Detecting Partially Fallen-out Magnetic Slot Wedges in AC Machines Based on Electrical Quantities only

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1 Deecing Pariay Faen-ou Magneic o Wedges in AC Machines Based on Eecrica Quaniies ony Goran ojčić 1, Rober Magne 1, Gojko Joksimović 2, Mario Vašak 3, Nedjejko Perić 3, Thomas M. Wobank 1 1 Deparmen of Energy ysems and Eecrica Drives, Vienna Universiy of Technoogy, Ausria 2 Facuy of Eecrica Engineering, Universiy of Monenegro Podgorica, Monenegro 3 Facuy of Eecrica Engineering and Compuing, Universiy of Zagreb, Croaia [email protected] Absrac- The winding sysem of high voage machines is usuay composed of pre-formed cois. To faciiae he winding fiing process saor sos are usuay wide opened. These wide opened sos are known o cause disurbances of he magneic fied disribuion. Thus osses are increased and machine s efficiency is reduced. A common way o counerac his drawback is given by pacing magneic so wedges in he sos. During operaion he wedges are exposed o high magneic and mechanica forces. As a consequence wedges can ge oose and finay fa ou ino he air-gap. ae-of-he-ar missing so wedge deecion echniques dea wih he drawback ha he machine mus be disassembed, wha is usuay very ime consuming. In his paper a mehod is invesigaed which provides he possibiiy of deecing missing magneic so wedges based ony on measuremen of eecrica quaniies and wihou machine disassembing. The mehod is based on expoiaion of machine reacion on ransien voage exciaion. The resuing curren response conains informaion on machine s magneic sae. This informaion is composed of severa machine asymmeries incuding he fau (missing wedge) induced asymmery. A specific signa processing chain provides a disinc separaion of a asymmery componens and deivers a high sensiive fau indicaor. Measuremens for severa fau cases are presened and discussed. A sensiiviy anaysis shows he high accuracy of he mehod and he abiiy o deec even pariay missing so wedges. I. INTRODUCTION The saor winding cois in medium and high voage machines are usuay manufacured ouside he machine. To faciiae he assembing process, saor sos are hus wide open. Bu during operaion hese open sos cause disurbances of he magneic air-gap fied and higher harmonic componens are induced. This eads o he increase of pusaions beween saor and roor sae variabes, ike orque pusaions, uneven fux densiy and higher noise eves. A in a, he power facor as we as he machine s efficiency is reduced. Pariay cosing of he open sos by magneic so wedges seems o counerac hese drawbacks [1]. Magneic wedges are used o simuae semi-cosed saor sos. The wedges ac as pahs for he eakage fux and reduce he effecive so opening. The properies of he wedge maeria srongy infuence he saring behavior of he machine [2]. Invesigaions have shown ha he maeria mus be of ow conduciviy and high permeabiiy [3]. As an exampe, a maeria wih his characerisic can be composied of 70% iron powder, 20% gass ma and 10% epoxy resin. Due o he reaive high percenage of iron powder in he maeria he wedges are reaivey brie. During operaion so wedges are exposed o high magneic and mechanica forces [2]. The acing forces are caused by he fundamena-wave as we as he oad profie. For exampe, a high number of moor sars or reciprocaing oad profie pu addiiona sresses on he wedges [4]. Furhermore he ife imier for he wedges is he epoxy resin. Especiay when he emperaure is rising due o overoad or high fux concenraion he aging process of epoxy is acceeraed. Thus aso he bonding beween saor eeh and wedges is affeced and he wedges are free o move [5]. As a resu he wedges can vibrae and sar geing oose, up o finay faing ou ino he air gap. The consequences are debris of gass ma, resin and iron powder in he air-gap, he winding sysem and end windings area. Especiay he iron powder can cause an increased discharge aciviy in high voage machines. Furher drawbacks are higher machine emperaure, higher inrush currens a saring and higher vibraions and noise. Up o now he mos effecive deecion echnique is he visua inspecion of he air gap area of he machine [5]. Anoher evidence for missing so wedges is he exisence of dus and gass maing debris in he machine s air cooing circui. However, a ieraure review has shown ha a pubished mehods dea wih he handicap of machine disassembing [4]. o, one or more faen ou so wedges can ony be idenified if pars of he machine are demouned. This procedure can be exremey ime consuming and couped wih high coss due o machine size, ocaion, and appicaion. A imey deeced missing so wedges can inform abou he reduced machine s performance. Thus machines sress can be reduced, efficiency can be increased, and a spreading ou of he fau can be avoided. This paper invesigaes a new mehod for deecion of missing so wedges. The ony inpus needed by he mehod are eecrica ermina quaniies, hus disassembing of he machine can be avoided. This advanage is based on he idenificaion of machines ransien reacance idenified by curren reacion o shor voage puse exciaion. The ransien curren reacion is mainy dominaed by he ransien eakage inducance. A considerabe par of he eakage fux is he saor so eakage. Assuming a symmerica machine he eakage fux passes hru he so wedges independen of he /12/$ IEEE 3867

2 spaia direcion of he exciaion puse. Missing so wedges change he so eakage pahs disincivey and hus induce an asymmery which can be deeced. A specia voage puse paern and specific signa processing of he curren signa provide a high sensiive fau indicaor which was aready inroduced in [6]. The focus of his paper is paced on a sensiiviy anaysis and means o increase appicabiiy of he mehod in fied. The invesigaions were reaized on an 11kW inducion machine wih a squirre cage. As voage puse generaor serves a voage source inverer which is conroed by a measuremen/conro sysem. The curren measuremens are performed by he inverer-bui-in curren sensors. According o he es machine s design wih semi cosed saor sos he so wedges were speciay adaped. The so wedges are made of he same indusry sandard maeria aso used for wedges in high power fied appicaions. II. AYMMETRY DETECTION BY EXPLOITING THE TRANIENT REACTION In he foowing he exciaion and signa processing seps for deerminaion of a fau indicaor wi be described. Parameer idenificaion of a dynamica sysem is commony done by observing he sep response. In case of an eecrica machine he exciaion is reaized by appying a voage sep o he machine erminas. The reacance wi hus be a curren sope, which can be measured and evauaed. An easy bu effecive way o reaize hese voage seps is given by inverer swiching. If he inverer sae is changed from inacive o any acive sae a voage sep is appied. The sep magniude is equa he dc ink voage and he resuing curren measuremen can be done by he bui-in curren sensors of he inverer. Machine s voage sep reacion can be described by he we known saor equaion (1) in he space phasor represenaion. di d R v r i (1) d d Basicay, he curren derivaive di /dτ is infuenced by differen parameers, namey he voage sep v (wih he magniude of he dc ink voage), he saor resisance r, he roor fux λ R and he eakage inducance. Considering, now ony he firs some en µs of he curren reacion he dominaing voage drop is he curren derivaive isef muipied by he ransien eakage inducance, denoed as,. In addiion, aso ime derivaive of he roor fux (back-emf) as we as he saor resisance voage drop infuence he curren sope. These disurbances have o be eiminaed o achieve a disinc idenificaion of,. The disurbance eiminaion is possibe by expoiing a specia voage paern. The main idea is given by appying wo shor voage puses (some en µs) wih differen inverer oupu saes. As he puses are subsequen, he fundamenawave poin of operaion can be considered as consan wihin his duraion. The back-emf as we as fundamena-wave saor curren i wi no change significany. Each voage puse can hen be described using (1). ubracion of boh equaions eads o a simpified coherence presened in (2). Thereby currens and voages of he firs and second voage puse are denoed by he indexes I and II. v, v v d i d v I I 0 r i r i, d i I d, I d R, I d R, II d d 0 d i d i I d d As saed, he fundamena-wave wi no change significany wihin he puse duraion. The saor curren as we as he back-emf can hus be assumed consan wihin his ime period. Due o he subracion, hese pars are eiminaed. The voage puses and heir spaia direcion are known from he inverer swiching saes. On he oher side he curren sope is obained from measuring he curren signa wihin he puse duraion. Therewih he ransien eakage inducance can be cacuaed by a simpe division. Uni now he ransien eakage inducance was considered as a scaar. Thus he resuing curren sope wi aways be parae o he exciaion voage puse. Now, considering a rea machine, he ransien eakage inducance has o be exended from a scaar o a spaia compex vaue,. Thus he inducance is geing dependen on he voage puse anguar posiion. As a consequence he voage puse and curren sope phasor wi no onger have he same direcion. The so inroduced non-scaar ransien eakage inducance aows considering hese anguar differences. I can be divided ino wo pars. Firs one is represening he symmerica machine and wi be denoed ( ) in he foowing. The second porion is of compex naure and correaed o he machines asymmeries. In furher descripion his par wi be denoed as he moduaed par or ( ). The composiion of he compex ransien eakage inducion is given in (3)., e j2 The ange γ of he asymmery porion gives he spaia posiion of he maximum inducance wihin one poe pair. The asymmery has hus a period of wo wih respec o one eecrica revouion and a fixed posiion wih respec o he saor. In Fig 1 hese coherences are depiced as an exampe for one missing so wedge. The saor is assumed o have 18 sos wihin one poe pair and he missing so wedges is ocaed a γ=60 (denoed missing wedge ). The ransien eakage inducance is represened as red soid ine. The asymmery caused by a missing so wedges eads o a sinusoida spaia moduaion of he inducance. The (2) (3) 3868

3 vaue is assigned o be he inducance mean vaue and as a superposed moduaion wih period of 2γ. On he oher side, he same moduaion is obained when a wedge is missing a anguar posiion γ+π=240. The porion can hus provide informaion on machine s asymmery and is he base for a fau indicaor. In case of no missing so wedges he moduaed par is no presen and he inducance is ony deermined by he par. negaive magniude of he dc ink voage U DC. The wo main puses are denoed as puse I and II. The curren sope measuremen is reaized wihin hese puses. The derivaive is obained from he difference of wo sampe poins wihin each puse, in he figure indicaed as curren sampe poins wih dashed gray ines. The reason for he wo shorer puses is hus o symmerize he resuing curren sope o he iniia curren vaue when he sequence is sared. Measuremens as we as he signa processing are execued on a digia signa processor. Due o he discree naure of signa processing he curren derivaive di /dτ is repaced by Δi /Δτ. Now he curren sope difference can be cacuaed by di Δ = Δi,I /Δτ- Δi,II /Δτ. Fig 1: paia disribuion of he ransien eakage inducance wihin one poe pair. Missing so wedge assumed a γ=60. According o he esimaion of he ransien eakage inducance in (2), aso he curren sope difference (di,i /dτdi,ii /dτ) can be separaed ino wo pars. When (3) is insered ino (2) and he received equaion is invered, (4) is obained. For shorer noaion he curren sope difference (di,i /dτdi,ii /dτ) is denoed as di Δ and voage difference as v Δ. The symmerica porion is deermined by he vaue y and moduaed by y. These vaues are obained from he inversion as shown in (5). di, di y v y v (4), y y e j(2 arg( v )) Thus i is sufficien o monior ony he resuing curren sope. The symmerica porion is poining in he direcion of he voage difference phasor. The moduaed par is depending on he anguar posiion of he maximum inducance and on is magniude. Therewih he moduaed par of he curren sope difference provides informaion on machine asymmery. Bu before his informaion can be used as a fau indicaor some furher signa processing seps are needed as wi be he described in he nex chaper. III. FAULT INDICATOR IGNAL PROCEING As was shown, he curren response o a specia es voage paern can be used o obain informaion on machine asymmeries. Voage puses as shown in Fig 2 are appied o he machines erminas. This exciaion signa consiss of wo shorer and wo onger puses each wih he posiive or (5) Fig 2: Voage signa appied o one main phase direcion (back) and curren response (red). The symmerica porion of di Δ is a disurbance when examining a fau indicaor and mus be eiminaed. In (4) i was shown ha he symmerica porion is parae o he exciaion direcion of he voage difference phasor v Δ. When appying he voage signa given in Fig 2 subsequen o he hree main phase direcions aso hree curren sope difference phasors di Δ are obained. The symmerica porion of each phasor hus aso poins in one main phase direcion. Combining he hree phasors by adding hem ogeher eads o ony one curren difference phasor. Therewih he shares of he symmerica porions eads o a zero sequence vaue ha is eiminaed. Now ony one phasor is remaining ha provides he informaion on machine s asymmery. This phasor is denoed asymmery phasor. In a rea machine asymmeries may no ony be fau induced. There are aso some inheren asymmeries, a presen in he asymmery phasor. Thus for fau deecion hese asymmeries mus be separaed. Eiminaion of inheren asymmeries is si possibe due o heir disinc behavior. Due o he focus of his paper on he inducion machine wih squirre cage he foowing expanaion is based on his machine ype. Neverheess his can be adaped aso o oher machine ypes. Main inheren asymmeries are caused by spaia sauraion and he soing of he saor/roor aminaion. The sauraion saiency arises from he differen eves of he sauraion aong he fux pahs of he fundamena wave. uaion period is equa wice he eecrica ange and corresponds o he machine s number of poes. The soing saiency has is source in he opening of he aminaion. The 3869

4 period is dependen on he anguar posiion of he roor wih respec o he saor. Wihin one revouion of he roor his asymmery wi have a period equa he number of roor bars. In case of an unskewed roor wih open roor sos his asymmery is usuay dominan. In case of skewed roor bars his asymmery wi decrease. Cosed roor sos wi reduce his moduaion down o he case ha i can be negeced. A. Eiminaion of inheren asymmeries In order o exrac a high sensiive fau indicaor he inheren asymmeries have o be eiminaed. Therefore a specific signa processing chain has o be execued which wi be described in he foowing. As a saring poin, voage puses (as shown in Fig 2) are appied o he machines erminas by inverer swiching. Wihin he puse duraion curren measuremen provide he curren derivaive difference phasor di Δ. The exciaion direcion sequeniay changes is direcion in he main phase direcions. Therewih one asymmery phasor can be obained from hree curren sope differences wih is offse porion eiminaed. ubsequeny he asymmery phasor is saved o a daa sorage arranged as an array. In he nex sep he roor posiion is changed and he asymmery phasor esimaion is repeaed. Each eemen of he sorage array hus corresponds o one anguar posiion of he roor. The whoe procedure is repeaed uni he roor has moved a eas one soing period. The so obained asymmery phasor se is now avaiabe for specra anaysis ike Fas Fourier Transform (FFT). The window size is chosen o one or a muipe of he roor soing period. Thus his moduaion can be ceary idenified. asymmery phasor is a compex vaue i is possibe o deec he fau eve by he magniude and he fau posiion by he ange vaue. The whoe fau indicaor esimaion procedure is presened as a bock diagram in Fig 3. IV. EXPERIMENTAL ETUP AND MEAUREMENT A. Experimena seup The machine under es is an 11kW inducion moor wih an unskewed roor. The number of roor sos is 44 and of saor sos 36, respecivey. The saor has 4 poes and a fu piched winding sysem. A voage source inverer serves as exciaion source. The measuremens are conroed by compuer sysem programmabe under Maab/imuink. aor so wedges are made of sandard wedge maeria aso used in indusria appicaions. The composiion consiss of abou 75% iron powder, 18% epoxy resin and 7% gass ma. Wih he uiizaion of indusria sandard wedge maeria he characerisics and behavior of appicaions in he fied are we simuaed. Geomery was speciay adaped o fi in he so openings of he machine. In Fig 4 a par of a sandard so wedge and an adaped so wedge for he es machine are shown. Fig 5 shows a cose view of he saor sos wih he adaped so wedges paced in he saor so openings. On he ef side a so wedge is missing. Fig 4: Indusria sandard magneic so wedge maeria and adaped so wedges. missing so wedge paced so wedge Fig 3: Bock diagram of he fau indicaor esimaion scheme A missing so wedge causes a fau induced saiency which is considered o be fixed wih he saor. In oher words: he asymmery arising from a missing wedge does no change is geomerica posiion wih respec o he saor. The puse exciaion as we as he curren measuremens is aso saor fixed. This indicaes ha he asymmery can be deeced in a specra anaysis as he offse of he specrum. Thus he offse of he FFT direcy serves as he fau indicaor. As he Fig 5: Cose view of saor sos wih speciay adaped so wedges. (one missing so wedge on ef side) B. Measuremen consrains The measuremens were carried ou wih he focus of fied appicabiiy. Basicay magneic so wedges are appied in high voage machines wih high power raing. The mehod is based on voage exciaion by an inverer. Fu size inverer in such power and voage regions are couped wih high coss and equipmen demands. Therefore he mehod is invesigaed wih he consrain ha he inverer power raing is ony a 3870

5 fracion of he machine s. A measuremens were carried ou on a non magneized machine wih zero fux and no oad. Curren reacance measuremen was reaized by he inverer bui-in curren sensors. These sensors are sandard indusria curren sensors and have no specific demands on very high resouion or cu-off frequency. In order o ge a cear impression of he so wedges posiions wih respec o he exciaion direcions Fig 6 shows a schemaic represenaion of a cu-ou of he saor aminaion wih he winding scheme. The cu-ou represens 120 of he saor aminaion. The winding sysem is doube ayer ype and cois of each phase are marked in differen coors. Red represens cois of phase U, green of V and bue of W, respecivey. The exciaion direcion is assumed o be in direcion U, indicaed by he red soid phasor denoed v Δ. In foowing invesigaions wedges in differen sos are removed o simuae missing so wedges. aring wih one whoe missing wedge he fau eve is decreased by removing ony pars of wedges in a so. Missing wedges are removed a he marked ocaions mw-u, mw-v and mw-w. Fau indicaor vaues are presened as do pos in he saor fixed frame. The unis are given as arbirary uni (a.u.) of he DP inner represenaion. The reference vaue for he fau indicaor is denoed fauess. The measuremen resu for a fu missing wedge in phase U is denoed wih 1 MW Ph U. The fau indicaor moves aong he rea axis which is equa he phase direcion U. Thus one whoe missing wedge can be ceary idenified ogeher wih is direcion. The nex cases were reaized by removing one wedge in phase V and phase W, respecivey. I has o be sressed han here is aways ony one singe so wedge removed whie a ohers are si in pace. The ocaions of each missing wedge are shown in Fig 6 The fau indicaor shows in each case he expeced behavior. Thus a missing so wedge can be ceary idenified in each phase. A measuremens were repeaed 72 imes o prove he mehod s accuracy. Hence, fau indicaor appears as big dos due o he po represenaion. Fig 7: Measuremen resus for one missing so wedge in each phase. Fig 6: Locaion of he so wedges wihin he saor aminaion. C. ensiiviy anaysis In a firs sep he machine was idenified in he fauess case. A wedges were paced in he sos. The so obained vaue of he fau indicaor serves as a reference. Basicay he fau indicaor can si have an offse vaue ahough no fau is presen. Considering a rea sysem here are aways some inheren asymmeries presen. o, aso he fau indicaor in he symmerica machine case shows a deviaion from he origin. This offse vaue can be caused by asymmeries in he measuremen sysem ike differen sensor ransfer funcions in he hree phases and evauaion eecronics. However, hese disurbances can be eiminaed in advance by offse eiminaion. For simuaion of a fau case wih a singe missing so wedge one wedge was removed in phase U and a oher wedges ef in he sos (in Fig 6 indicaed as mw- U ). Afer he measuremen was finished, he missing so wedge was paced back and anoher wedge removed in phase V ( mw-v ). ubsequeny he same was done for phase W ( mw-w ). The fau indicaor resus are depicured in Fig 7. Fied examinaions have shown ha usuay no a whoe wedge is faing ou or missing bu ony pars of i [7]. A reiabe mehod shoud provide he possibiiy o deec aso pariay missing so wedges. To prove he appicabiiy of he mehod for such fau cases furher measuremens were done. Thereby one wedge was cu ino four pars. Fig 4 shows exampes of spi wedges. Fig 8: Measuremen resus for pariay missing so wedge in phase direcion U. 3871

6 Powered by TCPDF ( A firs a four wedge pars were paced in he so and hen one by one par was removed and a measuremen execued. The fauy posiion was chosen in phase direcion U a posiion mw-u. The Fig 8 shows he fau indicaor vaues of his measuremen series. The fau cases are denoed wih 1/4 MW i 1 MW Ph U for 1/4 missing wedges up o one fu missing wedge, respecivey. The mehod shows si a good accuracy even if ony a sma par of 1/4 wedge is missing. D. Reducion of exciaion voage Up o now a measuremens were done by a fu size inverer. The dc ink voage was adjused o is nomina vaue of 440V. As aready menioned before, magneic so wedges are usuay appied in high voage machines. Reaizing a measuremen device based on his mehod woud require an inverer wih very high dc ink voage and currens. Bu his is basicay inked aso wih high coss of he inverer. To counerac his drawback he mehod was aso esed wih reduced exciaion voage magniudes and ceary reduced currens wih respec o raed. Fig 9: Fau indicaor magniude a differen exciaion voages. Χ 440V, 220V, 110V, 60V, * 30V. When he dc ink voage is reduced, so is aso he magniude of he voage puse. Thus he curren sope wi aso decrease and wih i aso he fau indicaor magniude. However, deecion of missing so wedges is si possibe due o he high reiabiiy and accuracy of he proposed mehod. In Fig 9 resus of measuremens a differen dc ink voage vaues are depicured. Cases under invesigaion were he same as in he previous subsecion wih ony pars of a singe wedge missing. The resus are presened as fau indicaor magniude versus he percenage of a missing wedge. Voage seps are shown in differen coors saring wih 440V (green) corresponding raed dc ink voage down o 30V (magena) corresponding o ~7% raed. Assuming raed dc ink voage, he curren change during he puse exciaion is in he range of 5% o 10% raed. This vaue is reduced down o 3,5 o 7 raed when owering he dc ink voage o 7% raed. Thus a machine wih a power raing of 1MW can be esed using a es inverer wih ony a fracion of he power (ess han 1 kw). Measuremens on a some hundred kw machine have shown ha he behavior of inheren saiencies (sauraion/roor soing) is he same as on he es machine. Hence, i can be expeced ha he fau deecion properies can be appied o machine of oher sizes. V. CONCLUION A mehod o deec missing magneic so wedges was invesigaed in his paper. An ousanding advanage of he mehod is given by he possibiiy o idenify so wedge faus wihou he usua disassembing he machine. Hence, high downime periods and herewih couped high financia osses can be reduced. The mehod is based on he expoiaion of he machine response on ransien voage puse exciaion. Measuremen of he curren sope provides informaion on he ransien eakages inducance. This informaion can be uiized o obain a fau indicaor wih high sensiiviy. A specific signa processing is appied o eiminae any inheren asymmery and hus o furher increase deecion accuracy. Measuremens on a sma machine were performed o verify he appicabiiy and accuracy of he mehod. I was proven ha even a remova of ony 25% of a singe so wedge can be accuraey deeced. In addiion i is aso possibe o idenify he posiion of he missing wedge. A furher advanage of he mehod is ha he inverer necessary for he exciaion can be down-sized o ony a fracion of he machines power raing. VI. ACKNOWLEDGMENT The work o his paper was suppored by he European Union in he EE-ERA.NET PLU framework. VII. REFERENCE [1] Mikami H., Ide K., Arai K., Takahashi M., and Kajiwara K., Dynamic harmonic fied anaysis of a cage ype inducion moor when magneic so wedges are appied, IEEE Trans. Energy Convers., vo. 12, no. 4, pp , Apr [2] Curiac, R.; Li, H.;, "Improvemens in energy efficiency of inducion moors by he use of magneic wedges," Peroeum and Chemica Indusry Conference (PCIC), 2011 Record of Conference Papers Indusry Appicaions ociey 58h Annua IEEE, vo., no., pp.1-6, ep [3] Kappaou, J.; Gyfakis, C.; afacas, A.; "A sudy of he effecs of he saor sos wedges maeria on he behavior of an inducion machine," Eecrica Machines, ICEM h Inernaiona Conference on, vo., no., pp.1-6, 6-9 ep [4] Hanna, R.A.; Hiscock, W.; Kinowski, P.;, "Faiure Anaysis of Three ow-peed Inducion Moors for Reciprocaing Load Appicaion," Indusry Appicaions, IEEE Transacions on, vo.43, no.2, pp , March-Apri [5] Davis M.; Probem and souions wih magneic saor Wedges, Iris Roaing Machine Conference., an Anonio, Texas, UA, June [6] ojicic, G.; amonig, M.; Nussbaumer, P.; Joksimovic, G.; Vasak, M.; Peric, N.; Wobank, T. M.; "A mehod o deec missing magneic so wedges in AC machines wihou disassembing," IECON h Annua Conference on IEEE Indusria Eecronics ociey, vo., no., pp , 7-10 Nov [7] Hanna, R.; chmi, D.W.;, "Faiure Anaysis of Inducion Moors: Magneic Wedges in Compression aions," Indusry Appicaions Magazine, IEEE, vo.18, no.4, pp.40-46, Juy-Aug