specialissuepaper 787

SPECIAL ISSUE PAPER 787

Surface analysis on rolling bearings afterexposure to defined electric stressT Zika1,3∗, I C Gebeshuber1,3, F Buschbeck1, G Preisinger2, and M Gröschl1

1Institute of General Physics, Vienna University of Technology, Vienna, Austria2SKF Österreich AG, Industrial Electrical Segment, Steyr, Austria3AC2T Research GmbH, Wiener Neustadt, Austria

The manuscript was received on 5 September 2008 and was accepted after revision for publication on 18 December 2008.

DOI: 10.1243/13506501JET538

Abstract: This article gives an overview about classical and frequency converter-inducedspurious bearing currents in induction machines and discusses typical damage patterns causedby the current passage. To investigate on the electric damage mechanisms, test bearings areoperated in a test rig and exposed to specific (classical low-frequency, and high-frequency) bear-ing currents. The induced damages to the surfaces are analysed visually and with the help ofan atomic force microscope, and compared for the different electric regimes applied. Further,the electrically damaged bearing surfaces are characterized by standard roughness parameters.The surface structure observable on certain test bearings shows good correlation to the struc-ture found with a bearing that had failed in the field under similar electric conditions. One ofthe investigated electric regimes applying high-frequency currents proved capable of generatingfluting patterns – as found in real applications – on the test rig. The experiments also indicate thathigh-frequency bearing currents, although in total dissipating less energy, are more dangerousto a bearing than continuous current flow. The presented method gives a good starting point forfurther investigation on electric current damage in bearings, especially regarding high-frequencybearing currents, and on bearing/grease lifetime under specific electric regimes.

Keywords: bearing currents, variable-speed drives, common-mode voltage, fluting, electricerosion

1 INTRODUCTION

Damages of rolling bearings in electric machinescaused by electric current passage are known fordecades [1]. The current paths for such parasitic d.c.and low-frequency a.c. bearing currents, which arepredominant, e.g. in mains-fed electric machines,have been studied and mitigating methods, such asinsulated bearings and shaft grounding brushes [2],have been developed.

Modern induction machines are controlled viafast-switching insulated gate bipolar transistor volt-age source converters that, in motors, provide thepossibility for precise control and adjustment of

∗Corresponding author: Institute of General Physics, Wiedner

Hauptstraße 8-10/E134, Vienna 1040, Austria.

email: [email protected]

rotational speed and torque as well as energy regen-eration at braking operations. In doubly fed inductiongenerators (DFIGs), a frequency converter feeds therotor windings to enable a constant line frequencyat varying rotor speed as well as active and reactivepower control [3]. In such electric machines, new typesof bearing currents are found. These parasitic high-frequency pulse currents (dV/dt currents, dischargecurrents, high-frequency circulating currents) affectbearing life to a great extent.

Nowadays, one faces a growing number of windpower applications, where DFIGs are used very fre-quently. Furthermore, there is an increasing demandfor energy-efficient driving applications, commonlyutilizing variable-speed drives comprising frequencyconverters. This leads to an increasing amount of reli-ability problems of bearings caused by high-frequencyelectric current passage.

Although both high- and low-frequency currentsmay reduce bearing lifetime significantly, the sources

JET538 © IMechE 2009 Proc. IMechE Vol. 223 Part J: J. Engineering Tribology

788 T Zika, I C Gebeshuber, F Buschbeck, G Preisinger, and M Gröschl

for these currents are different. For instance, thelow-frequency a.c. bearing currents are mainlycaused by magnetic asymmetries, whereas the high-frequency currents appearing in modern drive systemsare mainly caused by the high slew rates of the sig-nals and a non-vanishing common-mode voltage.Hence, the so-called dV/dt currents and other relatedparasitic effects arise [4].

This article gives an overview about bearing cur-rents in mains-fed as well as frequency converter-controlled induction machines, and their respectivesources. Further, the article focuses on the investi-gation of the damage done to the surfaces of rollingelements and rings of test bearings exposed to variouselectric regimes. These bearings (thrust ball bear-ings) were operated in a specially modified test rig,and got exposed to different types of artificial bear-ing currents (d.c., a.c., and high-frequency pulsecurrents (HFPCs)). The induced damages are com-pared on macroscopic and microscopic scales. Besidesvisual inspection of the macroscopic damage pat-terns (like fluting or frosting), the running tracks’surface topographies are recorded using an atomicforce microscope (AFM). The obtained surfaces arecharacterized and compared by using standard rough-ness parameters and, again, visual inspection. Thisallows for the first time to identify differences indamage induction between classical and modern fre-quency converter-induced bearing currents. Regard-ing high-frequency bearing currents, for comparisonto damages found in real applications also the sur-face topography of a bearing operated in wind turbinegenerator is displayed.

2 TYPES OF BEARING CURRENTS

There are three main causes for bearing currents,namely:

(a) electrostatic charging;(b) magnetic flux asymmetries;(c) common mode voltage in combination with high

slew rates of the voltage pulses.

The first two phenomena, electrostatic chargingand magnetic flux asymmetries, are well known andrepresent the ‘classical reasons’ for bearing currents.Non-vanishing common mode voltage in combina-tion with voltage pulses of high slew rates becameproblematic due to the introduction of modern, fastswitching, frequency converters.

In a set-up comprising either an induction genera-tor or motor, electrostatic charging effects may occurbecause of frictional electricity inside coupled devices.Without insulation of the shafts against each other,a d.c. voltage builds up at the motor/generator shaft

(referred to as frame or ground, respectively) and, thus,across the bearings.

Asymmetries in the magnetic field of an inductionmachine induce an a.c.voltage along the shaft in theaxial direction. When the bearings are in conductiveor resistive state of impedance [5], this voltage drivesa so-called circulating current along the shaft, acrossone bearing, through the frame of the machine, andacross the other bearing back to the shaft. The pathmay eventually extend across the bearing of a drivenmachine, depending on local impedance conditions.These currents are of low-frequency nature, accordingto the fundamental frequency of the phase voltages.They generate dissipative (active) power inside a bear-ing, which is locally converted into heat. Thereby, thesurfaces as well as the lubricant of the bearing couldbe damaged.

In modern frequency converter-controlled systems,the phase signals are composed of voltage pulses ofhigh slew rates (typically 2–10 kV/μs). Similar voltagetransitions are characteristic for the common modevoltage and the voltage at the motor star point. Ifa capacitance is present in the considered circuit,current pulses I = C dV1/dt are generated each timeduring the transitions of the phase voltage V1. (Thesame applies for phase voltages V2 and V3.) Thesecurrents are of high-frequency nature and are oftenreferred to in the literature as ‘dV /dt currents’. Fur-ther, according to Mütze [6] and Ollila et al. [7], thesedV /dt currents are capable of driving high-frequencycirculating currents with a current path including theshaft, both bearings, and the motor frame.

Figure 1 shows a sketch of an induction motorwith the associated stray capacitances (Fig. 1(a))and the respective electric equivalent circuit dia-gram, with respect to frequency converter-inducedcurrents (Fig. 1(b)). In such a set-up comprisingan induction motor, the frequency converter appliespulse width modulated (PWM) voltage patterns tothe stator windings. Hence, the voltages at the sta-tor windings and the (unavoidable) stray capacitancesbetween the stator windings and the rotor (shaft)are mainly responsible for the formation of parasiticcurrents.

During transitions of the phase voltage V1, thecurrent ISF = CSF dV1/dt flows via the stray capac-itance CSF from the stator windings to the motorframe (Fig. 1(b)). Accordingly, the current ISR flowsfrom the rotor winding through CSR to the rotor(shaft) and then splits into a branch through CRF

(IRF) and branches through the motor bearings(IB1, IB2), depending on the bearings’ actual stateof impedance. Therefore, a high-frequency pulse-shaped voltage VRF = VB1,2 is created between the rotor(shaft) and the machine’s frame and, thus, acrossthe bearings. If both bearings operate in the full-filmlubrication regime (capacitive state of impedance),VB1,2 can be estimated (with neglect of VPE1 see

Proc. IMechE Vol. 223 Part J: J. Engineering Tribology JET538 © IMechE 2009

Surfacae analysis on rolling bearings after exposure to defined electric stress 789

Fig. 1 (a) Sketch of an induction motor with associated stray capacitances and (b) electric equiv-alent circuit of an induction motor. Only the relevant electric parts responsible for HFPCsare shown

below) by considering the purely capacitive voltagedivider as

VB1,2

V1≈ CSR

CSR + CRF + CB1 + CB2

with the bearing capacitances CB1, CB2 replacing themore general impedances ZB1 and ZB2 in Fig. 1(b).From the motor frame, the summing current IPE1 =ISF + ISR = ISF + IRF + IB1 + IB2 flows back to the con-verter via the PE line. Thereby, it creates a voltage VPE1

across the inherent inductance LPE1 of the line, accord-ing to VPE1 = LPE1 dIPE1/dt . These voltage peaks VPE1 arecoupled to the rotor (shaft) via CRF. Basically, this hasno effect on the bearing voltage VB1,2, because it resultsin a common potential shift of the motor frame andshaft.

If a coupled device (like a gearbox or a drivenmachine) is present without an electrical insulationagainst the motor shaft, another branch exists for adV /dt current through the bearings of the coupleddevice. Depending on the grounding situation of thecoupled device, this may cause or pronounce parasiticcurrents, and also harm the bearings of the coupleddevice.

As long as a bearing operates in the full-film lubrica-tion regime and, thus, from the electrical point of viewacts as a pure capacitance, any current flowing throughthe bearing is purely reactive and does not cause anyphysical effect such as heating, etc. However, if thebearing operates in a resistive state of impedance [5],any current flow is connected to the generation ofactive power. Hence, the bearing could suffer dam-age to the lubricant and to its surfaces. Such activecurrent flow occurs if the bearing is not in the full-film lubrication regime at the time of the converter’sswitching operations, or if the insulating lubricant filmbreaks down while the stray capacitances inside theelectric machine are charged (this charging of the stray

capacitances is basically also caused by the dV /dtcurrents).

3 DAMAGE PATTERNS

Parasitic currents flowing through a bearing may beharmful in various ways. Classical low-frequency bear-ing currents induce damage on the surfaces of theraceways and rolling elements. This mainly results ina darkening of the running tracks (Fig. 2(b)), whichin the end may lead to a macroscopic washboard-like pattern, also called fluting (Fig. 2(d)). Kohaut [1]tries to explain this phenomenon by the bearing ballsbeing forced to ‘jump’ over electric pittings that werenot flattened ideally. This leads to oscillations of therolling elements and finally causes the raceway surfaceto develop a washboard pattern. Despite this qualita-tive explanation, Mütze et al. [8] states that it is stillunclear why certain bearings operated under identicalconditions show fluting and others do not. Further-more, also the grease of the bearing gets burned bythe current flow and shows a blackened appearance(Fig. 2(a)).

In electric drive systems comprising a frequencyconverter, the current flow through the bearings is notcontinuous, but occurs in discrete pulses. These pulsesare either synchronized with the switching opera-tions of the frequency converter (dV /dt currents)or distributed stochastically whenever the electricallyinsulating lubricant film between rings and rollingelements breaks down (discharge currents [9, 10]).These HFPCs result in a different visual appearanceof the damaged running tracks compared to the con-tinuous classical bearing currents. The running tracksmacroscopically show a grey/dull appearance, alsoknown as ‘frosting’ (Fig. 2(c)), caused by microscopiccraters all over the raceway surface. However, in theend this kind of surface damage may also lead to a



Fig. 2 Bearing damages: (a) blackened grease; (b) darkened running track; (c) grey/dull (frosting)running track; and (d) fluting pattern

washboard/fluting pattern (Fig. 2(d)) on the affectedsurfaces.

4 EXPERIMENTAL SET-UP

To investigate the different effects of various elec-tric regimes onto the bearings’ surfaces, a numberof test specimens have been created in several testruns. As specimens thrust ball bearings were used,mainly because of easier and more flexible experimen-tal handling (disassembling and reassembling). Thesebearings (type 51305) are made of 100Cr6 bearing-grade steel. All sample bearings were lubricated withthe same amount of standard grease consisting ofmineral base oil and lithium soap-based thickener.Furthermore, exactly the same experimental proce-dure in all test runs was ensured by fully automatedtest run control.

4.1 Mechanical set-up

A thrust ball bearing test rig available at SKF ÖsterreichAG, Steyr, Austria, was modified to allow the applica-tion of specific electric regimes on the test bearings(see Fig. 3). The support bearings were hybrid ones;the shaft (and hence the shaft washer of the test bear-ing) was contacted by a rotating mercury contact. Theloaded housing washer was insulated against the testrig using a polyamide disc and contacted by a brassscrew going through this insulating layer.The mechan-ical load was applied via a lever/spring combinationand measured with a load cell. The running temper-ature of the bearing could not be controlled but was

Fig. 3 Sketch of the test rig’s mechanical set-up

measured at the housing washer using a thermocou-ple. Finally, bearing vibrations were also monitoredutilizing a piezoelectric acceleration sensor (sensorBrüel & Kjaer-type 4367, conditional amplifier Brüel& Kjaer-type 2626). To allow comparison betweenexperiments, the RMS values of the measured accel-eration signal were normalized to the values foundat the very beginning of the respective experiment.These normalized values are marked as ‘vibrationlevel’ in Fig. 4.

4.2 Experimental procedure

From the mechanical point of view, all test bear-ings were operated at 1000 r/min rotational speed



Fig. 4 Samples for AFM analysis; left picture: sample cut out of a washer’s running track (approx.2 × 4 mm2, HFPC_2); right picture: two rolling elements (left: virgin condition, right:3 A a.c.)

and at a load of 1400 N. First, there was a running-in phase lasting for 2 h, where no electric stress wasapplied to the bearing. Afterwards, the desired elec-tric regime was engaged and the bearing was operatedfor 24 h under the respective electric regime. After26 h of total running time each test run was stoppedautomatically.

4.3 Electric regimes

The effects of five different electric regimes have beeninvestigated: for reference one without any current,two regimes applying continuous (classical) currents,and two regimes applying discrete current pulses (sim-ulating frequency converter-induced currents). Forthe two continuous (classical) current regimes, a cur-rent of 3 A (RMS) was chosen to induce damage inthe bearings. Considering the Hertzian contact areaof the test bearings, this resulted in a current densityof 1.8 A/mm2, which is above the limits for inductionof damage commonly stated in the literature [5]. Anoverview including the associated energy (per singlepulse and over the whole test run) and power (effectiveand peak) values is shown in Table 1.

The following designations will be used hereafterto specify the electric regime applied to the testbearing.

Table 1 Electric energy and power values for the vari-ous regimes applied to the test bearings

Single Total Effective PeakElectric pulse accumulated power powerregime energy (J) energy (kJ) (W) (W)

WO – 0 – –DC – 233 2.7 2.7AC – 181 2.1 3.5HFPC−1 9 × 10−7 0.778 0.009 8HFPC−2 6.8 × 10−5 58 0.68 50

1. WO: no active electric regime is applied to thebearing, so there is no current flow through thebearing.

2. DC: a 3-A continuous d.c. current source is con-nected to the bearing. The anode is at the housingwasher.The voltage drop across the bearing is about0.9V. This amounts to a power of 2.7W and a totalaccumulated energy of 233 kJ.

3. AC: a 3-A (RMS) continuous a.c. current with afrequency of 50 Hz is applied to the bearing. Mea-surements show a power of approximately 2.1Wand a total accumulated energy of 181 kJ.

To apply HFPC, specially designed electronics areattached to the bearing. These electronics simulatethe current pulses delivered by a frequency converterusing a capacitor for coupling such pulses to the testbearing. The electric energy is applied to the bearing ina short period of time with comparably long idle peri-ods in between the single pulses. This leads to big peakpower values (at the times of the pulses) and lower val-ues for the effective power over time (see Table 1). Inthis article, two different versions of such HFPCs arereported.

1. HFPC_1: the coupling capacitor is in the order ofsome nF. This results in a peak pulse current ofapproximately 1.5 A. The repetition frequency is10 kHz. The accumulated total energy amounts to778 J.

2. HFPC_2: The coupling capacitor is in the orderof a few μF. This results in a peak pulse currentof approximately 20 A. The repetition frequency is10 kHz. The accumulated total energy amounts to58 kJ.

It has to be noted that the electric impedance of abearing may fluctuate in the course of such a test run.Therefore, the data given in Table 1 are extrapolatedover the total test run duration using starting valuesmeasured at the beginning of the respective test run.



5 ANALYSIS PROCEDURE

To obtain a first impression of the electrically induceddamages to the test specimens, photographs of the testbearings were taken to assess the macroscopic damagepatterns. Then, AFM pictures of the respective surfacetopographies were recorded and flattened using asecond-order polynomial fit to remove the curvatureof the sample. For each sample, the running trackson the shaft and housing washer as well as on one ofthe rolling elements have been examined. To do so, thewashers had to be cut into smaller samples using a cut-off wheel, whereas the surfaces of the rolling elementscould be investigated without further manipulation(Fig. 4).

Using the topography data obtained with the AFM,the surfaces were also judged visually. Standard rough-ness parameters were used to characterize the surface:Rq (RMS roughness) and Rt (spread between the high-est and lowest height values) roughness values werecalculated using the full 30 × 30 μm2 AFM scan of

the surface, where Rq =√

1n

∑ni=1 y2

i and Rt = max yi −min yi (yi represent the height values of the scannedsurface).

For reference, topography analysis was also per-formed on a virgin bearing.

6 RESULTS

The data collected during the test runs show thatall test bearings had endured a mechanical load ofapproximately 1450 N throughout their respective testrun. A slight increase of the initially set load of 1400 Nwas most likely caused by the increase in bearing andtest rig temperatures. Bearing temperature data revealthat all bearings did run in a temperature range from45 to 55 ◦C.

Analysis of the online vibration measurement dataof the test runs (Fig. 5) shows that the specimenHFPC_2 exhibited a huge increase in vibration level,beginning after 8 h of running time. In contrast, all

Fig. 5 Progression of the vibration level of the samplebearings

other specimens (WO, AC, DC, and HFPC_1) showedless pronounced levels of vibration.

Figure 6 shows photos of the specimens HFPC_2,HFPC_1, and AC. For all test bearings, the shaftwashers have much broader running tracks than thehousing washers.With bearing HFPC_2 one can clearlysee a fluting pattern that developed on the shaftwasher only. This fluting pattern consists of brightflutes separated by darker areas in between them (thispattern is also observable in Fig. 4, left). The corre-sponding housing washer and the rolling elementsonly show a strong discolouring. Bearing HFPC_1shows a dull running track (frosting) on both washers.The rolling elements, however, do not show significant‘frosting’. Bearing AC shows a very dark discolouringon both washers and on the rolling elements. The DCbearing (not shown) looks very similar to the AC bear-ing, whereas the WO bearing only shows very slighttracks in the raceways (see Fig. 7). The virgin bearingREF naturally shows no signs of wear in the racewaysor on the rolling elements.

In Fig. 7, the surfaces of a virgin bearing’s shaftwasher and the shaft washer of the test bearing WO,run without any current passage, are shown. In bothcases one can clearly see the initial honing pattern,which has its origin in the production process. TheWO bearing’s surface shows some marks. It also hasless surface roughness (Rq and Rt , see Tables 2 and 3).This is also valid for the housing washers of these bear-ings, whose topographies have a similar appearance tothose of the shaft washers. The rolling elements do ini-tially have very smooth surfaces. However, even herethe surface roughness (Rq and Rt) is reduced furtherdue to bearing operation without electric stress.

Figure 8 depicts the surface of the HFPC_2 bear-ing’s shaft washer. In the left picture the bright zoneof the fluting pattern is shown. The honing pattern,as seen on the bearings REF and WO, has completelyvanished. The surface shows discrete sphere-shapedfeatures up to approximately 2 μm in diameter. Theright picture shows the dark zone of the fluting pattern.Here, discrete features are less distinct. Furthermore,the roughness is much less compared to the brighterzone (see also Tables 2 and 3). The housing washerand the rolling elements of this bearing show similarstructures as the shaft washer.

The surface of the HFPC_1 bearing’s shaft washer isshown in Fig. 9 (left picture). It reveals similar featuresas the HFPC_2 bearing, but on a smaller scale. Thediscrete sphere-shaped features have an approximatediameter of 1 μm. From the optical impression, thisbearing’s rolling element (see Fig. 6) shows only littlechange to its original surface. Nonetheless, the sur-face topography (Fig. 9, right) already reveals damagedareas, which are not covering the whole surface, yet.In the other regions the rolling element still exhibitssimilar structures as for the WO bearing.



Fig. 6 Photos of the test bearings (HFPC_2, HFPC_1, and AC) demonstrating the induced damage(left to right: housing washer, rolling elements+grease, shaft washer)

Fig. 7 Comparison of the shaft washers’ surface topography of bearings REF (left picture) and WO(right picture)

Table 2 Rq roughness of washers and rolling elements oftest bearings

Housing Shaft RollingRq (nm) washer washer element

REF 112 112 10WO 61 64 7DC 56 91 81AC 60 78 83HFPC−1 62 63 24HFPC−2 (bright spot) 90 128 58HFPC−2 (dark spot) – 45 –

Table 3 Rt roughness of washers and rolling elements oftest bearings

Housing Shaft RollingRt (nm) washer washer element

REF 843 754 116WO 419 399 98DC 685 921 833AC 624 792 944HFPC−1 475 450 293HFPC−2 (bright spot) 606 838 585HFPC−2 (dark spot) – 480 –



Fig. 8 Comparison of surface topography of HFPC_2 test bearing; left picture: bright spot in flutingpattern; right picture: dark spot in fluting pattern

Fig. 9 Topography of HFPC_1 test bearing; left picture: surface of the shaft washer; right picture:surface of a rolling element

Fig. 10 Comparison of the shaft washers’ surface topography of bearings DC (left picture) and AC(right picture)

In Fig. 10, the shaft washer surfaces of the test bear-ings DC and AC are compared. Both bearings’ surfacesdo not show honing patterns anymore. In contrast tothe HFPC test bearings, these two specimens do notshow such well-defined features, but exhibit a cloudysurface structure.

As a reference to the field, Fig. 11 shows the sur-face topography of a deep-grove ball bearing that was

operated in a wind turbine generator. The surface ofthis bearing’s outer ring shows clear similarities to theHFPC test bearings’ surfaces. The discrete features inthis case have a diameter of 3–5 μm.

The Rq roughness values of the various samples arelisted in Table 2. The WO bearing shows reduced Rq

values compared to the virgin bearing’s (REF) values.Especially the washers’ values are nearly halved.



Fig. 11 Surface topography of a large-size deep-groveball bearing’s outer ring that was operated in thefield (wind turbine generator)

The DC and AC bearings show very similar Rq val-ues for housing washer and rolling elements. The Rq

values of the rolling elements are higher than the REFbearing’s value.

The Rq roughness of bearing HFPC_1 is still very sim-ilar to the one of bearingWO, even though the structureon the surface is significantly different (compare Figs 7and 9). Only the rolling elements show a significantdifference in the Rq value.

Inside the bright spots of bearing HFPC_2, the Rq

values on the washers are significantly increased withrespect to the other test bearings and are comparableto the values of the REF bearing. The roughness valuesof the rolling element do not reach the ones of the ACand DC bearings, but are high compared to all othertest bearings. The dark spot in the fluting pattern, onthe contrary, shows low Rq values, even lower than thatfor the WO bearing’s shaft washer.

The same applies to the Rt roughness values seen inTable 3, with the exception that the values for HFPC_2are lower than the ones for AC and DC.

7 DISCUSSION

Various test runs were conducted in which bearingswere damaged by different electric regimes. In thecourse of such a test run, the AC and DC electricregimes applied more electric energy to the bearingthan the HFPC_2 regime. Having this in mind, it isastonishing that the HFPC_2 test bearing is the onlybearing showing a fluting pattern in one of the race-ways. The generation of this fluting pattern can beclearly observed from the vibration measurement data(Fig. 5), where after 8 h of running time a significantrise in vibration level sets in. All other test bearingsare inconspicuous in terms of vibration level andshow similar values. These findings of on-line vibra-tion monitoring are confirmed by the macroscopicimpression of the test bearings after the test runs. All

other test bearings do not show a fluting pattern in anyraceway.

In general, the HFPC regimes (HFPC_1 and HFPC_2)apply the electric energy in a very short period of time(during the current pulse, 50 ns up to some μs) withlong idle times without any current flow in betweenthe single pulses. This results in comparatively loweffective power and energy values, in contrast to highpeak power values. These peak power values are higherthan the ones of the continuous current (AC or DC)regimes (see values in Table 1). The fact that theHFPC_2 test bearing suffered the most severe dam-age (fluting) indicates that not only the overall appliedenergy but also the peak power is of importanceregarding the damaging process. The HFPC regimesdeposit the electric energy in a short period of time.Therefore, the affected surface area in a single pulse issmall and well localized. However, after a certain timeof running many of these single localized areas accu-mulate and the damage is spreading across the wholesurface. With the continuous current regimes, on theother hand, the energy is more evenly distributedacross the surfaces of the bearing’s running tracksand rolling elements immediately. However, furtherresearch is necessary to entirely clarify why the pulsedapplication of electric energy is capable of inducing afluting pattern in shorter time than higher-energetica.c. or d.c. currents.

The AFM topography pictures of the test bearingsclearly bring out the difference in damage mechanismbetween continuous current flow and pulsed currentflow. The surfaces of the AC and DC bearings sharea cloudy structure without discrete features. BothHFPC test bearings, on the contrary, show distinctsphere-shaped features on their surfaces, presum-ably induced by the discrete current (energy) pulses.For reference, also a deep-grove ball bearing thathad been operated in a wind turbine generator wasexamined. In this type of application, bearings areexposed to HFPCs. Although the geometric dimen-sions and other mechanical parameters between thisbearing and the other sample bearings investigatedhere are not directly comparable, the surface struc-tures observed on both HFPC test bearings show goodcorrelation to the features found on the bearing fromthe field. The bearing run without any current stillexhibits the honing structure originating in the pro-duction process. However, the test run did, as onewould expect, burnish the surface roughness com-pared to the topography of a virgin bearing. All testbearings that were exposed to any electric regime donot show signs of this honing pattern anymore, whichis a hint that complete remelting of the surfaces hasoccurred. The HFPC_1 bearing’s rolling element showsonly certain areas that had been exposed to the pulsedcurrents and other areas exhibiting the initial surface.Using longer running times, one could expect that thewhole surface will be covered with electrically induced



damage pattern. It is also interesting that the areasshowing damage consist of cavities below the sur-rounding undamaged surface, but also hills reachingabove the initial surface.

Unfortunately, the typical roughness parameters Rq

and Rt only represent an insufficient description of thebearings’ surfaces regarding current damage, becausethese parameters are neglecting the actual surfacestructure to a great extent. The WO bearing’s surfacesare burnished by the mechanical running. This can beobserved by a reduction of the WO bearing’s rough-ness parameters compared to the REF bearing. Bothroughness parameters do show increased values forthe AC, DC, and HFPC_2 bearings when compared tothe WO bearing. However, the virgin bearing (REF),due to the distinctive honing structure shows valuesof similar level to the AC, DC, and HFPC_2 test bear-ings. Furthermore, the values of the HFPC_1 bearing’swashers do not significantly differ from the WO bear-ing’s ones, only the roughness values for the rollingelements are elevated. This is neglecting the fact thatthe complete surface structure of the HFPC_1 bearinghas been transformed by the electric stress.

The HFPC_2 fluting pattern is macroscopicallymade up of bright (topographically lower) and dark(topographically higher) regions. It is remarkable thatthe respective surfaces also have a considerably differ-ent microscopic appearance, regarding surface topog-raphy and roughness. Although the bright areas doshow a structure with discrete features (as discussedabove) and also high Rq and Rt roughness values, thedark areas do not show many distinct features andthe roughness parameters are very low. Here, alsoother microscopic parameters, e.g. micro-hardnessand chemical composition, might be of interest inorder to further understand the mechanism behindthe generation of this fluting pattern.

8 CONCLUSION

An overview was given about the different typesof bearing currents in induction machines. Typicalmacroscopic damage patterns caused by electric cur-rent passage have been discussed. Test bearings wereexposed to various electric regimes (a.c., d.c., andHFPCs) and the resulting damages were assessed byvisual inspection and by using an AFM. The differ-ences of classical continuous bearing currents (d.c. ora.c.) to the more recent frequency converter-inducedHFPCs have been exemplified with the help of AFMsurface topography measurements on the test bear-ings. Furthermore, it could be illustrated that bothHFPC test bearings show similar features like a ref-erence bearing that had been exposed to frequency-converter-induced currents in the field. Damagedbearing surfaces have been characterized by typicalsurface roughness parameters Rq and Rt . However,

these parameters turned out not to be fully suitable tocharacterize damage by electric current passage, sincethe surface structure is neglected to a great extent.Therefore, other ways of surface characterization haveto be found.

The experiments also showed that HFPCs are capa-ble of generating fluting damage to the surface inshorter times compared to continuous currents ofhigher energy. However, especially regarding the flut-ing damage, closer investigations are still necessary toclarify the exact mechanism of its induction. Here, thesurface topography pictures give a first hint for a dif-ferent surface structure in the bright and in the darkareas of the pattern.

To sum up, the presented experiments are a goodstarting point for further investigation on electriccurrent damage in bearings, regarding the damag-ing mechanism of HFPCs, thresholds for damageinduction, generation of fluting patterns, and bear-ing/grease lifetime under specific electric regimes.

ACKNOWLEDGEMENTS

This work was funded from the Austrian Kplus-Program and has been carried out within the AustrianCenter of Competence for Tribology, Wiener Neustadt,Austria. The authors wish to thank H. Köttritsch andH. Weninger from SKF Österreich AG, DevelopmentCentre Steyr, Austria, and H. Stache, R. Spallek andS. Grundei from Klüber Lubrication München KG,Munich, Germany, for valuable discussions and theirsupport of this work.

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APPENDIX

Notation

CB1, CB2 capacitance of a bearingCRF capacitance between the rotor and the

frameCSF capacitance between the stator and the

frame

CSR capacitance between the stator and therotor

IB1, IB1 current flowing through the bearingIPE1 current flowing in the protective earth

lineIRF current flowing from the rotor to the

frameISF current flowing from the stator to the

frameISR current flowing from the stator to the

rotorLPE1 inductance of the protective earth lineRq RMS roughnessRt spread between the highest and the

lowest surface height valuesVB1, VB2 voltage across a bearingVPE1 voltage drop in the protective earth lineVRF voltage between the rotor and the frameV1, V2, V3 phase voltages of the induction

machineyi height values of the surface

topographyZB1, ZB1 impedance of a bearing


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