Shaft Currents Research Articles

Exploring tribo-electro-chemistry mechanisms of h-BNO@PDA as lubricant additives under electrified conditions for electrical vehicles powertrain

The study explores the use of polydopamine-functionalized hexagonal boron nitride (h-BNO@PDA) as a lubricant/insulator additive in polyalphaolefin (PAO6) oil. This article's novelty lies in the formulation of an innovative lubricant that is free from sulfated ash, phosphorus, and sulfur (SAPS), exhibiting super-lubrication and desirable dielectric properties under electrified conditions. The h-BNO@PDA nanolubricant is specifically designed to mitigate the negative effects of shaft currents and enhance powertrain lubrication for electric vehicles (EVs). Herein, the physicochemical, dielectric strength, and tribological properties of h-BNO@PDA nanolubricant are compared to commercial fully synthetic oil (ATF-6S) used in EVs. The SRV-IV tribometer was used to assess tribological performance under various currents (0–10 A). Subsequently, characterization techniques were utilized to illustrate wear mechanisms and explore tribochemical reactions. The h-BNO@PDA nanolubricant presented superior physicochemical and dielectric properties. The h-BNO@PDA nanolubricant reduced the friction coefficient by 18–20 % and the wear rate by 4–72 % compared to ATF-6S oil under electrified conditions at temperatures of 27 °C and 100 °C. These findings can be attributed to the positive effects of oxidational wear (Fe3O4) and h-BNO@PDA on tribolayer formation under electrification conditions. Eventually, our findings will offer promising insights to enhance the durability of EVs powertrain.

Inverter-Fed Motor Drive System: A Systematic Analysis of Condition Monitoring and Practical Diagnostic Techniques

Due to their efficiency and control capabilities, induction motors fed with inverters have become prevalent in various industrial applications. However, ensuring the reliable operation of the motor and diagnosing faults on time are crucial for preventing unexpected failures and minimizing downtime. This paper systematically analyzes condition monitoring and practical diagnostic techniques for inverter-fed motor drive systems. This study encompasses a thorough evaluation of different methods used for condition monitoring and diagnostics of induction motors, with the most crucial faults in their stator, rotor, bearings, eccentricity, shaft currents, and partial discharges. It also includes an assessment of their applicability. The presented analysis includes a focus on the challenges associated with inverter-fed systems, such as high-frequency harmonics, common-mode voltages causing the bearing currents, and high voltage gradients (dv/dt) due to fast switching frequency, which can impact the motor operation, as well as its faults analysis. Furthermore, this research explores the usefulness and efficiency of various available diagnostic methods, such as motor current signature analysis and other useful analyses using advanced signal processing techniques. This study aims to present findings that provide valuable insights for developing comprehensive condition monitoring strategies, and practical diagnostic techniques that enable proactive maintenance, enhanced system performance, and improved operational reliability of inverter-fed motor drive systems.

Lubricated sliding wear of gear material under electrification – A new approach to understanding of the influence of shaft currents in the wear of EV transmissions

Electrification of drivelines of electric vehicles (EVs) can give rise to damaging shaft voltages/currents that can potentially impair the tribological performance of sliding and rolling elements in these driveline systems. In fact, previous research has shown that the existence of shaft voltages/currents in electric machines is indeed highly detrimental for the service life of tribological components of EVs. Such currents/voltages originate from magnetic flux asymmetry, inverter-induced voltage and triboelectrification and may also interfere with the normal sliding condition of other electrically conductive tribological components connected to the electric motors in addition to those that are comprised in the driveline, e.g., gears. In the past, the influence of shaft voltages/currents in gear systems has been explored scarcely. Accordingly, this work aims to investigate specifically the lubricated sliding friction and wear behavior of gear materials under the influence of DC electrification. For this, many tests were conducted on a gear material in a two-electrode-cell-based pin-on-disc tribometer representing the sliding contact of a gear interface. The tests were run under different currents (0, 1.5 and 3 A) and using different lubricants (pure mineral base oil, gear oil and transformer oil). The differences in sliding contact temperature, coefficient of friction (CoF), wear volumes and underlying mechanisms, as well as the chemical changes occurred in the lubricating oils by electrification are analyzed and discussed. Overall, the results confirmed that electrification can indeed have a strong influence on friction and wear of gear materials besides increasing contact temperature and thus accelerating the oxidation at the sliding interfaces for all the lubricants. The increase in wear is most likely due to three-body abrasion caused by increased oxidation and/or oxide-based debris formation under electrification. CoF was found to decrease by electrification, which was ascribed to the formation of a larger wear scar (that reduced contact pressure) and continuous replenishment of an oxide layer affording Lower Friction To Sliding Surfaces.

Electrified four-ball testing – A potential alternative for assessing lubricants (E-fluids) for electric vehicles

The increasing worldwide demand for hybrid and electric vehicle technology has brought new challenges for the global lubricant industry. The new lubricants for electric vehicles, also called E-fluids or EV-fluids, are expected to meet a new set of requirements including withstanding the much severe operating conditions of EV powertrains. High starting torques, high RPMs and uncontrollable shaft currents passing through the contact interfaces are some of the most challenging powertrain conditions that can impact the performance of these lubricants. Although there have been some papers already reporting on significant alteration of tribological properties of lubricants under electrification, so far there are no standard test protocols aimed at the fast and reliable screening of base oils, additives and formulated lubricants under such conditions. Thus, this research work focuses on exploring the popular four-ball ASTM-D4172 standard method to evaluate the tribological behavior of lubricants under electrified conditions. Specifically, a conventional four-ball tester was instrumented with a DC power source and a resistance data logger was used to explore the friction and wear performance of various lubricants (including a neat base oil, two automatic transmission fluids (ATF-3 and ATF-5) and a gear oil) with different dielectric strength under the conditions of ASTM-D4172 standard with and without DC currents (0, 1.5 and 3 A). The changes in average wear scar diameter, specific wear modes and mechanisms, friction coefficients, electrical contact resistance and temperature were analyzed and reported. Besides, the chemical changes in oils during the electrified tests were evaluated via IR spectroscopy. The main findings suggest that passing current through the four-ball contact interfaces significantly alters friction, wear and temperature in comparison to the conventional standard method without electrification. Thus, the proposed test method with electrification could be a potential new alternative for testing E-fluids under electrical environments.

ANALYTICAL CALCULATION OF SHAFT CURRENTS IN THE MOTORS WITH HOMOGENEOUS YOKE

Literature dealing with the matter of shaft currents mostly elaborates on the methods of motor diagnostics which only observe and analyse harmonics frequencies characteristic for shaft currents. The analytic calculation of the amplitudes of those currents has not been analysed so far for homogenous yoke motors and, in order for correct protection measures to be undertaken, it is necessary to determine exactly the parameters which influence those currents. Shaft currents are an important component of the currents which electrically damage the bearings and usually represent the dominant contribution. This is especially important for low-voltage induction motors which, according to IEC’s data, are the motors with the widest application range in the industry. In this work, the analytic calculation of shaft currents is explained for those motors.

Анализ повреждений подшипников тяговых машин на электроподвижном составе при питании от преобразователей частоты и напряжения

The technical and economic efficiency of electric rolling stock (ERS) is determined by the characteristics of its traction electric drives, the key ones of them being the manufacturing cost, repair and maintenance costs, service life, specific energy consumption, and operational reliability. The modern ERS uses induction electric drives with pulse-width control of the voltage and frequency control of the rotation frequency. The field experience gained from operation of electric drives equipped with semiconductor converters has shown a growing number of cases involving accelerated wear of the traction machine bearings. Currents through the bearings or shaft currents, which usually flow from the electrical machine shaft through the bearings, have been known since the time the electrical machines were invented. Owing to recent achievements made in power electronics, the application field of inductor motors has become much wider. In particular, the use of inverters with pulse-width modulation (PWM) and high switching frequency make it possible to operate an electric drive with a lower acoustic noise and with more efficient energy conversion; however, the use of inverters also entails the generation of bearing currents in induction motors. The article analyzes possible factors causing the occurrence of bearing currents. Typical changes occurring in the bearings as a result of electric current flowing through them are shown: the rolling surface becomes dull, grooves on the ball race occur, and lubrication becomes degraded. The influence of operating parameters on the frequency of the occurring breakdowns and the distribution of forces in the bearing system depending on the load are considered. A conclusion is drawn that the potential problem of bearing currents should be solved using design, structural, and algorithmic methods at different stages of design and operation of electrical systems involving adjustable electric drives.

Анализ повреждений подшипников тяговых машин на электроподвижном составе при питании от преобразователей частоты и напряжения

The technical and economic efficiency of electric rolling stock (ERS) is determined by the characteristics of its traction electric drives, the key ones of them being the manufacturing cost, repair and maintenance costs, service life, specific energy consumption, and operational reliability. The modern ERS uses induction electric drives with pulse-width control of the voltage and frequency control of the rotation frequency. The field experience gained from operation of electric drives equipped with semiconductor converters has shown a growing number of cases involving accelerated wear of the traction machine bearings. Currents through the bearings or shaft currents, which usually flow from the electrical machine shaft through the bearings, have been known since the time the electrical machines were invented. Owing to recent achievements made in power electronics, the application field of inductor motors has become much wider. In particular, the use of inverters with pulse-width modulation (PWM) and high switching frequency make it possible to operate an electric drive with a lower acoustic noise and with more efficient energy conversion; however, the use of inverters also entails the generation of bearing currents in induction motors. The article analyzes possible factors causing the occurrence of bearing currents. Typical changes occurring in the bearings as a result of electric current flowing through them are shown: the rolling surface becomes dull, grooves on the ball race occur, and lubrication becomes degraded. The influence of operating parameters on the frequency of the occurring breakdowns and the distribution of forces in the bearing system depending on the load are considered. A conclusion is drawn that the potential problem of bearing currents should be solved using design, structural, and algorithmic methods at different stages of design and operation of electrical systems involving adjustable electric drives.

Conversion of UEP Signatures Between Different Environmental Conditions Using Shaft Currents

In this paper, we investigate an approach that allows naval vessels to continuously estimate their underwater electric potential (UEP) signature strength, by converting a beforehand measured reference UEP signature to the present environmental conditions. The approach is intended to improve risk assessment during mine counter measure (MCM) operations and to be used within integrated ship signature management systems (ISSMS). It is therefore designed to depend only on data available in these scenarios and to be simple and fast, rather than just being as accurate as possible. After explaining the concept and deducing the conversion relations, the approach is tested using numerical simulations and ranged UEP data of a ship (CFAV Quest). The results show that it is well suited to convert UEP signatures between different water conductivities and sensor depths, while it is not capable of converting between different seabed conductivities. The signature conversion is best for intact hull coatings, while hull-to-anode currents lower the correlation between signature and shaft currents, and hence impair the conversion quality.

PWM drive related bearing failures

The phenomenon of motor shaft voltages producing circulating shaft currents has been recognized since the 1920s. When a motor is operated by sinusoidal power, shaft voltages are caused by alternating flux linkages with the shaft. This article investigates the phenomenon of induced shaft voltages caused by PWM AC variable speed drives and discusses methods of mitigating their harmful effects.

Comparison of techniques for measurement of shaft currents in rotating machines

Irregularities in the magnetic circuits of motors may result in spurious voltages that lead to shaft currents through the shaft, bearings, bearing supports and closing through the machine framework. The IEEE Standard Test Procedure for Polyphase Induction Motors and Generators discusses the shaft current and presents a measurement method for recording either the voltage across the ends of the shaft or the current. This paper discusses an alternative measurement approach and its application to the identification of shaft current in a large induction machine. Procedures were developed for measuring the shaft current. The procedures include the shunt current method and the measurement using the Rogowski coil. Only the Rogowski coil measurement yields accurate measurements of shaft currents whereas the other method either yields inaccurate measurement or may result in other problems for the machine's integrity. The theory and justification for the superiority of the Rogowski coil method is presented along with supporting test data.

Generator eccentricity, vibration, and shaft currents

ECCENTRICITY of the rotor causes pulsating unbalanced magnetic forces; induced currents in rotor winding, wedges, and surface; and circulating currents between stator circuits with consequent increased losses and temperatures. Inequality in the gap also affects the fringing fluxes at the core end region, and may result in high temperatures in clamping plates or retaining rings.

Eccentricity, Vibration, and Shaft Currents in Turbine Generators [includes discussion

Eccentricity, Vibration, and Shaft Currents in Turbine Generators [includes discussion

Eddy current losses in armature conductors

James Rurke: With reference to Fig. 14 of the paper showing the offset segments, our company has used offset segments uniformly and continuously for 18 years in segmental punching machines and has not experienced any trouble from shaft currents.

Shaft currents in electric machines

W. F. Dawson: I can endorse one point which Mr. Alger made, namely that shaft currents are often associated with high saturation in the stationary armature cores of synchronous generators.

Shaft Currents in Electric Machines

This paper describes the causes of, and remedies for, the existence of ``shaft currents'' or ``bearing currents'' which sometimes flow across the rubbing surfaces of the bearings of electric machinery, thereby gradually damaging the shaft and bearings. Up to the present time the only cause of shaft currents that has attracted any particular attention has been the use of sectionalized stators, and the published discussions have been chiefly confined to synchronous alternators. Fleischman <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> and others have shown that sectionalizing causes shaft currents for the reason that the extra reluctance of the joints causes an unequal division of the flux between the clockwise and counter-clockwise paths in the yoke, thus giving a resultant flux linking the shaft. Applying the same method of reasoning used in the case of sectionalizing to the general case of any machine with segmental punchings, the following facts are shown: 1. A principal cause of shaft currents in revolving electric machines is the use of poles and segments in certain ratios. 2. The frequency of the shaft current due to joints in the stator yoke is an odd mnultiple of the frequency of the stator flux, the frequency of the shaft currents due to rotor joints is an odd multiple of the rotor frequency, and these frequency multiples are determined by the ratios of poles to segments. 3. Machines with 4, 8, 16, 24, 32, etc., poles are especially likely to have shaft currents, and machines with 6, 10, 14, 22, etc., poles are relatively immune. 4.

Shaft currents in electric machines

This paper describes the causes of, and remedies for, the existence of “shaft currents” or “bearing currents” which sometimes flow across the rubbing surfaces of the bearings of electric machinery, thereby gradually damaging the shaft and bearings. Up to the present time the only cause of shaft currents that has attracted any particular attention has been the use of sectionalized stators, and the published discussions have been chiefly confined to synchronous alternators. Fleischman <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> and others have shown that sectionalizing causes shaft currents for the reason that the extra reluctance of the joints causes an unequal division of the flux between the clockwise and counter-clockwise paths in the yoke, thus giving a resultant flux linking the shaft. Applying the same method of reasoning used in the case of sectionalizing to the general case of any machine with segmental punchings, the following facts are shown: 1. A principal cause of shaft currents in revolving electric machines is the use of poles and segments in certain ratios. 2. The frequency of the shaft current due to joints in the stator yoke is an odd multiple of the frequency of the stator flux, the frequency of the shaft currents due to rotor joints is an odd multiple of the rotor frequency, and these frequency multiples are determined by the ratios of poles to segments. 3. Machines with 4, 8, 16, 24, 32, etc., poles are especially likely to have shaft currents, and machines with 6, 10, 14, 22, etc., poles are relatively immune. 4. By the proper choice of the number of segments for use with any machine, or by the use of segments with offset dovetails, or both, shaft currents can be effectively eliminated in most cases. The possibilities of shaft currents being caused by homopolar action as the result of magnetic flux flowing in the shaft, or by other means, are discussed, and it is concluded that such causes are seldom important. A possible useful application for the theory of shaft currents in the design of a high-current transformer is mentioned, and the possibility of obtaining multiple frequencies from a stationary transformer in this way is shown to be dependent upon the presence of magnetic saturation. A table of combinations of poles and segments that will cause shaft currents is given, and a bibliography of the subject is appended.