine only the preence o quadratic phae coupling (QPC) rom the harmonically related requency component o, and. It neglect the poibility that the occurrence o may be alo o due to the nonlinearity between and. Becaue o thi, it i not adequate to decribe modulation ignal uch a the motor current ignal in equation.(6). Several approache uch a [,, 3] have been tudied on the ue o bipectrum or modulation ignal and howed more compact reult in analying ignal. To improve the (8) (9) 3

5 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 perormance o the conventional bipectrum in decribing the motor current ignal, thi tudy ue the ignal bipectrum (MSB) uggeted in [7]: B MS * * (, ) E[ X ( ) X ( ) X ( ) X ( )] Unlike equation.(9), thi equation conider both ( ) and ( ) concurrently or poible QPC in MS ignal. It illutrate that a bipectral peak will be clear at birequency B MS, ) i ( ( ) and ( ) are both due to QPC between and. Thi i more accurate and eective in repreenting the MS ignal eature. The total phae o MS bipectrum in equation (9) i MS (, ) ( ) ( ) ( ) ( ) when two component and are in coupling, their phae are related by ( ( ) ( ) ( ) ( ) ) ( ) By ubtituting () into () the total phae o MS bipectrum will be zero and it amplitude will be the product o the our magnitude, which i the maximum o the complex product. Thereore, a bipectral peak will appear at (, ). Epecially, equation. (9) now take into account both ( + ) and ( - ) imultaneouly or meauring the nonlinearity o modulation ignal. I ( + ) and ( - ) are both due to nonlinear eect between and, a bipectral peak will appear at birequency (, ). Thi i more accurate and apparent in repreenting the ideband characteritic o modulated ignal. or the current ignal in equation (5), it MS bipectrum phae can be, ) MS ( Equation.(3) how that the bipectrum phae i connecting to only ideband component i.e. the angular poition o ault and motor deign and operating parameter, but exclude the phae o the undamental component. It mean that MS bipectrum i independent o the angular poition o the rotor or the tart point o a ignal acquired. Thi will allow an average o MS bipectrum etimation to be perormed uing a data et collected or ramed at any time. The average in turn will uppre random noie and unrelated component to obtain a reliable etimation o MS bipectrum and o the hidden modulating ignal. 3.. Sideband detection MSB Becaue a motor current ignal with mechanical ault contain a erie o ideband component which appear mainly around the upply component, a bipectrum lice at the upply requency i uicient to characterize the ideband or ault detection. By etting o equation. (9) into a contant requency value uch a =5Hz, MSB at upply requency can be expreed a: B MS E X * * X X X () () () (3), (4) To extract ideband only, the magnitude inluence o the upply component on the reultant bipectrum amplitude can be removed by normalizing the upply component by it magnitude: X X / X, which ha unity magnitude but maintain the ueul phae or averaging. In n thi way equation (3) become a ideband MSB: B n MS E X * * X X X, n n (5) 4

6 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 And in the orm o magnitude-phae, the ideband MSB i B n MS jms (, ), E X X e Thee equation how that the magnitude o normalized MSB i only the product o the ideband component. Under the condition that the amplitude o the ideband i the ame, the amplitude rom equation (4) or (5) can be directly compared with the ideband amplitude o power pectrum in equation. (7). However, the amplitude rom equation. (4) i obtained by including phae inluence. It will uppre any component with phae inconitency due to random noie. On the other hand, the ideband amplitude include noie inluence becaue the power pectrum i unable to ue phae inormation. Correponding MSB bicoherence or modulation ignal only in [7] can be impliied a b n MS, B E X (, X to check the degree o random noie inluence and hence conirm the exitence o MSB peak Perormance evaluation To evaluate the perormance o the ideband MSB in detecting and diagnoing weak modulating component, a ynthetic ignal i created to have a carrier requency o 5Hz, two modulating component o 5±4 Hz and 5±5Hz, an additional component o Hz and a high level o Gauian noie. igure (a) how the power pectrum o the ignal. To highlight the mall modulating component, the igure doe not diplay the ull amplitude at 5Hz and at Hz, but how the detail o the diband change with and without noie induced. It i clear that the pectrum ha a high background level through the whole requency range due to the high noie induced. Moreover, the amplitude at the maller and the larger ideband are higher than thoe o the noie ree ignal. Thee higher amplitude obviouly will degrade detection perormance. In contrat, the MSB lice at 5Hz hown in igure (b) preent a very clean repreentation or the ignal with noie. It ha an almot zero background loor and how clearly the two requency component accounting or the ideband in the pectrum. In addition, thee two component are conirmed by high amplitude o MSB coherence hown in igure (c). Mot importantly, the two amplitude rom MSB are very cloe to thoe o the noie-ree cae. It give a more accurate etimation o the modulating ignal becaue MSB take phae inormation during the average proce. In addition, the error o peak etimation i in a imilar range, le than % or both the larger and maller modulating ignal. On the other hand, the peak etimation o power pectrum produce igniicantly greater error and become wore when the ignal-noie ratio (SNR) i lower or the cae o mall modulating ignal. Thi how that the MSB pectrum can produce more accurate detection and diagnoi. In addition, MSB can alo exclude any component that are not rom modulation eect. The component at Hz ha a mall peak at 4Hz in MSB due to limited accuracy in numerical calculation. However, becaue the amplitude o MSB coherence i very low, it i eay to identiy that thi i not a real modulating component. Thi imulation tudy enhance undertanding that MSB can provide a much cleaner repreentation o the modulating proce which create a reliable bai or reliable detection and accurate etimation o modulating ignal. n MS ) (6) (7) 5

7 Amplitude Amplitude Amplitude 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 x -6 (a) Power Spectrum 8 6 Noie-ree SNR= -8dB requency(hz) x -6 (b) MSB at 5Hz Pxx error or large peak: 4.5% MSB error or large peak:.9% Pxx error or mall peak: 85.5% MSB error or mall peak:.95% non-qpc requency(hz) (c) MCB Coherence at 5Hz requency(hz) igure. Comparion o detection and diagnoi between power pectrum and MSB. 4. Tet Setup To evaluate the perormance o MSB analyi in diagnoing gearbox ault, a number o electric current ignal were meaured rom a two tage gearbox tet rig under dierent gear ault cae and operating condition. 4.. Tet rig igure how the gearbox tet rig ued to tet dierent gear condition uing motor current diagnoi in thi reearch. It conit o a 3-phae induction motor, a two tage helical gearbox, lexible coupling and a DC motor which act a a mechanical load. The induction motor (kw, 465 rpm and our magnetic pole) wa upplied by a 3-phae power upply through a variable peed drive which allow dierent operating condition to be peciied. The chematic o the tet rig etup i 6

8 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 hown in igure. The technical peciication o the helical gearbox are given in Table. In general the rig i uiciently large to repreent many indutrial application or evaluating the perormance o current ignal baed condition monitoring. In addition, the t tage i conigured a a peed increaer to explore detection perormance in a wider requency range. igure. Gearbox tet trig. igure 3. Schematic layout o the gearbox tet rig. 4.. ault imulation The gear box ault examined in thi tudy are tooth breakage and hat mialignment, which occur commonly in gear tranmiion and ha been tudied extenively in CM in urace vibration baed ault detection. Vibration reult how tooth breakage create hort tranient vibration repone due to dicontinuity in meh tine. Obviouly, the local meh tine change will alo caue rotational ocillation. A dicued in Section, the rotational ocillation will lead to electric current change in a 7

9 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 orm o modulation. Thu by detecting the degree o modulation, it i poible to ind the level o tooth breakage. To tudy the detection perormance o MSB, our ault cae were examined. A detailed in table, the ault were created with dierent everity and location. A both the tooth breakage and hat mialignment can induce torque ocillation, but at dierent requencie and location, it i poible or MSB to detect and eparate between thee two type o ault and diering everity. Table. Speciication o two-tage helical gearbox. Decription irt Stage (Input Shat ) Second Stage (Output Shat 3) Reduction ratio.83 (peed increae) (peed decreae) Number o teeth Z /Z =58/47 Z 3 /Z 4 =3/59 Overlap ratio Contact ratio Shat diameter 4mm 4mm Shat requency r=4.4hz r=3.6hz r3=6.46hz Meh requency 46.36Hz 39.76Hz Table. Teted gear et and ault detail. Gear ID ault location ault decription ault everity Gear Pinion on hat 5% tooth breakage on one pinion tooth mall Gear Pinion on hat 5% tooth breakage on one pinion tooth large Gear7 Gear on hat 5% tooth breakage on one gear tooth mall Gear Pinion on hat Healthy but with mialignment on hat 3 3 grade 4.3. Data acquiition Electrical current ignal are meaured by a hall-eect enor with a linear requency repone rom Hz to 4kHz, which allow the content in a wide requency range, epecially around the upply undamental o 5Hz, to be meaured accurately. To examine the inluence o the operating condition on ault diagnoi perormance, electrical current ignal were meaured under load: %, %, 3%, 38% and 44% o the ull operating load o the AC motor. In addition, to create a uicient number o average and obtain reliable reult, data acquiition or each gear condition wa repeated three time. To benchmark the perormance o the current ignal baed ault detection, vibration rom the gearbox caing and intantaneou angular peed were alo meaured. All o the meaurement were ampled imultaneouly at a rate o khz and with a data length o 6 econd per channel at each acquiition. 5. Reult and Dicuion The dataet or the 4 gear et are proceed to obtain their correponding MSB repectively. To reduce the inluence o pectral leakage, the egment ize o ourier Tranorm (T) i et to be 48 point, coving nearly cycle o the upply component. A Blackman-Harri window i applied to the egment to minimie the idelope o T. In addition, the number o average i 9 or each MSB calculation. 8

10 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 5.. Characteritic o MSB rom a multitage gearbox To ind bet detection requency component, MSB ha been explored up to 3Hz. However, it wa ound that the mot ueul inormation lie in the range below 5Hz where gear hat related component how high amplitude and exhibit high correlation with operating condition. Thi conirm the at attenuation modulation with requency in equation.(5). Thereore, the component in the low requency are ocued to develop diagnotic eature. igure 4 how typical pectra o the current ignal rom Gear7 under both baeline and aulty cae. igure 4(a) how that MSB o the baeline ha two primary component at 4.63Hz and 3.396Hz. Thee component are the hat requencie o r and r repectively and hence link to the tranmiion perormance at the t tage o the gearbox. A both o them have high amplitude at the baeline cae it how that they are mainly caued by manuacturing error uch a gear eccentricity on the gear et, although r may have ome contribution rom the rotor eccentricity o the motor. Neverthele, the exitence o thee two component how that the current ignal contain ueul inormation about the condition o the gear et. or more detailed undertanding, the MSB pectrum i preented with a magniication o the low amplitude component a hown in igure 4(b). It how that the MSB ha a erie o econdary component due to gear tranmiion in the low amplitude range. Although they have low amplitude in MSB, their MSB coherence o igure 4(c) are igniicantly high to upport the exitence o the component. irtly, the component at r3=6.767hz and it high order harmonic can be clearly een, howing that the current ignal alo contain the inormation o the tranmiion at the econd tage, even though it i urther away rom the driving motor. Secondly, two more component due to the interaction o hat and hat can be oberved at r-r=5.7648hz and at xr-r=8.867, which how again the high error o the gear et at the t tage. inally, a component at 9.455Hz i ound to be the higher order harmonic o rotor bar pa requency (BP), which are identiied by invetigating the requency variation characteritic with load in high requency range. Comparing thee identiied component between the baeline and the aulty cae ha ound that the amplitude o the component linking to r, r and their interaction: r-r and xr-r ha a clear and humongou increae due to more ocillation rom the ault. On the other hand, the amplitude at r3 and it harmonic how an increae only at r wherea a decreae at r and 4xr, which are not o conitent with the ault eect. Thee change characteritic indicate that the aulty tooth create more ocillation on the irt tage and thi can be ued to determine the preence, the location and everity o the ault. In particular, the mot igniicant change due to the ault are at r and r and they have high SNR. Thereore, they are taken in the ollowing ection a key eature to examine the MSB perormance o diagnoing gear tooth breakage at the irt tage. However, the ditinctive component at 39.5Hz or the aulty cae i not rom gear tranmiion becaue it ha very low coherence amplitude. It ha demontrated again that the MSB analyi ha the capability o excluding intererence component. 5.. MSB peak with load variation igure 5 how MSB peak and their diagnoi perormance under dierent load condition baed on Gear and Gear. It i clear that rom igure 5 (a) and (b) that MSB peak at r and r vary with load in a nonlinear behaviour. With load increae they increae in the mall load range and then decreae in the high load range. Epecially, the maximum value o r i at % load wherea that o r i at 3% load. Thee behaviour are not o conitent with the ideband amplitude decreaing with increae in load due to an eect o load damper in [3]. Moreover, the ditribution o amplitude at r, r and r3 i very dierent in that MSB peak are inverely proportional to the quare o requency value, hown in equation. (5), providing that the inertia dierence and error dicrepancy between the three hat i relatively mall. Becaue o thee inconitencie and very high amplitude at r and r compared with that o r3 component, it ha ound that the gearbox under teting may have poible torional reonance in the requency range rom 4Hz to 3Hz. 9

11 Amplitude Amplitude(A ) xr-xr xr-xr Amplitude(A ) 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 Baed on thee reonance eect, the nonlinear variation o MSB peak with load can be undertood a a combined eect o the load damper and torional reonance. or r component, it peak tart with lower amplitude at 3% load. When the load increae to 3%, the requency value decreae and move cloer to it reonance, which reult in the maximum MSB peak. A the load increae urther, the requency move even lower and move away rom the reonant point, which reult in lower MSB amplitude. In a imilar way the component at r how the highet at 3% load where a econd reonance may exit (a) MSB at 5Hz-Gear7 at Load % Baeline Tooth Breakage r r requency(hz) 5 x -5 (b) MSB at 5Hz 4 3 xr3 xr3 Bar Pa req. 4xr requency(hz).8 (c) MSB Coherence at 5Hz requency(hz) igure 4. requency content o motor current MSB reult Gear tooth breakage diagnoi rom igure 5 (a) and (b), it can alo be een that baeline MSB amplitude at r and r are very dierent between the two gear et. In particular, the amplitude o Gear i everal time higher than that o Gear due to inherent manuacture error. However, compared with the baeline or each gear et, MSB peak at r or correponding aulty cae and become higher under nearly all load condition. However, peak at r how an increae in a ewer cae. In addition, the amplitude

12 Dierence(%) Dierence(%) Dierence(%) Dierence(%) Dierence(%) 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 ditribution with load alo how a ditinctive dierence. Thee how that the aulty cae can be dierentiated rom their baeline in pite o the high manuacturing error. However, becaue the peak value o Gear are much higher than that o Gear, it i diicult to tell which gear et ha a evere ault uing peak value directly. Intead, relative change have to be employed or meauring the everity. igure 5 (c) and (d) how relative change with load at r and r repectively. It can be een that the change at r are much higher, compared with that at r, howing that the ault i located at hat. Moreover the change o Gear at r are higher than that o Gear, howing that the ault at Gea i evere. A. 8 x (a) MSB Peak Value at r Gear Gear Gear Gear A..3.. (b) MSB Peak Value at r Gear Gear Gear Gear (c) MSB Peak Dierence at r (d) MSB Peak Dierence at r igure 5. Gear tooth breakage diagnoi under dierent load. A. 6 x -3 (a) MSB Peak Value at r A. 4 x -3 (b) MSB Peak Value at r 3 A. x -4 (c) MSB Peak Value at r3 Gear-Out Shat-.mm Gear-Out Shat-.mm Gear-Out Shat-.4mm Gear-Out Shat-.6mm (a) MSB Peak Dierence at r (b) MSB Peak Dierence at r 8 (c) MSB Peak Dierence at r igure 6. Gear hat mialignment diagnoi under dierent load.

13 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364// Gear hat mialignment diagnoi To conirm the diagnotic perormance, ucceive mialignment are induced to the end o output hat end by adding dierent thicknee o waher underneath the bae o gearbox. Becaue o the rigid coupling connection, the mialignment will caue load ocillation at X. A hown in igure 6 (c) MSB peak at r3 increae gradually with the degree o mialignment. However, the peak at r and r do not how uch a gradual increae change. Thi may how that the ault i located at hat 3. Thi can be urther conirmed by the relative change preented in graph o the bottom row. Epecially, relative change at r3 alo how the gradual change but with a much higher amplitude. On the other hand, the change at r and r are mall. Thi conorm urther that the ault i at hat 3 and that MSB o current meaurement i able to produce a good detection and diagnoi o dierent mechanical ault. 6. Concluion Mechanical ault occurring in an electrical motor drive reult in a modulation eect to current ignal. However, the modulating component i very mall and diicult to be etimated accurately by conventional power pectrum analyi. The modulation ignal bipectrum (MSB) uggeted in thi paper i able to include the phae eect o modulating component and hence uppre noie inluence to obtain an accurate etimation o the modulating component. Experimental tudy baed on a kw two tage gearbox driven by a three phae induction motor, how that MSB peak at dierent rotational requencie can be ued to quantiy the everity o gear tooth breakage and the degree o hat mialignment. In addition, the type and location o a ault can be determined baed on the requency at which the relative change o MSB peak i the highet among dierent rotational requencie. Reerence [] Benbozid M H A review o induction motor ignature analyi a a medium or ault detection IEEE. tranaction on indutrial electronic vol. 47 no. 5 pp [] Kar C and Mohanty A R 4 Monitoring gear vibration through motor current ignature analyi and wavelet tranorm, Mechanical Engineering Department, Indian Intitute o Technology, Kharagpur, 7 3. India. [3] Kia, S.H.;Henao, H.;Capolino, G.A., 7,Gearbox Monitoring Uing Induction Machine Stator Current Analyi,: Diagnotic or Electric Machine, Power Electronic and Drive, SDEMPED, [4] Rgeai M 7 Helical gearbox ault detection uing motor current ignature analyi. PhD Thei. Univerity o Mancheter. [5] Naid A 9 ault detection and diagnoi o reciprocating compreor uing motor current ignature analyi. PhD Thei. Univerity o Hudderield. [6] Naid A, Gu S, Yimin Hu, Al-Arbi, S and Ball A 9 Bipectrum Analyi o Motor Current Signal or ault Diagnoi o Reciprocating Compreor. Key Engineering Material, pp ISSN [7] Gu S, Y Hu, N Naid A and Ball A Electrical motor current ignal analyi uing a modiied bipectrum or ault diagnoi o downtream mechanical equipment, Mechanical Sytem and Signal Proceing,Vol. 5, no.,issn [8] Obaid R R, Habetler T G and Tallam R M 3. Detecting load unbalance and hat mialignment uing tator current in inverter-driven induction motor, IEEE int. Elec. Mach. and Drive Con., pp [9] Bellini A, ilippetti, rancechini G, Taoni C, and Kliman G B Quantitative evaluation o induction motor broken bar by mean o electrical ignature analyi IEEE. Tranaction on Indutry Application, Vol. 37, No. 5, pp [] Jouny I and Moree R L 994 Bipectra o modulated tationary ignal IEE. Electronic

14 5th International Congre on Condition Monitoring and Diagnotic Engineering IOP Publihing Journal o Phyic: Conerence Serie 364 () 5 doi:.88/ /364//5 Letter Vol.3 No pp [] Wong D, Cliton D A and Taraenko L An introduction to the bipectrum or EEG analyi. Biomedical Signal Proceing Group. Intitute o Biomedical Engineering. Univerity o Oxord. Oxord UK. [] Colli W B, White P R and Hammond J K 997 Higher-Order Spectra: The Bipectrum and Tripectrum. Mechanical Sytem and Signal Proceing Vol. No.3 pp [3] Jouny I and Moree R L 994 Bipectra o modulated tationary ignal IEE. Electronic Letter Vol.3 No pp