Shaft Current Research Articles

The evolution process of fluting damage on bearing outer races during rolling with a shaft current

This study focuses on the evolution of fluting damage in bearing outer races. Fluting damage first occurred at the loading position and then extended along outer races. This non-uniform distribution of fluting damage is related to the non-uniform load inside bearing. It is speculated that the alteration of bright and dark areas in fluting is related to the periodic destruction and reconstruction of lubricating film. The phase transition from α′-Fe to α-Fe is observed beneath erosion surface. At microscale, non-uniform damage of the fluting appears as the dark area exhibiting more severe erosion, lower oxidation, greater roughness, deeper depth, and lower hardness. The results can provide deeper understanding of the failure process and damage mechanism of bearings in electric fields.

A Model Predictive Control Scheme with Minimum Common-Mode Voltage for PMSM Drive System Fed by VSI

Common-mode voltage (CMV) brings shaft voltage and shaft current, and corrodes the bearings of the permanent-magnet synchronous machine (PMSM), which affects the reliability of the whole PMSM drive system. Since the CMV applied by the zero voltage vectors (ZVVs) is three times that applied by the active voltage vectors (AVVs), a modulation scheme achieving minimum CMV without ZVV is proposed and introduced into the model predictive control structure for the PMSM drive system. Firstly, the whole modulation range is divided into three regions, including the low voltage modulation region (LVMR), high voltage modulation region (HVMR), and over-voltage modulation region (OVMR). Meanwhile, the regional boundary expression is derived. Then, the active zero-state pulse width modulation (AZSPWM) is adopted in LVMR. To improve the steady-state performance, near-state pulse width modulation (NSPWM) without opposite ZVVs is applied to the HVMR. Furthermore, when the reference voltage vector (VV) is located in OVMR, an optimal scheme is proposed to improve the dynamic response. Under the premise of no ZVV existing in the whole modulation region, simulation and experimental results show that the proposed hybrid modulation method can improve the steady-state and dynamic performance of the PMSM drive system.

Brushless DC Motor Common-mode Voltage Analysis

Brushless DC motors (BLDCMs) with their own performance of obvious advantages: good speed regulation performance and high efficiency. BLDCMs have continuously replaced brushed DC and induction motors in many competitive fields and obtained more extensive applications. The common-mode voltage (CMV) in the motor parasitic capacitance coupling in the motor shaft induces the shaft voltage. It then triggers the shaft current, resulting in electrical corrosion of the bearing. It is necessary to conduct a systematic scientific analysis of its CMV. For different modulation methods, the phase voltages under three conditions without PWM modulation, HPWM_LPWM modulation, and HPWM_LON modulation are studied, FFT, and then the CMV is analyzed. Then, the correctness of the analysis is verified by simulation analysis.

Study on Electrical Pitting Prevention Device of a Rotating Shaft Using Automatic Control Potential Balancing.

A rotating body consisting of a rotating shaft and bearings inevitably generates voltage and current. The potential difference between the bearing and the shaft is the main cause of electrical corrosion, which causes motor failure, shortened bearing life, and many safety issues. To prevent corrosion, passive shaft-grounding devices use conductive materials and brushes; however, these devices cannot be completely grounded, so there is a difference in local potential, and brush friction generates a shaft current. The cumulative effect causes electrical corrosion; therefore, in this study, an electrical corrosion protection device for the rotating power supply shaft was developed. It detected current and potential difference and established a feedback system on the rotating shaft. It also energized the rotating shaft using an external power supply to eliminate the potential difference on the shaft and reduce electrical corrosion. The result was prolonged motor life and improved stability, operating efficiency, and operability of related equipment. In this study, a rotating-shaft test rig was set up, and a constant current was applied to simulate the potential difference and verify the performance of the anti-corrosion device. Gradually, the design scheme was optimized; the potential difference on the rotating shaft was accurately quantified; and the goal of controlling the potential difference within 2 mV was achieved. Finally, the electrical corrosion protection device was applied to the rotating shaft of a merchant ship, and the current and potential difference on the rotating shaft were monitored for 30 days. The results showed that the device had excellent performance in reducing the potential difference on the rotating shaft and preventing electrical corrosion.

Maximum Power Point Tracking Control for Non-Gaussian Wind Energy Conversion System by Using Survival Information Potential.

In this paper, a wind energy conversion system is studied to improve the conversion efficiency and maximize power output. Firstly, a nonlinear state space model is established with respect to shaft current, turbine rotational speed and power output in the wind energy conversion system. As the wind velocity can be descried as a non-Gaussian variable on the system model, the survival information potential is adopted to measure the uncertainty of the stochastic tracking error between the actual wind turbine rotation speed and the reference one. Secondly, to minimize the stochastic tracking error, the control input is obtained by recursively optimizing the performance index function which is constructed with consideration of both survival information potential and control input constraints. To avoid those complex probability formulation, a data driven method is adopted in the process of calculating the survival information potential. Finally, a simulation example is given to illustrate the efficiency of the proposed maximum power point tracking control method. The results demonstrate that by following this method, the actual wind turbine rotation speed can track the reference speed with less time, less overshoot and higher precision, and thus the power output can still be guaranteed under the influence of non-Gaussian wind noises.

Experimental study on electric corrosion damage of bearing and solution

During the operation of the bearing in the motor, the induced current will inevitably be generated. The bearing raceway will be damaged by current. In this paper, the oil film thickness and capacitance in the bearing are analyzed in detail, and the relationship between oil film thickness, rotational speed, and load is obtained. The oil film thickness increases with the increase of bearing rotation speed and decreases slightly with the increase of load. The minimum oil film thickness of the bearing is between 0.4 μm and 2.9 μm. The capacitance of bearing is influenced by Hertz contact area, oil film thickness, and lubricant quality. It is between 1.9 pF and 34.7 pF. A shaft current test rig was developed. Based on the analysis results, orthogonal tests are set up to study the influence of voltage, rotation speed, and load on bearing operation. By using range analysis and variance analysis, the same conclusion is obtained: the most dominant factors affecting the raceway damage width, surface roughness, and vibration amplitude are voltage, load, and rotation speed, respectively. The mixed ceramic bearing and full-ceramic bearing are used instead of steel bearing to solve the problem of shaft current. Add conductive grease and non-conductive grease, respectively, and analyze and compare the advantages and disadvantages of different combinations from the perspective of temperature and friction and wear. The order from inferior to superior is steel bearing, hybrid ceramic bearing with ceramic outer ring, hybrid ceramic bearing with ceramic inner ring, hybrid ceramic bearing with ceramic ball and full-ceramic bearing. By using non-conductive grease, the damage is reduced by 16%–29%, surface roughness is reduced by 90%–122%, and vibration is reduced by 23%–176%. Under the same conditions, the bearings using non-conductive grease are better than those using conductive grease.

Research on bearing current damage description model of variable frequency motor

The bearing failure seriously affects the normal operation of the motor. In this paper, the early damage of bearing wear caused by the shaft current in the operating state of the variable frequency motor is proposed. From the perspective of the bearing capacitance storage capacitor charge, the method based on analyzing the relationship between pitting pit size and shaft current per unit area of bearing track surface is proposed, so that the model can directly judge whether the bearing is in an absolutely safe running state. In this paper, the description model between the size of the pitting pit per unit area of the bearing track surface and the mechanical and electrical parameters is derived based on the assumption that the charge required for the formation of pitting pits per unit area of the bearing track surface. It is shown that the shaft current can promote the bearing to produce pitting pit, which leads to the early damage of the bearing, and the size of the pitting pit is positively related to the shaft current. Matlab/Simulink simulation verifies the correctness of the description model.

Bearing fault severity estimation using time-based descriptors for rotating electric machines

This paper evaluates the bearing fault severity in rotating electric machines based on the time-based descriptors derived from machine vibration signal. Time-based descriptors are independent of bearing dimensions and machine dynamics. To establish the relationship between the time-based descriptor and bearing fault severity, a wide range of defective bearings are examined on an experimental set-up and the factors contributing to the value of time-based descriptor are determined. The application of this technique to the bearing faults such as rolling element, inner-race and outer-race fault, has confirmed that time-based descriptor successfully measures the severity of faults. The significance of time-based descriptors has been evaluated and tested by developing online bearing failure situation via shaft current. Since the time-based descriptors are independent of dimensions and characteristic frequencies of the bearing, the severity estimation is less sensitive to operating conditions for data acquisition. The laboratory investigations on 10-hp cage induction motor are presented.

A Survey on Shaft Voltage of Large Generator

Shaft voltage is widely existed in the operation of generator and needed to pay more attentions. Based on the mechanism of generator shaft voltage, this paper analyses and expounds the harmfulness of shaft voltage and shaft current, then summarizes the prevention and control measures of shaft voltage and the research status. The direction of generator shaft voltage is prospected for further research.

Fault Classification and Fault Signature Production for Rolling Element Bearings in Electric Machines

Most condition monitoring techniques for rolling element bearings are designed to detect the four characteristic fault frequencies. This has lead to the common practice of categorizing bearing faults according to fault location (i.e., inner race, outer race, ball, or cage fault). While the ability to detect the four characteristic fault frequencies is necessary, this approach neglects another important class of faults that arise in many industrial settings. This research introduces the notion of categorizing bearing faults as either single-point defects or generalized roughness. These classes separate bearing faults according to the fault signatures that are produced rather than by the physical location of the fault. Specifically, single-point defects produce the four predictable characteristic fault frequencies while faults categorized as generalized roughness produce unpredictable broadband changes in the machine vibration and stator current. Experimental results are provided from bearings failed in situ via a shaft current. These results illustrate the unpredictable and broadband nature of the effects produced by generalized roughness bearing faults. This issue is significant because a successful bearing condition monitoring scheme must be able to reliably detect both classes of faults.

Shaft current in AC induction machine. An online monitoring system and prediction rules

Shaft current in AC induction machine. An online monitoring system and prediction rules

Bearing damage analysis in a large oil-ring-lubricated induction machine

In this paper, an alternative method of online detection capable of discerning machine failure modes resulting in shaft current is proposed. The relationship between shaft current and fault conditions such as asymmetrical flux due to joints in the lamination segments, broken rotor bars, air-gap eccentricity, saturation, and slot harmonics are investigated. The diagnostic equipment used in the investigation of a three-phase medium-voltage squirrel-cage induction machine is described. The experimental results using the alternative method of online detection of shaft current are presented.

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.

Rotor winding short detection

The paper presents a review of four techniques which are used for detecting, and in some instances locating, short-circuited turns in the rotors of large turbogenerators. The information is supplemented by experience with some of the techniques gained in a manufacturing establishment. The four techniques described are airgap search coil, recurrent surge oscillograph (RSO), circulating stator current, and rotor shaft current.

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.