A Brief Review on the Tribological Effects of Electrically Induced Bearing Damage

Tribology, the fundamental science and study of friction, wear, and lubrication, plays a vital role in ensuring the reliable and efficient operation of mechanical systems. At the heart of many tribological systems are bearings, which serve as critical components for improving mechanical efficiency through minimizing friction and supporting loads. Bearings enable smooth rotation or linear motion by reducing contact between moving parts and distributing forces evenly. They are found in a wide range of applications, from automotive engines, industrial machinery to aerospace systems and household appliances. Understanding the behavior and performance of bearings is essential for optimizing system efficiency, reducing wear, and enhancing the overall reliability and lifespan of mechanical systems. Reliable bearings are crucial for not only noise and vibration reduction but also for the safe operation of the machinery by providing stable and predictable motion thereby reducing the risk of failures. Through advancements in material science, lubrication strategies, and experimental techniques, researchers and engineers continue to push the boundaries of tribology and bearings, paving the way for more innovative and sustainable solutions in various industries [1,2].

Each year, approximately 10 billion bearings are produced globally. However, providing the bearings are correctly installed, fit for the sized operating conditions, and free from contamination only a small percentage of these bearings actually fail. The majority of bearings, around 90%, outlast the equipment in which they are installed, and some bearings are replaced as a precautionary measure for preventive reasons. Only about 0.5% of bearings are replaced due to damage or failure. This translates to approximately 50 million bearings being replaced annually as a result of damage or failure [3]. Generally, bearings fail as a result of fatigue and poor lubrication, some fail due to seal failure and contamination aftereffects, and some fail due to improper handling and installation, incorrectly sized for the operating load and environment. In each application and industry, some failure modes are more prominent than others, for example, in wind turbines, one of the major causes of bearings failures is related to overloading as a result of random sudden gusts of wind whereas in the paper and pulp industry, it is contamination [3,4]. One topic of bearing damage that has recently got a lot of interest in the research community is the electrically induced bearing damage (EIBD) as found in rolling element bearings. Electrically induced bearing damage refers to the harmful effects caused by stray currents in electric motors that flow through the motor bearings, leading to the thermal decomposition of lubricants and hardware damage [5–7]. Mitigating this damage is essential for ensuring the durability and reliable performance of electric motors in diverse applications such as electric vehicles, wind turbines, and some aviation applications [5,8–10]. In the last 15 years, there have been many comprehensive literature reviews on the EIBD phenomena, to name a few [5,8,11–15] and these published media include academic studies and industrial development. This article primarily focuses on briefly summarizing these literature reviews and discusses recent published work pertaining to the tribological effects of electrically induced rolling element bearing wear, including both academic and industrial development. Furthermore, it supplements the previous reviews by emphasizing pertinent work that has been published earlier and aims to examine the impact of stray currents on lubricants, hardware damage, and the overall performance and durability of bearings in electric motor applications. While not aiming to be exhaustive and a rewrite of previous recent literature reviews, this article offers a summary of the different aspects explored in the existing literature concerning the subject of bearing currents. Additionally, it will provide tribological implications and reasoning at relevant intervals. For further in-depth reading on the topic of bearing currents, it is recommended that the readers consult the comprehensive reviews as referenced earlier.

The first working electric motor was introduced in the earlier half of the nineteenth century by an American Blacksmith called Thomas Davenport [16], and the first scientific papers on bearing currents started to emerge in the early 1900s [17–20] which discussed initial thoughts on the shaft current causes and damages, and the first known mention of lubricant influence on the EIBD was in the latter half of the twentieth century [21,22].

Before we discuss the source of electrical bearing failures in electric motors, it is important to highlight that the stray currents in bearings are sometimes inaccurately referred to as electric discharge machining (EDM) currents. However, it should be clarified that EDM is a deliberate and controlled post-processing machining method where spark erosion/machining takes place in metal fabrication to achieve a desired final shape [23]. EIBD and EDM currents are a phenomenon that is not desired and is a result of electric motor shaft voltages. This author’s thoughts on correct terminology resonate with Hansen [24], however, to keep consistency with current literature “EDM” will be used throughout this article.

To understand how electric motor bearings are damaged through electric current discharge effects, we must first understand the source of the shaft voltages. In such systems, there are various types of frequencies, including both low frequencies and high frequencies, ranging from a hundred kilohertz to several megahertz [25,26]. Starting with low-frequency current first, He et al. [8] and Plazenet et al. [26] discussed in depth the source of shaft voltages and provided extensive reference to other researchers, and this article will aim to summarize some of their reading and provide references for further reading. The generation of shaft voltages can be categorized into three distinct parts: (a) magnetic flux asymmetry—caused by variations in magnetic pole distribution or shaft position which ultimately leads to low-frequency sinusoidal voltage and current waves [11,27–29]. An unbalanced flux can generate low-frequency shaft voltage that can create circulation currents in the complete system [11]; (b) electrostatic effects—electrostatic charge affects through a tribocontact of dissimilar materials [22,30–32]. This type of contact can lead to charge build up in the systems that can electrostatic discharge in the bearings [11]; and (c) inverter-induced voltage effects—high-frequency switching of variable frequency drive inverters that use pulse-width modulation (PWM) induce an in-balanced common mode voltage (CMV) on the motor shaft [7,11,33].

Many researchers [34–42] have discussed at length the phenomenon of bearing currents with those relating to “circulating” and “non-circulating” types. These are higher in frequency, greater than 100 kHz [25,26], and associated with the PWM inverters that operate with fast switching transistors, such as silicon-based IGBT and MOSFET types that are used in VFDs of electric vehicles to provide variation in speed control [8,33]. Some of these transistors are now being replaced by silicon-carbide-based transistors and gallium-nitride-based transistors and are believed to offer improved inverter efficiency and thereby improved vehicle range [43]. High-frequency inverter switching is desired for better motor efficiency and in terms of an electric vehicle this can translate to improved vehicle range; however this results in greater amounts of stray currents [44]. There are four main types of what the literature [26,45] refers to as “inverter-induced bearing currents.” Two non-circulating types are (a) known as dV/dt, at low-speed small capacitive type currents in the milliampere range (max—200 mA) can occur but due to their insignificancy with other bearing currents these are usually considered negligible [40], and (b) EDM bearing currents which are also capacitive in nature and build up charge over time in the bearing contact. The BVR [25,46] provides some indication of the likelihood of bearing damage as the bearing voltage increases. The lubricating film between the bearing and the associated raceway and any other interface where there is an oil gap (or sometimes an air gap) such as the cage-to-rolling element interface [10], charges as a result of CMV and acts as a capacitor and eventually as the voltage exceeds the breakdown voltage, which is the threshold at which the voltage forces itself through an insulator, a discharge up to ∼3 A [26,40,47] between the interface occurs. It is highly likely that in a rolling element bearing contact for an electric vehicle, EDM currents will occur consistently and at random times due to; (1) the potential difference at the bearing being generally greater than the breakdown voltage of the lubricant film thickness, (2) the lubricant film thickness between the element and the raceway is generally only a couple of micrometers thick [48] so only small voltages sometimes as low as 3 V are required (assuming an electrical field intensity or breakdown voltage of the lubricant is approximately 15 V/µm and the lubricant film thickness is as low as 0.2 μm) [46], (3) due to the nature of operation and unpredictable slight change in the gaps between the mechanical bearing components (raceways, rolling elements, and cage) [10] which will inevitably alter the threshold of the breakdown voltage, and (4) load and speed demands which can impact the previous three comments. In one study [47], it was found that increased axial loading reduced the voltage of EDM discharges to less harmful values, thereby reducing the likelihood of any discharges damaging the bearing. This is possibly a result of reduced film thickness in the contact. On the topic of oil film thickness, a couple of research articles [49,50] have presented their thoughts on the likelihood of EDM effects based on film thickness and both articles agree that a thick elastohydrodynamic (EHD) film is enough to prevent an EDM event thereby increasing mechanical life, which also resonates with an earlier study by Kaufman and Boyd [51]. However, recent findings by Li et al. [52] where an AC was employed between an optical ball-on-disk tribometer found that a thicker elastohydrodynamic film can have a reduced capacitance and consequently have a greater frequency of discharge events which counter supports previous research findings. Gonda et al. [50] suggested that the elastohydrodynamic lubrication (lubricant film separation between non-conformal contacts with no metal-to-metal surface asperity contacts such as that found in rolling element bearings) is the worst-case scenario for the average number of EDM events especially as the film thickness reduces due to the breakdown voltage of the film being penetrated at lower voltages. As the film thickness reduces the resistive nature of the film switches to more of an ohmic one as more metal-to-metal contact occurs. Schneider et al. [53] suggested that the film thickness in a rolling element contact that is subjected to an electrical field can be considered as a two-plate capacitor. An electrical model of a tribological contact as a function of lubricating film thickness was drawn by Martin [54] and coupling this with the work by Gonda et al. [50], Schneider et al. [53], and Hansen [24] it is possible to draw the Stribeck curve and the lubrication regimes as a function of contact resistance as shown in Fig. 1. It has also been illustrated on the Stribeck curve that the elastohydrodynamic regime where there is a sufficient lubricant film to prevent metal-to-metal contact is also favorable for EDM discharge events for the reasons discussed earlier.

The film thickness in an elastohydrodynamic point contact is generally “horseshoe” like and the capacitance across this contact was modeled by Schneider et al. [53]. This work concluded that within such contacts themselves there are different possibilities of creating discharge events and that the minimum lubricant film thickness in the contact should be considered negligible when understanding the influence on contact capacitance (Fig. 2). This has also recently been experimentally observed as constant rays of purple flashlight across an elastohydrodynamic contact by Li et al. in their optical ball-on-disk tribometer [52]. Elastohydrodynamic lubrication in rolling elements was once considered a relatively safe operation in terms of limiting wear as there is no metal-to-metal contact, however the work from Busse et al., Gonda et al., and Li et al. [49,50,52] would suggest that in such contacts that are in the presence of an electrical field and susceptible to EDMs events that this is a now a lubrication regime that is potentially not safe and could lead to premature rolling contact fatigue [8] and therefore should be an extension of durability concerns for original equipment manufacturers (OEMs), tier suppliers, and lubricant additive companies. In addition to this, and something that should prioritize this durability concern, is that it is likely the number of EDM events will increase (and circulating currents) as the electric motor power is increased [55,56], this would therefore of great importance to heavy-duty applications [24]. Circulating currents begin to dominate at higher power outputs and this is related to increased capacitance between the stator frame and the motor windings as the motor size increases [57] and this influences the motor ground currents which consequently lead to greater flux and circulating bearing currents [40].

He et al. [8] conveniently classified the different types of failure modes based on bearing morphological damage (frosting, fluting, pitting, spark erosion, and welding) and lubrication failures. Figure 3 and Tischmacher [7] provided a rating scale for electrically induced bearing damage and lubricant condition, (Fig. 4). Such research articles help in understanding and early detection of electrically induced bearing damage. In basic principle, a discharge event carries energy in the form of electrical current and voltage and this rapid release of electrical energy converts to heat energy. Weak discharges can lead to a matte gray finish known as frosting [32], and discharges with greater energy can lead to random pits on the running surfaces [58–60]. Ultimately, such failure modes can result in reduced bearing service life [61].

The introduction of a high-energy discharge has enough energy to vaporize or melt the bearing raceway material and leave a pit [7,60]. Since the local area of discharge is usually surrounded by a lubricant medium, the pit is then instantly quenched which leads to material change and hardened crater edges that can be brittle and break off exacerbating the problem of third-body wear and rolling contact fatigue [62] (Fig. 5). Since the bearing can be subjected to circulating periodic currents, a pattern known as “fluting” is a common damage mode that has the appearance of evenly distributed ripples that are usually perpendicular to the axis of rotation [21,38,63]. It is known that damage to the bearing surfaces will introduce mechanical vibration and the combined effects of vibration and periodic electrical charging are responsible for this rhythmic damage pattern [8,64]. Other mechanical damage modes include residual “spark tracks” that are long and sharp on the surface [32,65]. Welding of bearing components in the housing when a large amount of current flows can occur, however, it is not generally attributed to the current flow in EDM events as these are usually smaller in nature [32]. As well as mechanical material damage, the impact on the lubricant has been studied in detail and has been found to degrade, change its composition, and ability to provide effective lubrication [6,7,66–68]. The arc energy has the potential to cause chemical reactions and prematurely degrade the lubricant [66] and create free radicals that can react with the oxygen to form carboxylic acids [69] and/or form hydrogen radicals that can penetrate the bearing surfaces [4] and lead to hydrogen embrittlement and white etch cracking (WEC) phenomena [6]. These are in addition to base oil oxidation and other lubricant additives that can increase the acidity of the lubricant leading to reduced performance [8,70]. The presence of electric fields can result in microbubbles forming [71] as a result of the additional heat energy within the contact these microbubbles could collapse and reduce the lubricant film strength thereby leading to metal-to-metal contact [72,73]. Zinc dialkyldithiophosphate (ZDDP) additive was introduced to liquid paraffin that was subjected to an electric field and it was found that the addition of ZDDP can impact the formation of microbubbles believed to be caused by changes in the electroviscous effects [74]. It has also been demonstrated that the WEC is triggered leading to premature bearing failure when an applied electrical current flows through a contact [6]. The wetting behavior of a liquid on a surface, and hence the interfacial tension, has also been observed to be altered by the presence of an electric current which could have adverse impacts on bearing lubrication [75,76].

Bearing current prevention is the best solution and each motor will require different mechanisms for mitigation. This is often due to the motor architecture employed and the design of it, the power outputs and sometimes even the ease of installation for mitigating hardware will play a role [11]. There are many reported mitigating techniques to help reduce or completely eliminate bearing currents and therefore prolong bearing life [38,46,49,77–89]. The majority of the solutions that have been either reported or used in industry fall in two distinct categories [40]; (1) mitigating strategies on the inverter which includes mitigation either through the inverter and/or the connection between the inverter and the motor, and (2) mitigation strategies for bearing currents inside the motor. Reducing the electric field is an outcome of the first category and this may include the use of filtration on the inverter outputs and/or CMV signals [90–92], modulating the voltage [93–95], and shielded cabling [49,96]. The use of multiphase systems has also been suggested as a means to reduce bearing currents [97]. Inside the motor insulating the bearings to prevent current flow is one option [87,96,98] and alternative options include the use of hybrid ceramic elements in the bearing assemblies [11,99], Faraday shields around the rotors [100,101], and shaft grounding brushes [87,102]. More on these mitigation strategies can be found in the references provided, and Plazenet et al. [26] provided a summary of the different mitigation strategies and what type of currents are then impacted (Table 1).

The previous section provides some information on the hardware and software solutions to limit or completely mitigate bearing currents. There is an increasing focus on the lubricant and whether that can be of use in preventing such currents. The lubricant itself is considered a dielectric medium [103] so would seem an obvious choice in the prevention of bearing currents. The lubricant properties, such as rheological, tribological, and electrorheological, can all be influenced by the base oil type and the additives used [29]. There are many electrical characteristics of a lubricant, and in the case of bearing currents the more influencing properties according to Ref. [5] are “relative permittivity”—the ability of the dielectric lubricant to be polarized in an electric field, in other words, this refers to the lubricant's capacity to act as a capacitor and store an electric charge; “dielectric strength”—the ability of the lubricant to withstand a breakdown voltage; and the “electrical specific resistivity”—the ability of the lubricant to actually conduct or pass an electric current. In one study [104], it has even been shown that the electric properties of the lubricant can sometimes be more influential than the film thickness between a rolling element raceway. The lubricant chemistry can influence these electrical characteristics and a more in-depth description of these properties can be found in Ref. [103]. It must be noted that while there are some industrial standards for measuring the electrical properties of lubricants, many were developed using transformer oils [50]. Examples include the ASTM D1816 test [105], which provides a good measurement of the dielectric breakdown voltage of a lubricant. However, it would be beneficial for system models if the breakdown voltage were measured under conditions similar to the actual application. This includes factors such as electrode spacing, shape, roughness, and material; oil temperature, pressure, and flowrate; as well as input voltage waveform, polarity, frequency, and ramp rate. The practical difficulties associated with testing under conditions encountered in electric vehicle drivetrain rolling element bearings, however, are substantial.

The use of conductive lubricants, with much focus on grease lubricated contacts, has recently been of interest as a means to reduce bearing currents [50,106]. In terms of chemistry influence on conductivity, McFadden et al. experimentally measured that additives can contribute to increasing the conductivity of base oil by up to six orders of magnitude [107]. This conductivity was found to increase with temperature because of viscosity decrease and increase in the ability of the charged particles to move and carry a charge. The research also found that smaller polar molecules such as detergents have the greatest ability to reduce the conductivity of lubricant whereas larger, less polar chemistries such as viscosity modifiers had very little effect on conductivity. In an attempt to further improve the conductivity of a fluid with additives, one reported article used nanocarbon particles and demonstrated a reduction in fluting damage [108]. Carbon black addition to both over-based calcium sulfonate and lithium-based grease was found to increase the conductivity of the greases [109]. The wear performance and the mechanical efficiency, through friction reduction, were also improved. Most of the published articles are focused on greased contacts, but there is an increasing interest in ionic liquids due to their excellent conductivity and ability to reduce bearing discharge events [110,111]. Other studies [112,113] have explored the use of silver nanoparticles to enhance lubricant conductivity and mitigate electrical discharges with some success. Water contamination has been shown to reduce the dielectric strength and hence improve the conductivity of PAO coolants [114] and although while not directly applicable to bearing lubrication in an electric motor, the insights gained from this study can offer valuable perspectives. It must be remembered that any water contamination in a bearing would cause other tribological performance concerns. The relationship between dielectric strength and conductivity on bearing damage has been conveniently illustrated in Fig. 6.

It has been shown that the lubricants, with most research based on greased bearing tests, are prone to premature aging or oxidation and tend to discolor when under the influence of electric potentials [7,66,69,104], and shown to have increased carboxyl components [69], reduced dielectric strength, and change in rheological properties [66] which will impact the rate of electrical discharge and/or the mechanical load carrying capacity of the grease ultimately leading toward early bearing failure. It has been reported that increased temperatures in the contact area [29] can lead to the loss of base oil due to the evaporation of lower molecular weight components, resulting from their higher volatility [116] and as mentioned earlier, microbubbles that can then no longer support mechanical load [71,72]. As a result, both the thermal and electrical performance of the lubricant need to be considered [117], which is in addition to corrosion properties, material compatibility, wear control, and aeration [118].

There has been an increasing interest in benchtop tribology testing of contacts under the influence of an electric field and the general consensus is that the wear of the contact changes and usually increases [50,67,68,119–129]. The change in wear has been linked to many theories. The formation of harder pitted areas has been postulated as mentioned earlier [62], which can sometimes create hardened oxide-rich and carbide wear particles that lead to third-body wear [119,122]. Some of these third-body wear particles were found to correspond to hematite (α-Fe₂O₃) which has a similar hardness to AISI 52100 bearing steel [119]. Spikes [130] discussed at great length about “Triboelectrochemistry” a term first introduced in the early 1980s [131]. Spikes described multiple mechanisms by which the tribological conditions can be impacted; lubricant additives that are surface active are generally polar components and the adsorption/desorption of these additives can change because of the electrical potential. Other effects include the change in nature of chemical reactions more specifically redox reactions at the surface [130]. A change in tribofilm formation under the influence of an electrical potential as reported in Refs. [132,133] would result in a change in overall tribological performance. Furthermore, a change in polarity impacted wear performance and it was found that wear on a cathode surface in a two-electrode system consisting of a ball and disk system was reduced [68]. Changing the form of electrical potential from DC to AC has shown to have varied results on tribology test component wear [121,129]. Some studies that have completed a deeper surface chemistry evaluation have discovered more carbonaceous species in the wear tracks [122–124]. A modified micropitting rig with electrical potential applied across the contact suggested macropitting performance improved, hence reducing the likelihood of rolling contact fatigue, as the current increased above 75 mA [134]. This suggests that there could be cases where the tribological performance can be tuned for longevity. The “tuning” of friction performance was seen in aqueous solutions of conducting ceramic contacts [135].

Over the past two decades, there has been a significant increase in interest from both the research and industrial communities regarding the source, premature damage failures, and mitigation techniques for electrical stray currents in bearings. This growing interest is fueled in part by efficiency and power density improvements in electric motor design that have led to the widespread adoption of electric motors in electric vehicles and the use of electric generators in sustainable energy sources like wind turbines where such electrical–mechanical damages can occur, impacting bearing life and lubrication.

In this article, a brief overview of the source of the electrically induced bearing damage and failure modes, and the impact on tribological factors such as bearing life and lubrication was discussed. Elastohydrodynamic lubrication for bearings offers low friction and low wear performance, however under the influence of an electric field this has become an area of durability concern, especially as the power intensities increase and better technology is employed for improved inverter efficiencies, leading to more stray currents.

Current findings and challenges: Recent fundamental benchtop tribology testing has shown that a lubricated contact under the influence of an electric field will generally introduce more wear due to the oxidation of both the contact materials and the lubricant and affect the adsorption/desorption of surface-active lubricant chemistries thereby impacting the complete tribological performance. There has been some academic works on improving the electrical properties of lubricants such as the conductivity and dielectric strength through changes in formulation with some success, but this has yet to be seen viable in commercial applications.

Future research directions: Despite all the research to date, further research is needed in the following areas:

1. Online hardware conditioning monitoring:

   • The development of advanced methods for vibration detection due to roughness change as a result of the formation of surface pits through either acoustic means [136] or current and impedance measurement [5,137].

   • Investigation into the changes in lubricant electrical properties through accelerated degradation as given by Romanenko et al. [66] could prove another promising means for online assessment of the lubricant if appropriate sensors are utilized.

2. Machine learning applications:

   • Employment of machine learning techniques to analyze and predict outcomes from condition monitoring data, which could help prolong bearing life and optimize maintenance schedules.

3. Tribotronics:

   • Tribotronics [138] is a promising research area where control of the tribological behavior of a contact can be changed through precise electronic control could be an advantageous approach especially when considering triboelectrochemistry, as suggested by Spikes [130].

   • Some of this electronic control has already seen some benefits in tribological performance [120,135,139], but more work is required to understand its impact on surface-active chemistries and lubrication.

4. Real-world simulation and collaboration:

   • Benchtop tribological testing benefits best when the application of interest is closely simulated in terms of operating contact conditions and there needs to be more reference to real-world scenarios.

   • A closer relationship between OEMs, tier supplies, lubrication additive companies, and academic groups would facilitate this and lead to better screener testing of hardware, software, and even lubrication formulation strategies to mitigate EIBD. Based on this review, it becomes clear that a one-size-fits-all approach may not be appropriate in this case. Instead, a collaborative effort would be the most effective way to predict and model the extent of electric bearing damage in operating electric motors and determine their service life.

5. Industry standards:

   • The development of industry standards that accurately measure the electrical properties of electric vehicle lubricants, including those with driveline performance additives, is essential. These standards should ensure consistency and reliability in performance evaluation by considering application-specific conditions such as electrode spacing, shape, roughness, and material; oil temperature, pressure, flowrate; as well as input voltage waveform, polarity, frequency, and ramp rate.

By addressing these research directions, significant advancements in the understanding and mitigation of the electrically induced bearing damage can be made. Improved condition monitoring and predictive maintenance strategies will enhance the reliability and lifespan of bearings in electric motors and generators. Machine learning and tribotronics have the potential to offer innovative solutions for real-time control and optimization of tribological performance. Finally, establishing industry standards will ensure that new formulations are both effective and commercially viable, leading to broader adoption and improved performance of electric vehicles and sustainable energy applications.

The author would like to thank Dr. Chris McFadden from The Lubrizol Corporation for their invaluable assistance in enhancing the author's understanding of the electrical properties of lubricants, which significantly contributed to the development of this article.

There are no conflicts of interest.

The authors attest that all data for this study are included in the article.

t =

time

A =

ampere

V =

voltage

mA =

milliampere

AC =

alternating current

BVR =

bearing voltage ratio

CMV =

common mode voltage

DC =

direct current

EIBD =

electrically induced bearing damage

EDM =

electric discharge machining

IGBT =

insulated-gate bipolar transistor

MOSFET =

metal-oxide-semiconductor field-effect transistor

OEM =

original equipment manufacturer

PAO =

polyalphaolefin

VFD =

variable frequency drive

WEC =

white etch cracking

ZDDP =

zinc dialkyldithiophosphate