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The prevalent motor bearing premature failures due to the high frequency electric current passage
Engineering Failure Analysis 45 (2014) 118–127
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
The prevalent motor bearing premature failures due to the high frequency electric current passage William Liu ⇑ Industrial Services (NDT & Materials), SGS New Zealand Ltd., Auckland, New Zealand
a r t i c l e
i n f o
Article history: Received 10 April 2014 Received in revised form 2 June 2014 Accepted 25 June 2014 Available online 9 July 2014 Keywords: Bearing current Fluting Microcrater Skin effect
a b s t r a c t Bearing premature failures are prevalent in VFD (Variable Frequency Drives) motors. The three common symptoms of the modern bearing current appeared on the bearings of a group of pump motors. The bearing failure mechanisms have been studied from the tribological point of view. The natures of the three common symptoms have been uncovered. A hypothesis of the skin effect has been proposed to explain the three common symptoms. The discovery of the serial microcraters reveals the discharge with Fourier series features. The nature of the fluting patterns excluded the theories of both EDM (Electrical Discharge Machining) and discharge corrugation. VA (Vibration Analysis) detected the harmonics of the BPOR (Ball Pass Outer Ring) frequency which associated with the fluting on the outer raceway. This discovery may be applied to detect fluting in condition monitoring. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Statistical data shows that over 60% of motor failures were bearing failures [1]. Since the application of Variable Frequency Drives (VFD) in contemporary motors, motor bearing premature failures have been prevalent due to the electrical current passage [2]. The bearing failures due to the electric current passage have been recognised for almost a century. The fluting phenomena in motor bearings were first discovered in the 1920s [3]. Subsequently, the bearing current issues in AC motors have been studied [4–7]. The various bearing symptoms have been listed in the bearing manufacturer’s documents [8–11]. The practical solutions to bearing current, such as shaft grounding to bypass the current [2], ceramic-coated bearings [12] and hybrid bearings [13] for electric insulation, appeared to be effective. However, with the application of the fast switching PWM inverter in contemporary motors, the traditional solutions seems to be no longer effective for the high frequency bearing current [14]. In order to differentiate these two different kinds of bearing current, for the convenient discussion in this paper, the sinusoidal AC bearing current is referred to as the classic bearing current, and the high frequency non-sinusoidal current as the modern bearing current. The research of modern bearing current had once been an active topic. Most of the studies focused on the bearing current sources from the electronic perspective [15–17], such as common mode voltage. However, fundamental research in modern bearing current issues from the tribological perspective has been very scarce. The difficulty lays in the complex failure mechanisms. In laboratory simulation tests, it is extremely difficult to separate a single mechanism from the complex ⇑ Tel.: +64 9 635 0303; fax: +64 9 636 6250. E-mail address: [email protected] http://dx.doi.org/10.1016/j.engfailanal.2014.06.021 1350-6307/Ó 2014 Elsevier Ltd. All rights reserved.
W. Liu / Engineering Failure Analysis 45 (2014) 118–127
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Nomenclature f t A L d
l q
frequency of the current (Hz) time (s) half amplitude of the current intensity (ampere) – I/2 1/2 the period (2L = 1/f) skin depth (m) relative permeability of the medium resistivity of the medium (X m)
mechanisms. Disregarding the simulation tests, on the other hand, scenarios and examples encountered in industry can be another path to understanding the fundamental failure mechanisms. This paper presents some research on the motor bearing failure mechanisms due to modern bearing current passage from an industrial case study. The bearing premature failures occurred in a group of pump motors in the steamfields of a geothermal power plant in New Zealand. The motors were 250 kW with pulse width modulated (PWM) inverter VFD. The bearing numbers were 6318 (NDE) and 6320 (DE). Most of the bearings failed in approximately two years of service. Visual inspection found that the bearing grease had been hardened or partially solidified. The bearing surfaces had been deposited with a brown film. From the symptoms of the brown deposit and the grease hardening, bearing failures due to the high frequency electric current passage was certain. Hence, the failed bearings can be applied in researching the fundamental failure mechanism. 2. Experimental methods There were in total 20 of such motors. Vibration analysis in the condition monitoring programmes detected bearing deterioration. Before the motors break down, the problematic bearings had been replaced. After the bearing grease had been ultrasonically cleaned, bearing coupons were cut from both the inner and outer rings. Fig. 1 shows a typical bearing coupon. Various analyses have been conducted on the failed bearing coupons. 2.1. LOM LOM (Light Optical Microscopes), both stereomicroscopic and metallographic microscope, had been applied to examine the bearing coupons. 2.2. ESEM/EDS The bearing coupons had also been examined under ESEM (Environmental Scanning Electronic Microscope); the model was an FEI Quanta 200 F with field emission gun. The EDS (Energy Dispersive Spectroscope) detector was EDAX brand SiLi (Lithium drifted) with a Super Ultra Thin Window.
Fig. 1. The failed DE bearing covering with the brown deposit.
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2.3. FTIR The brown deposit on the failed bearing had been examined with FTIR (Fourier Transform Infrared Spectrometry), Model Nicolet 8700. The full spectra were recorded in ATR mode. 2.4. Particle analysis Filtergram (Patch Test) method had been applied for examination of the particles in the bearing grease. The grease sample was dissolved in a solvent and filtered with 1.0 lm membrane. Then the particles were examined under optical microscope. 2.5. Vibration analysis The vibration data was collected from the motors and the spectra were analysed by the VA (Vibration Analysis) software of the E-Monitor. 3. Analysis results In modern bearing current issues, brown deposit, corrugation marks and fluting patterns are the three common symptoms. Fig. 2 displays the three common symptoms on an outer ring segment. The natures of each symptom have been studied. 3.1. The brown deposit and the arcing marks The grease hardening and the brown deposit in the steamfield motor bearings were prevalent. Fig. 3a shows the brown deposit on bearing outer ring. The brown deposit on the outer raceway was examined under stereomicroscope. Some microarcing marks left on the deposit can be identified (Fig. 3b). The similar arcing marks can also be found on the ball surface (Fig. 3c). The arcing marks are the sound evidence of electric discharge, confirming the electric current passage in the bearing. The brown deposit under ESEM is shown in Fig. 3d. The deposit was a thin film. In situ EDS analysis results on the deposit film are shown in Fig. 4. 3.2. FTIR analysis on the brown deposit The brown deposit film has also been analysed with FTIR spectrometer. The analysis results are shown in Fig. 5. 3.3. The oil degradation particles Some oil degradation particles had been found on the filtergram. A typical varnish particle and a carbonaceous particle are shown in Fig. 6a and b respectively. 3.4. The microscopic features of the corrugation marks The corrugation marks under ESEM is shown in Fig. 7. A series of different sizes of the microcraters had been discovered. The microcrater diameters in this study ranged from < 2 lm to > 30 lm. Some smaller microcraters formed within a large
Fluting Pattern
Brown Deposit Film
Corrugation Marks
Fig. 2. The three symptoms of the modern bearing current (brown deposit, corrugation marks and fluting patterns) on the bearing segment.
W. Liu / Engineering Failure Analysis 45 (2014) 118–127
(a)
121
(b)
Micro-arcing Marks
Brown Deposit Film
(c)
(d) The Brown Deposit Film
EDS The Corrugation Marks (microcrater)
The Micro-aching Marks
Fig. 3. (a) The brown deposit on the outer ring; (b) the micro-arcing marks left on deposit film (stereomicroscope 10); (c) the corrugation mark (microcrater) and the micro-aching marks on the ball surface (optical microscope 100); (d) the brown deposit film at low magnification (ESEM 51).
Fig. 4. In situ EDS analysis results on the brown deposit film.
microcrater. The largest microcrater was up to 60 lm. Some ‘‘mud-crack’’ oxide film nearby the corrugation marks had been found (Fig. 8). 3.5. The microscopic features of the fluting patterns The fluting pattern did not appear on all the raceways. Only less than a quarter of the outer raceway had the fluting pattern. Fig. 9a shows the fluting pattern under stereomicroscope. At low resolution, the fluting pattern is composed of the ‘‘dark stripes’’ and the ‘‘bright stripes’’ alternatively. At high resolution (ESEM), the fluting pattern is composed of scratch marks (Fig. 9b). Fig. 10 shows a scratch mark at high magnification. The debris left on the bottom of the scratch mark has been identified. It is clear that the scratch marks were formed by three-body abrasive wear. In situ EDS1 analysis results on the debris (Fig. 11a) suggest that the debris was generated from the bearing steel. EDS2 analysis results on the bearing steel shows the similar results (Fig. 11b). The debris was most likely generated from the microcraters during the bearing current discharge.
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W. Liu / Engineering Failure Analysis 45 (2014) 118–127 *Bearing surface
0.40
C=C Band 2935.3 2850.9
0.35 0.30
Log (1/R)
CH2 Group
The Oil Degradation Bands
0.45
0.25
Nitration Band Oxidation Band
0.20
C-H Band
0.15
O-H Band 0.10 Thickener Band 0.05 0.00 -0.05 -0.10 4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1) Fig. 5. The FTIR spectra of the brown deposit film.
Fig. 6. The oil degradation particles under optical microscope (200): (a) a varnish particle (L150 lm); (b) a carbonaceous particle (L600 lm).
Fig. 7. The serial features of microcraters on the corrugation marks (ESEM 3000).
3.6. Vibration analysis The VA in the condition monitoring programme detected that the harmonics of Ball Pass Outer Ring (BPOR) frequency. Fig. 12 shows the frequency domain from a bearing with fluting pattern on the outer raceways. The two high peaks in the high frequency range were coincident with the harmonics of BPOR.
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Fig. 8. The ‘‘mud crack’’ oxide film nearby the corrugation mark (ESEM 6000).
(a) The “Dark Stripes”
(b) Less Dense Scratch Marks (The “Bright Stripe”)
The “Bright Stripe”
The “Bright Stripe”
Dense Scratch Marks (The “Dark Stripe”)
Fig. 9. The fluting patterns under microscopes: (a) the ‘‘dark stripes’’ and the’’ bright stripes’’ in fluting pattern (stereomicroscope 20); (b) the ‘‘dark stripe’’ with dense scratch marks and the ‘‘bright stripes’’ with less dense scratch mark (ESEM 600).
4. Discussions All of above analysis results were associated with the characteristics of the modern bearing current passage. Differentiating from the classic bearing current, the high frequency of modern bearing current was the root cause. In order to explain these characteristics, a hypothesis of bearing current skin effect was proposed. 4.1. The hypothesis of bearing current skin effect In PWM inverter, almost perfect square wave current was applied in the motors. The Fourier series for a square wave can be expressed as below [18]:
f ðtÞ ¼
1 X A npt sin L p n¼1;3;5;... n 4
According to above equation, a periodic square wave can be interpreted as a fundamental frequency imposed by infinite harmonics frequencies. Hence, the high frequency components in the modern bearing current are the key issue. Therefore, the skin effect of the high frequency current must be considered. The skin effect of high frequency current is well-known in electronics. But it has been previously neglected in interpretation of the motor bearing failures. The skin effect is a phenomenon of high frequency current to pass only on the conductor surface. The higher the frequency, the shallower the current distributes on the conductor surface. The skin depth is the parameter to describe the skin effect. The practical formula for skin depth is as follows [19]:
d 503
rffiffiffiffiffiffiffiffiffi
q lf
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EDS1
EDS2
Fig. 10. The scratch marks formed by the three-body abrasive wear and the debris on the bottom of the scratch mark (ESEM 5000).
Fig. 11. In situ EDS analysis results: (a) on the debris; (b) on the bearing steel.
The Harmonics of BPOR Frequency
Fig. 12. The BPOR harmonics peaks in the frequency domain from a bearing with the fluting pattern on the outer raceway.
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For steels, if the frequency is 100 kHz, the skin depth is approximately 30 lm. If the frequency increases to 1 MHz, the skin depth decreases to approximately 10 lm [19]. In PWM inverter, the frequencies can be in the MHz range [20]. Hence, modern bearing current mainly passes on the very shallow steel surface. Even though the bearing current intensity is low to the mA range, the in-situ heat concentration on the very shallow surface cannot be neglected. The accumulation of heat due to the bearing current skin effect, plus the heat generated from the roller friction, would result in the in-situ high temperatures. Hence, the grease/oil degradation is accelerated by the bearing current skin effect. Literature indicates that a current of 0.01 A is capable of reducing the service life of the bearings up to 20% [21]. Without the bearing current skin effect being taken into account, it is difficult to understand how such low current can significantly damage the bearings. Rolling element bearings work in EDL (elastohydrodynamic lubrication) regime [22]. The effective thickness of the oil film for EDL [23] is within the order of the above skin depth. From this point, the heat of the bearing current skin effect would mainly concentrate on the effective oil film. Hence, the lubrication deterioration would be accelerated by the bearing current skin effect. Sooner or later, the bearings would fail due to improper lubrication. Therefore, the main culprit of lubrication destruction in the modern bearing current was the skin effect. 4.2. The root cause of the grease hardening and the brown deposit Although some studies on the grease hardening in other motor bearings had been done [24,25], the brown deposit on bearing surface has not been reported. FTIR analysis on the harden grease showed that the grease had suffered from high temperatures [24]. But the root cause of the high temperature had not been pinpointed. Bearing grease hardening is more prevalent in VFD motors. The reason why the bearing grease is prone to be hardened had not been well established. However, with the hypothesis of bearing current skin effect, the mystery of grease hardening can be easily uncovered. The high temperatures due to the skin effect severely degrade the grease/oil, resulting in the grease hardening and the brown deposit. From the EDS analysis results (Fig. 4), the majority carbon content indicated that the deposit was organic substances generated from the bearing grease. Significant amount of oxygen suggests the severe oxidation of the grease/oil. Other elements, i.e. Ca, S, K, Na and Mg were from the grease/oil additives. 4.3. FTIR spectra The FTIR analysis results (Fig. 5) further confirm that the deposit was from the oil degradation. The band at 3441 cm1 is associated with the hydroxystearate thickener in the grease [24]. The stretching bands between 2800 cm1 and 3100 cm1 are the CAH bond [26]. Also the bands at 1450 cm1, 1375 cm1 and 730 cm1 associated with the CH2 groups. These data confirms the organic nature of the deposit. The band between 1800 cm1 and 1670 cm1 is in the carbonyl (C@O) region, an indication of oil oxidation (Oxidation Index) [27]. The band between 1650 cm1 and 1600 cm1 is the Nitration Index [27]. The Oxidation Index and Nitration Index are widely applied for quantifying the oil degradation in used oil analysis. The two significant high and overlapped bands at Oxidation and Nitration Indexes were the evidence of the grease/oil severe degradation, suggesting that the deposit was the degradation products. In the oil degradation process, varnish particles would form due to polymerization of the oil degradation products. Nitration Index is an indication of high temperatures. In used lubricant analysis, the peak of Oxidation Index band is usually higher than that of Nitration Index. However, in this case study the peak of Nitration Index was much higher than that of Oxidation Index. This unusual result suggests that the grease/oil had been subjected to extremely high temperatures. The bands of 1570 cm1 is C@C double bond [28]. The high peaks in this band suggested that aromatic compounds had formed. The aromatic reaction is the final stage of oil degradation [29,30]. The oxidation band, nitration band and the C@C band compose of the oil degradation bands, indicating the severe oil degradation in the bearing grease. Aromatic reaction generates carbonaceous deposits. Some black solid products in the harden grease could be the carbonaceous substances. If aromatic reaction occurred, some carbonaceous particles would be present in the grease. The varnish particles and the carbonaceous particles found in the filtergram (Fig. 6) further confirm the oil degradation process. From all above discussion, it can be concluded that due to the heat generated by the skin effect, the lubricating oil in the grease had been ‘‘cooked’’, resulting the brown deposit and the grease hardening. 4.4. The serial features of the microcraters The corrugation marks (macroscale) are the symptoms of bearing current discharge [14,31], which exist in both classic [32] and modern bearing current issues. In microscale, the corrugation marks composed of microcracters. The microcrater sizes in classic bearing current ranged from 1 to 4 lm [14]. The average microcrater diameters were 2.9 lm and 2.3 lm on the rollers and on the outer raceway respectively [33]. The serial features of the microcraters (Fig. 7) of the modern bearing current were different from the microcraters of classic bearing current. To the author’s knowledge, this series feature of the microcraters has not been previously reported.
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These serial characteristics of the microcraters can also be explained with the aforementioned Fourier series equation. The microcrater diameters are associated with discharge energy, i.e. the current intensity. As the low frequency component has high current intensity, while the high frequency component has low current intensity, therefore, the large microcraters associate with the low frequency components while the smaller microcraters with the high frequency components. The ‘‘mud-crack’’ oxide film near the corrugation mark was the evidence of high temperature (Fig. 8). The ‘mud-crack’ oxide scale is FeO, which is formed at high temperatures [34]. Hence, the lubricant’s damage due to high temperatures can be indirectly confirmed.
4.5. The nature of the fluting pattern The fluting, also called ‘‘washboard’’, is a common symptom in classic bearing current failures. However, the fluting pattern in modern bearing current is not as obvious as in the classic bearing current. In literature, there are 4 mechanisms to describe the fluting formation. However, the mechanisms of the fluting have not been well established. In the first mechanism, fluting is believed to be generated by EDM (Electrical Discharge Machining) [3]. This mechanism has been once accepted by the industry. In the second mechanism [6,7], it was proposed that the fluting was formed by discharge corrugation. The corrugation was developed from the slip bands. The process from slip bands to fluting pattern was accelerated by the passage of the current. In fact, both of the above mechanisms are incorrect as they cannot be proven through microscopic evidence. Furthermore, both mechanisms cannot explain the periodic characteristics of the fluting patterns. Based on the periodic characteristics of the fluting pattern, other mechanisms associated to bearing vibrations have been developed. In the third mechanism [33], the two stages of fluting formation had been proposed. In the first stage, the microcraters have formed due to the sparks. In the second stage, when the rolling elements are rolling over microcraters, the mechanical resonance forms the fluting pattern. In the fourth mechanism, there are two types of fluting [10]. Type 1 is the fluting caused by electric current; and Type 2 is the fluting induced by vibration. Type 1 is found on the dark bottom of the corrugations, as opposed to the bright or rusty appearance at the bottom of the Type 2. However, at which frequencies and how the bearing vibrations generate the fluting have not described in aforementioned mechanisms. According to above descriptions, Type 1 fluting in the fourth mechanism should be the same as the third mechanism. In the author’s opinion, the classic bearing current is prone to form the Type 1 fluting, while modern bearing current likely form Type 2 fluting. It is practically well-known that the occurrence of the fluting pattern yields noisy bearings [21]. The noise could be due to the lubricant destruction by the bearing current skin effect. In this case study, the fluting pattern (Fig. 2) was generated by ball vibrations, being similar to the Type 2 in the fourth mechanism. The ball vibration enhanced the scratch marks on the outer raceway. It is obvious that there is neither an EDM mark nor corrugation mark (microcrater) in the fluting patterns. Therefore, the first 3 aforementioned mechanisms are not justifiable in this case study. The nature of the fluting pattern was the different dense of scratch marks in this study. The ‘‘dark stripe’’ had dense scratch marks while the ‘‘bright stripe’’ had less dense scratch marks (Fig. 9). As less light is reflected from the dense scratched area, the ‘‘dark stripes’’ are observed. This reason is vice versa for the ‘‘bright stripes’’. The fluting patterns were due to the light reflection contrast between the two alternative stripe patterns.
4.6. The ball vibration The harmonics of Ball Pass Outer Ring (BPOR) frequency was detected (Fig. 12), indicating that the fluting pattern on the outer raceway was associated with the ball vibration. The application of VA to monitoring fluting has not been reported. This discovery may be applied to detect the fluting in VA condition monitoring. VA analysis further confirms that this type of fluting was induced by the ball vibration. The ball vibration on the outer ring enhanced the three-body wear, forming the stripes of dense scratch marks (Fig. 9b). Therefore, the fluting pattern was formed by the three-body abrasive wear. The Ball Pass Outer Ring vibration enhanced the wear. How the bearing current induces the ball vibrations upon the raceway is unknown.
5. Conclusions The prevalent bearing premature failures in VFD motors were due to the high frequency current passage. ESEM/EDS and FTIR analyses indicated that the brown deposit was from the grease/oil degradation. In FTIR spectra, the oxidation band, nitration band and the C@C band compose of the oil degradation bands, indicating the severe oil degradation in the bearing grease. A hypothesis of bearing current skin effect has been proposed to explain lubrication destruction. This theory can explain why bearing grease is so prone to be hardened in VFD motors.
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The serial features of microcraters in the corrugation marks have been discovered. These serial features can be explained by Fourier series. The large microcraters associated with the low frequency components while the smaller microcraters with the high frequency components. The nature of the fluting patterns has been uncovered in this study, excluding the previous theories of both EDM (Electrical Discharge Machining) and discharge corrugation. Vibration analysis detected the harmonics of the BPOR (Ball Pass Outer Ring) frequency which associated with the fluting on the outer raceway. The fluting pattern was composed of the scratch marks generated by the three-body wear. The ball pass vibration enhanced the wear. Acknowledgements The author is grateful to Catherine Hobbis at Research Centre for Surface and Material Science, The University of Auckland for her assistance in the ESEM/EDS operations. The author acknowledges for using the vibration database collected by the condition monitoring team at SGS New Zealand Ltd in their routine work. Reference [1] Toshiba’s Informative application guideline – Commom causes of motor failure. http://toshont.com/ag/mtrapplication/AG04%28CausesofMotor Failure%29.pdf. [Attracted May 2014]. [2] Bearing currents in modern AC drive systems, ABB Technical Guide No. 5, 1999. [3] Charoy A, Dunand P, Bearing current induced by a power drive. Automative power electronics, September 26 & 27, 2007-Paris. [4] Schiferl R, Melfi M, Avoiding current-induced bearing failure in electric motor-driven paper mill systems. Pulp & Paper, October 2002. [5] Oh HW, Willwerth A. Shaft grounding – A solution to motor bearing currents, ASHRAE Transaction V. 114, Part 2, p. 246–50. [6] Prashad H. Analysis of the effects of an electric current on contact temperature, contact stress and slip band initiation on the roller tracks of roller bearings. Wear 1989;V131:1–14. [7] Prashad H. Determination of time span for the appearance of flutings on the track surface of rolling-element bearings under the influence of electric current. Tribol Trans 1998;V41(1):103–9. [8] Bearing failures and their causes, SKF Publication PI 401 E, 1994. [9] Care and Maintenance of bearings, NTN Corporation, CAT.NO.3017/E, 1996. [10] SKF website, Bearing failures and their causes, Passage of electric current. [Attracted October 2013]. [11] Evens RD. Classic bearing damage modes. The Timken Company; 2011. [12] Electrically insulated bearings from SKF, SKF Publication 6160EN, May 2006. [13] Hybrid bearings for electrical machinery, SKF Publication 5128E, May 2001. [14] Weigand M. Lubrication of rolling bearings – technical solutions for critical running conditions. Machine Lubrication Magazine, January 2006. [15] Chen S, Lipo T, Novotny D. Circulating type motor bearing current in inverter drives. IEEE – IAS Conference San Diego, CA October; 1996. [16] Busse D et al. An evaluation of the electrostatic shielded induction motor: a solution for rotor shaft voltage buildup and bearing current. IEEE IAS Conference San Diego, CA October; 1996. [17] Erdman J et al. Effective of PWM inverter on AC motor bearing currents and shaft voltage, IEEE APEC Conference Dallas. TX March; 1995. [18] Arfken G. Fourier series. Ch. 14 in mathematical methods for physicists, 3rd ed. Orlando, FL: Academic Press; 1985. p. 760–93. [19] Hart HW. Engineering electromagnetics. 7th ed. New York: McGraw Hill; 2006. p. 83–4. [20] Inverter-driven induction motors shaft and bearing current solutions, Baldor Electric Company, Industry white paper, RAPS 868. [21] Summers-Smith JD. A tribology casebook. Bury St. Edmunds: Mechanical Engineering publications; 1997, ISBN 1-86058-041-6. [22] Szeri AZ. Hydrodynamic and elastohydrodynamic lubrication. Bhushan B. Modern Tribology Handbook, vol. 1. CRC Press; 2001. p449. [23] Rolling bearing lubrication. FAG Kugelfischer Georg Schäfer AG, Industrial Bearings and Services, Publ. No. WL 81 115/4 EA, p. 4. . [24] Zhi-Qiang Yu, Zhen-Guo Yang. Fatigue failure analysis of a grease-lubricated roller bearing from an electric motor. J Fail Anal Preven 2011;11:158–66. [25] Ost W, Baets PD. Failure analysis of the deep groove ball bearings of an electric motor. Eng Fail Anal 2005;12:772–83. [26] Diaby M et al. Impact factors for the degradation of engine oil causing carbonaceous deposits in the piston’s grooves of diesel engine. Fuel 2013;V107:P90–101. [27] Monitoring oil degradation with infrared spectroscopy. Wear Check Technical Bulletin Issue 18; 2000. http://www.wearcheck.com/literature/techdoc/ WZA018.pdf. [28] Diaby M et al. Understanding carbonaceous deposit formation resulting from engine oil degradation. Carbon 2009;V47:355–66. [29] Kouame SD et al. Deposit formation from the lubricant degradation: a uniform layer deposition model, Lubrication Science; 2013. . doi: 10.1002/1s.1247. [30] Kouame SD, Eric Liu E. Characterization of fully and partially additized lubricant deposits by temperature programmed oxidation. Tribol Int 2014;V72:P58–64. [31] Boyanton H. Bearing damage due to electric discharge – electric damage machining of bearings. Albany, OR: Shaft Grounding Systems, Inc.; 1995. http://electrical fluting.com/techdocs/BDED_pg1. [32] Prashad H. Appearance of craters on track surface of rolling element bearings by spark erosion. Tribol Int 2001;V34:39–47. [33] Didenko T, Pridemore WD. Electrical fluting failure of a Tri-lobe roller bearing. J Fail Anal Preven 2012;12:575–80. [34] Liu W. Failure analysis on the economisers of a biomass fuel boiler. Eng Fail Anal 2013;31:101–17.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
The prevalent motor bearing premature failures due to the high frequency electric current passage William Liu ⇑ Industrial Services (NDT & Materials), SGS New Zealand Ltd., Auckland, New Zealand
a r t i c l e
i n f o
Article history: Received 10 April 2014 Received in revised form 2 June 2014 Accepted 25 June 2014 Available online 9 July 2014 Keywords: Bearing current Fluting Microcrater Skin effect
a b s t r a c t Bearing premature failures are prevalent in VFD (Variable Frequency Drives) motors. The three common symptoms of the modern bearing current appeared on the bearings of a group of pump motors. The bearing failure mechanisms have been studied from the tribological point of view. The natures of the three common symptoms have been uncovered. A hypothesis of the skin effect has been proposed to explain the three common symptoms. The discovery of the serial microcraters reveals the discharge with Fourier series features. The nature of the fluting patterns excluded the theories of both EDM (Electrical Discharge Machining) and discharge corrugation. VA (Vibration Analysis) detected the harmonics of the BPOR (Ball Pass Outer Ring) frequency which associated with the fluting on the outer raceway. This discovery may be applied to detect fluting in condition monitoring. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Statistical data shows that over 60% of motor failures were bearing failures [1]. Since the application of Variable Frequency Drives (VFD) in contemporary motors, motor bearing premature failures have been prevalent due to the electrical current passage [2]. The bearing failures due to the electric current passage have been recognised for almost a century. The fluting phenomena in motor bearings were first discovered in the 1920s [3]. Subsequently, the bearing current issues in AC motors have been studied [4–7]. The various bearing symptoms have been listed in the bearing manufacturer’s documents [8–11]. The practical solutions to bearing current, such as shaft grounding to bypass the current [2], ceramic-coated bearings [12] and hybrid bearings [13] for electric insulation, appeared to be effective. However, with the application of the fast switching PWM inverter in contemporary motors, the traditional solutions seems to be no longer effective for the high frequency bearing current [14]. In order to differentiate these two different kinds of bearing current, for the convenient discussion in this paper, the sinusoidal AC bearing current is referred to as the classic bearing current, and the high frequency non-sinusoidal current as the modern bearing current. The research of modern bearing current had once been an active topic. Most of the studies focused on the bearing current sources from the electronic perspective [15–17], such as common mode voltage. However, fundamental research in modern bearing current issues from the tribological perspective has been very scarce. The difficulty lays in the complex failure mechanisms. In laboratory simulation tests, it is extremely difficult to separate a single mechanism from the complex ⇑ Tel.: +64 9 635 0303; fax: +64 9 636 6250. E-mail address: [email protected] http://dx.doi.org/10.1016/j.engfailanal.2014.06.021 1350-6307/Ó 2014 Elsevier Ltd. All rights reserved.
W. Liu / Engineering Failure Analysis 45 (2014) 118–127
119
Nomenclature f t A L d
l q
frequency of the current (Hz) time (s) half amplitude of the current intensity (ampere) – I/2 1/2 the period (2L = 1/f) skin depth (m) relative permeability of the medium resistivity of the medium (X m)
mechanisms. Disregarding the simulation tests, on the other hand, scenarios and examples encountered in industry can be another path to understanding the fundamental failure mechanisms. This paper presents some research on the motor bearing failure mechanisms due to modern bearing current passage from an industrial case study. The bearing premature failures occurred in a group of pump motors in the steamfields of a geothermal power plant in New Zealand. The motors were 250 kW with pulse width modulated (PWM) inverter VFD. The bearing numbers were 6318 (NDE) and 6320 (DE). Most of the bearings failed in approximately two years of service. Visual inspection found that the bearing grease had been hardened or partially solidified. The bearing surfaces had been deposited with a brown film. From the symptoms of the brown deposit and the grease hardening, bearing failures due to the high frequency electric current passage was certain. Hence, the failed bearings can be applied in researching the fundamental failure mechanism. 2. Experimental methods There were in total 20 of such motors. Vibration analysis in the condition monitoring programmes detected bearing deterioration. Before the motors break down, the problematic bearings had been replaced. After the bearing grease had been ultrasonically cleaned, bearing coupons were cut from both the inner and outer rings. Fig. 1 shows a typical bearing coupon. Various analyses have been conducted on the failed bearing coupons. 2.1. LOM LOM (Light Optical Microscopes), both stereomicroscopic and metallographic microscope, had been applied to examine the bearing coupons. 2.2. ESEM/EDS The bearing coupons had also been examined under ESEM (Environmental Scanning Electronic Microscope); the model was an FEI Quanta 200 F with field emission gun. The EDS (Energy Dispersive Spectroscope) detector was EDAX brand SiLi (Lithium drifted) with a Super Ultra Thin Window.
Fig. 1. The failed DE bearing covering with the brown deposit.
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2.3. FTIR The brown deposit on the failed bearing had been examined with FTIR (Fourier Transform Infrared Spectrometry), Model Nicolet 8700. The full spectra were recorded in ATR mode. 2.4. Particle analysis Filtergram (Patch Test) method had been applied for examination of the particles in the bearing grease. The grease sample was dissolved in a solvent and filtered with 1.0 lm membrane. Then the particles were examined under optical microscope. 2.5. Vibration analysis The vibration data was collected from the motors and the spectra were analysed by the VA (Vibration Analysis) software of the E-Monitor. 3. Analysis results In modern bearing current issues, brown deposit, corrugation marks and fluting patterns are the three common symptoms. Fig. 2 displays the three common symptoms on an outer ring segment. The natures of each symptom have been studied. 3.1. The brown deposit and the arcing marks The grease hardening and the brown deposit in the steamfield motor bearings were prevalent. Fig. 3a shows the brown deposit on bearing outer ring. The brown deposit on the outer raceway was examined under stereomicroscope. Some microarcing marks left on the deposit can be identified (Fig. 3b). The similar arcing marks can also be found on the ball surface (Fig. 3c). The arcing marks are the sound evidence of electric discharge, confirming the electric current passage in the bearing. The brown deposit under ESEM is shown in Fig. 3d. The deposit was a thin film. In situ EDS analysis results on the deposit film are shown in Fig. 4. 3.2. FTIR analysis on the brown deposit The brown deposit film has also been analysed with FTIR spectrometer. The analysis results are shown in Fig. 5. 3.3. The oil degradation particles Some oil degradation particles had been found on the filtergram. A typical varnish particle and a carbonaceous particle are shown in Fig. 6a and b respectively. 3.4. The microscopic features of the corrugation marks The corrugation marks under ESEM is shown in Fig. 7. A series of different sizes of the microcraters had been discovered. The microcrater diameters in this study ranged from < 2 lm to > 30 lm. Some smaller microcraters formed within a large
Fluting Pattern
Brown Deposit Film
Corrugation Marks
Fig. 2. The three symptoms of the modern bearing current (brown deposit, corrugation marks and fluting patterns) on the bearing segment.
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(a)
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(b)
Micro-arcing Marks
Brown Deposit Film
(c)
(d) The Brown Deposit Film
EDS The Corrugation Marks (microcrater)
The Micro-aching Marks
Fig. 3. (a) The brown deposit on the outer ring; (b) the micro-arcing marks left on deposit film (stereomicroscope 10); (c) the corrugation mark (microcrater) and the micro-aching marks on the ball surface (optical microscope 100); (d) the brown deposit film at low magnification (ESEM 51).
Fig. 4. In situ EDS analysis results on the brown deposit film.
microcrater. The largest microcrater was up to 60 lm. Some ‘‘mud-crack’’ oxide film nearby the corrugation marks had been found (Fig. 8). 3.5. The microscopic features of the fluting patterns The fluting pattern did not appear on all the raceways. Only less than a quarter of the outer raceway had the fluting pattern. Fig. 9a shows the fluting pattern under stereomicroscope. At low resolution, the fluting pattern is composed of the ‘‘dark stripes’’ and the ‘‘bright stripes’’ alternatively. At high resolution (ESEM), the fluting pattern is composed of scratch marks (Fig. 9b). Fig. 10 shows a scratch mark at high magnification. The debris left on the bottom of the scratch mark has been identified. It is clear that the scratch marks were formed by three-body abrasive wear. In situ EDS1 analysis results on the debris (Fig. 11a) suggest that the debris was generated from the bearing steel. EDS2 analysis results on the bearing steel shows the similar results (Fig. 11b). The debris was most likely generated from the microcraters during the bearing current discharge.
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0.40
C=C Band 2935.3 2850.9
0.35 0.30
Log (1/R)
CH2 Group
The Oil Degradation Bands
0.45
0.25
Nitration Band Oxidation Band
0.20
C-H Band
0.15
O-H Band 0.10 Thickener Band 0.05 0.00 -0.05 -0.10 4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1) Fig. 5. The FTIR spectra of the brown deposit film.
Fig. 6. The oil degradation particles under optical microscope (200): (a) a varnish particle (L150 lm); (b) a carbonaceous particle (L600 lm).
Fig. 7. The serial features of microcraters on the corrugation marks (ESEM 3000).
3.6. Vibration analysis The VA in the condition monitoring programme detected that the harmonics of Ball Pass Outer Ring (BPOR) frequency. Fig. 12 shows the frequency domain from a bearing with fluting pattern on the outer raceways. The two high peaks in the high frequency range were coincident with the harmonics of BPOR.
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Fig. 8. The ‘‘mud crack’’ oxide film nearby the corrugation mark (ESEM 6000).
(a) The “Dark Stripes”
(b) Less Dense Scratch Marks (The “Bright Stripe”)
The “Bright Stripe”
The “Bright Stripe”
Dense Scratch Marks (The “Dark Stripe”)
Fig. 9. The fluting patterns under microscopes: (a) the ‘‘dark stripes’’ and the’’ bright stripes’’ in fluting pattern (stereomicroscope 20); (b) the ‘‘dark stripe’’ with dense scratch marks and the ‘‘bright stripes’’ with less dense scratch mark (ESEM 600).
4. Discussions All of above analysis results were associated with the characteristics of the modern bearing current passage. Differentiating from the classic bearing current, the high frequency of modern bearing current was the root cause. In order to explain these characteristics, a hypothesis of bearing current skin effect was proposed. 4.1. The hypothesis of bearing current skin effect In PWM inverter, almost perfect square wave current was applied in the motors. The Fourier series for a square wave can be expressed as below [18]:
f ðtÞ ¼
1 X A npt sin L p n¼1;3;5;... n 4
According to above equation, a periodic square wave can be interpreted as a fundamental frequency imposed by infinite harmonics frequencies. Hence, the high frequency components in the modern bearing current are the key issue. Therefore, the skin effect of the high frequency current must be considered. The skin effect of high frequency current is well-known in electronics. But it has been previously neglected in interpretation of the motor bearing failures. The skin effect is a phenomenon of high frequency current to pass only on the conductor surface. The higher the frequency, the shallower the current distributes on the conductor surface. The skin depth is the parameter to describe the skin effect. The practical formula for skin depth is as follows [19]:
d 503
rffiffiffiffiffiffiffiffiffi
q lf
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EDS1
EDS2
Fig. 10. The scratch marks formed by the three-body abrasive wear and the debris on the bottom of the scratch mark (ESEM 5000).
Fig. 11. In situ EDS analysis results: (a) on the debris; (b) on the bearing steel.
The Harmonics of BPOR Frequency
Fig. 12. The BPOR harmonics peaks in the frequency domain from a bearing with the fluting pattern on the outer raceway.
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For steels, if the frequency is 100 kHz, the skin depth is approximately 30 lm. If the frequency increases to 1 MHz, the skin depth decreases to approximately 10 lm [19]. In PWM inverter, the frequencies can be in the MHz range [20]. Hence, modern bearing current mainly passes on the very shallow steel surface. Even though the bearing current intensity is low to the mA range, the in-situ heat concentration on the very shallow surface cannot be neglected. The accumulation of heat due to the bearing current skin effect, plus the heat generated from the roller friction, would result in the in-situ high temperatures. Hence, the grease/oil degradation is accelerated by the bearing current skin effect. Literature indicates that a current of 0.01 A is capable of reducing the service life of the bearings up to 20% [21]. Without the bearing current skin effect being taken into account, it is difficult to understand how such low current can significantly damage the bearings. Rolling element bearings work in EDL (elastohydrodynamic lubrication) regime [22]. The effective thickness of the oil film for EDL [23] is within the order of the above skin depth. From this point, the heat of the bearing current skin effect would mainly concentrate on the effective oil film. Hence, the lubrication deterioration would be accelerated by the bearing current skin effect. Sooner or later, the bearings would fail due to improper lubrication. Therefore, the main culprit of lubrication destruction in the modern bearing current was the skin effect. 4.2. The root cause of the grease hardening and the brown deposit Although some studies on the grease hardening in other motor bearings had been done [24,25], the brown deposit on bearing surface has not been reported. FTIR analysis on the harden grease showed that the grease had suffered from high temperatures [24]. But the root cause of the high temperature had not been pinpointed. Bearing grease hardening is more prevalent in VFD motors. The reason why the bearing grease is prone to be hardened had not been well established. However, with the hypothesis of bearing current skin effect, the mystery of grease hardening can be easily uncovered. The high temperatures due to the skin effect severely degrade the grease/oil, resulting in the grease hardening and the brown deposit. From the EDS analysis results (Fig. 4), the majority carbon content indicated that the deposit was organic substances generated from the bearing grease. Significant amount of oxygen suggests the severe oxidation of the grease/oil. Other elements, i.e. Ca, S, K, Na and Mg were from the grease/oil additives. 4.3. FTIR spectra The FTIR analysis results (Fig. 5) further confirm that the deposit was from the oil degradation. The band at 3441 cm1 is associated with the hydroxystearate thickener in the grease [24]. The stretching bands between 2800 cm1 and 3100 cm1 are the CAH bond [26]. Also the bands at 1450 cm1, 1375 cm1 and 730 cm1 associated with the CH2 groups. These data confirms the organic nature of the deposit. The band between 1800 cm1 and 1670 cm1 is in the carbonyl (C@O) region, an indication of oil oxidation (Oxidation Index) [27]. The band between 1650 cm1 and 1600 cm1 is the Nitration Index [27]. The Oxidation Index and Nitration Index are widely applied for quantifying the oil degradation in used oil analysis. The two significant high and overlapped bands at Oxidation and Nitration Indexes were the evidence of the grease/oil severe degradation, suggesting that the deposit was the degradation products. In the oil degradation process, varnish particles would form due to polymerization of the oil degradation products. Nitration Index is an indication of high temperatures. In used lubricant analysis, the peak of Oxidation Index band is usually higher than that of Nitration Index. However, in this case study the peak of Nitration Index was much higher than that of Oxidation Index. This unusual result suggests that the grease/oil had been subjected to extremely high temperatures. The bands of 1570 cm1 is C@C double bond [28]. The high peaks in this band suggested that aromatic compounds had formed. The aromatic reaction is the final stage of oil degradation [29,30]. The oxidation band, nitration band and the C@C band compose of the oil degradation bands, indicating the severe oil degradation in the bearing grease. Aromatic reaction generates carbonaceous deposits. Some black solid products in the harden grease could be the carbonaceous substances. If aromatic reaction occurred, some carbonaceous particles would be present in the grease. The varnish particles and the carbonaceous particles found in the filtergram (Fig. 6) further confirm the oil degradation process. From all above discussion, it can be concluded that due to the heat generated by the skin effect, the lubricating oil in the grease had been ‘‘cooked’’, resulting the brown deposit and the grease hardening. 4.4. The serial features of the microcraters The corrugation marks (macroscale) are the symptoms of bearing current discharge [14,31], which exist in both classic [32] and modern bearing current issues. In microscale, the corrugation marks composed of microcracters. The microcrater sizes in classic bearing current ranged from 1 to 4 lm [14]. The average microcrater diameters were 2.9 lm and 2.3 lm on the rollers and on the outer raceway respectively [33]. The serial features of the microcraters (Fig. 7) of the modern bearing current were different from the microcraters of classic bearing current. To the author’s knowledge, this series feature of the microcraters has not been previously reported.
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These serial characteristics of the microcraters can also be explained with the aforementioned Fourier series equation. The microcrater diameters are associated with discharge energy, i.e. the current intensity. As the low frequency component has high current intensity, while the high frequency component has low current intensity, therefore, the large microcraters associate with the low frequency components while the smaller microcraters with the high frequency components. The ‘‘mud-crack’’ oxide film near the corrugation mark was the evidence of high temperature (Fig. 8). The ‘mud-crack’ oxide scale is FeO, which is formed at high temperatures [34]. Hence, the lubricant’s damage due to high temperatures can be indirectly confirmed.
4.5. The nature of the fluting pattern The fluting, also called ‘‘washboard’’, is a common symptom in classic bearing current failures. However, the fluting pattern in modern bearing current is not as obvious as in the classic bearing current. In literature, there are 4 mechanisms to describe the fluting formation. However, the mechanisms of the fluting have not been well established. In the first mechanism, fluting is believed to be generated by EDM (Electrical Discharge Machining) [3]. This mechanism has been once accepted by the industry. In the second mechanism [6,7], it was proposed that the fluting was formed by discharge corrugation. The corrugation was developed from the slip bands. The process from slip bands to fluting pattern was accelerated by the passage of the current. In fact, both of the above mechanisms are incorrect as they cannot be proven through microscopic evidence. Furthermore, both mechanisms cannot explain the periodic characteristics of the fluting patterns. Based on the periodic characteristics of the fluting pattern, other mechanisms associated to bearing vibrations have been developed. In the third mechanism [33], the two stages of fluting formation had been proposed. In the first stage, the microcraters have formed due to the sparks. In the second stage, when the rolling elements are rolling over microcraters, the mechanical resonance forms the fluting pattern. In the fourth mechanism, there are two types of fluting [10]. Type 1 is the fluting caused by electric current; and Type 2 is the fluting induced by vibration. Type 1 is found on the dark bottom of the corrugations, as opposed to the bright or rusty appearance at the bottom of the Type 2. However, at which frequencies and how the bearing vibrations generate the fluting have not described in aforementioned mechanisms. According to above descriptions, Type 1 fluting in the fourth mechanism should be the same as the third mechanism. In the author’s opinion, the classic bearing current is prone to form the Type 1 fluting, while modern bearing current likely form Type 2 fluting. It is practically well-known that the occurrence of the fluting pattern yields noisy bearings [21]. The noise could be due to the lubricant destruction by the bearing current skin effect. In this case study, the fluting pattern (Fig. 2) was generated by ball vibrations, being similar to the Type 2 in the fourth mechanism. The ball vibration enhanced the scratch marks on the outer raceway. It is obvious that there is neither an EDM mark nor corrugation mark (microcrater) in the fluting patterns. Therefore, the first 3 aforementioned mechanisms are not justifiable in this case study. The nature of the fluting pattern was the different dense of scratch marks in this study. The ‘‘dark stripe’’ had dense scratch marks while the ‘‘bright stripe’’ had less dense scratch marks (Fig. 9). As less light is reflected from the dense scratched area, the ‘‘dark stripes’’ are observed. This reason is vice versa for the ‘‘bright stripes’’. The fluting patterns were due to the light reflection contrast between the two alternative stripe patterns.
4.6. The ball vibration The harmonics of Ball Pass Outer Ring (BPOR) frequency was detected (Fig. 12), indicating that the fluting pattern on the outer raceway was associated with the ball vibration. The application of VA to monitoring fluting has not been reported. This discovery may be applied to detect the fluting in VA condition monitoring. VA analysis further confirms that this type of fluting was induced by the ball vibration. The ball vibration on the outer ring enhanced the three-body wear, forming the stripes of dense scratch marks (Fig. 9b). Therefore, the fluting pattern was formed by the three-body abrasive wear. The Ball Pass Outer Ring vibration enhanced the wear. How the bearing current induces the ball vibrations upon the raceway is unknown.
5. Conclusions The prevalent bearing premature failures in VFD motors were due to the high frequency current passage. ESEM/EDS and FTIR analyses indicated that the brown deposit was from the grease/oil degradation. In FTIR spectra, the oxidation band, nitration band and the C@C band compose of the oil degradation bands, indicating the severe oil degradation in the bearing grease. A hypothesis of bearing current skin effect has been proposed to explain lubrication destruction. This theory can explain why bearing grease is so prone to be hardened in VFD motors.
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The serial features of microcraters in the corrugation marks have been discovered. These serial features can be explained by Fourier series. The large microcraters associated with the low frequency components while the smaller microcraters with the high frequency components. The nature of the fluting patterns has been uncovered in this study, excluding the previous theories of both EDM (Electrical Discharge Machining) and discharge corrugation. Vibration analysis detected the harmonics of the BPOR (Ball Pass Outer Ring) frequency which associated with the fluting on the outer raceway. The fluting pattern was composed of the scratch marks generated by the three-body wear. The ball pass vibration enhanced the wear. Acknowledgements The author is grateful to Catherine Hobbis at Research Centre for Surface and Material Science, The University of Auckland for her assistance in the ESEM/EDS operations. The author acknowledges for using the vibration database collected by the condition monitoring team at SGS New Zealand Ltd in their routine work. Reference [1] Toshiba’s Informative application guideline – Commom causes of motor failure. http://toshont.com/ag/mtrapplication/AG04%28CausesofMotor Failure%29.pdf. [Attracted May 2014]. [2] Bearing currents in modern AC drive systems, ABB Technical Guide No. 5, 1999. [3] Charoy A, Dunand P, Bearing current induced by a power drive. Automative power electronics, September 26 & 27, 2007-Paris. [4] Schiferl R, Melfi M, Avoiding current-induced bearing failure in electric motor-driven paper mill systems. Pulp & Paper, October 2002. [5] Oh HW, Willwerth A. Shaft grounding – A solution to motor bearing currents, ASHRAE Transaction V. 114, Part 2, p. 246–50. [6] Prashad H. Analysis of the effects of an electric current on contact temperature, contact stress and slip band initiation on the roller tracks of roller bearings. Wear 1989;V131:1–14. [7] Prashad H. Determination of time span for the appearance of flutings on the track surface of rolling-element bearings under the influence of electric current. Tribol Trans 1998;V41(1):103–9. [8] Bearing failures and their causes, SKF Publication PI 401 E, 1994. [9] Care and Maintenance of bearings, NTN Corporation, CAT.NO.3017/E, 1996. [10] SKF website, Bearing failures and their causes, Passage of electric current.