Yonghyun Park

Monitoring of journal bearing faults based on motor current signature analysis for induction motors

Most of the failures in oil-lubricated journal bearings are associated with mechanical instabilities produced by lubrication system problems or bearing wear, and result in increased shaft vibration. Therefore, monitoring of the journal bearing condition in the field mainly relies on analysis of proximity probe or accelerometer signals. However, not all motors with journal bearings are equipped with such mechanical sensors due to cost or environmental restrictions. In this paper, the feasibility of applying motor current signature analysis (MCSA) for remote monitoring of mechanical instabilities in journal bearings produced by oil whirl and bearing clearance problems, is evaluated. In addition, a small-scale journal bearing test setup that can be implemented in low voltage motors for testing journal bearing faults under controlled conditions, is presented. An experimental study on a sealless fuel pump motor operating in the field, and on the custom-built journal bearing motor test setup shows that MCSA is capable of providing remote monitoring of journal bearing oil whirl and increased clearance for potentially preventing motor failure. The results of vibration analysis are also presented for comparative evaluation.

Monitoring Journal-Bearing Faults: Making Use of Motor Current Signature Analysis for Induction Motors

Most of the failures in oil-lubricated journal bearings are associated with mechanical instabilities produced by lubrication-system problems or bearing wear and result in increased shaft vibration. Therefore, monitoring the journal-bearing condition in the field relies mainly on the analysis of proximity-probe or accelerometer signals. However, not all motors with journal bearings are equipped with such mechanical sensors due to cost or environmental restrictions. In this article, we evaluate the feasibility of applying motor current signature analysis (MCSA) for the remote monitoring of mechanical instabilities in journal bearings produced by oil-whirl and bearing-clearance problems. In addition, we present a small-scale journal-bearing test setup that can be implemented in low-voltage motors for testing journal-bearing faults under controlled conditions. An experimental study on a sealless fuel-pump motor operating in the field and on the custom-built journal-bearing motor test setup shows that MCSA can remotely monitor journal-bearing oil whirl and excessive clearance to prevent motor failure. The results of vibration analysis are also presented for comparative evaluation.

Permanent magnet temperature estimation in PM synchronous motors using low cost hall effect sensors

Knowledge of the permanent magnet (PM) temperature in PM synchronous machines (PMSMs) is of great importance both for control and monitoring purposes. Increase in PM temperature during motor operation can degrade the magnetic flux strength and consequently the machines torque production capability, also can cause irreversible demagnetization of the PM. Direct measurement of the PM temperature is not viable in practice, due to both cost and reliability issues. Indirect PM temperature estimation methods recently studied can require knowledge of thermal or electrical model parameters or can have undesired effects on motor operation. In this paper, the feasibility of using low cost hall-effect sensors for PM temperature estimation is investigated. Hall sensors are present for detecting the initial position of the rotor in majority of PMSM applications for which incremental encoders are used for control. The proposed method can therefore be implemented with low or no additional cost. Experimental results on two IPMSMs show that the method is capable of providing non-invasive estimation of the PM temperature without a priori motor parameter information for monitoring and protection against excessive increase in temperature.1

Influence of Blade Pass Frequency Vibrations on MCSA-Based Rotor Fault Detection of Induction Motors

Motor current signature analysis (MCSA) has recently become widespread in industry for on-line detection of rotor cage faults in induction motors for preventing forced outages. Although it can provide low cost, remote monitoring of rotor faults, cases of false indications have been reported, where the causes of some false indications are still unknown. It is shown for the first time in this work that high-amplitude blade pass frequency (BPF) vibrations produced in pumps, fans, or compressors can cause false rotor fault indications, if the number of motor poles is an integer multiple of the number of blades. The influence of BPF vibration on MCSA based rotor fault detection is analyzed, and it is shown that the interaction between BPF vibration and rotor faults can produce false positive and negative fault indications. Alternative test methods capable of separating the influence of the BPF vibration and rotor faults are suggested for avoiding false MCSA alarms. The claims made in the paper are verified experimentally on a custom-built 380 V induction motor-centrifugal pump system setup.

Permanent Magnet Temperature Estimation in PM Synchronous Motors Using Low-Cost Hall Effect Sensors

Knowledge of the permanent magnet (PM) temperature in PM synchronous machines (PMSMs) is of great importance both for control and monitoring purposes. Increase in PM temperature during motor operation can degrade the magnetic flux strength and consequently the machines torque production capability, and can also cause irreversible demagnetization of the PM. Direct measurement of the PM temperature is not viable in practice due to both cost and reliability issues. Indirect PM temperature estimation methods recently studied require knowledge of thermal or electrical model parameters or can have undesired effects on motor operation. In this paper, the feasibility of using low-cost Hall-effect sensors for PM temperature estimation is investigated. Hall sensors are present for detecting the initial position of the rotor in majority of PMSM applications for which incremental encoders are used for control. The proposed method can, therefore, be implemented with low or no additional cost. Experimental results on two interior PMSMs show that the method is capable of providing noninvasive estimation of the PM temperature without a priori motor parameter information for monitoring and protection against excessive increase in temperature.

Influence of blade pass frequency vibrations on MCSA-based rotor fault detection of induction motors

Motor current signature analysis (MCSA) has recently become widespread in industry for on-line detection of rotor cage faults in induction motors for preventing forced outages. Although it can provide low cost remote monitoring of rotor faults, cases of false indications have been reported, where the causes of some false indications are still unknown. It is shown for the first time in this work that high-amplitude blade pass frequency (BPF) vibrations produced in pumps, fans, or compressors can cause false rotor fault indications if the number of motor poles is an integer multiple of the number of blades. The influence of BPF vibration on MCSA-based rotor fault detection is analyzed, and it is shown that the interaction between BPF vibration and rotor faults can produce false positive and negative fault indications. Alternative test methods capable of separating the influence of the BPF vibration and rotor faults are suggested for avoiding false MCSA alarms. The claims made in the paper are verified experimentally on a custom-built 380 V induction motor-centrifugal pump system setup.

Electrical monitoring of mechanical defects in induction motor driven V-belt-pulley speed reduction couplings

V-belt-pulley couplings are commonly used for speed reduction in induction motor driven industrial applications since they provide flexible transmission of power at low cost. However, they are susceptible to mechanical defects such as belt wear or crack that can cause slippage or damage of the belt and lead to decrease in efficiency and lifetime of the system. There are many limitations to applying existing tests such as visual inspection, thermal or mechanical monitoring as they require visual or physical access to the system and/or costly sensors. Considering the large quantity of belt-pulley systems employed in industry, the impact of the economic loss incurred by low-efficiency operation and unplanned process outages is significant. In this paper, electrical monitoring of belt-pulley coupling defects based on the analysis and trending of the stator current frequency spectrum under steady-state and starting conditions is presented. The proposed method is verified on 1) 6.6 kV motor driven pulpers, and on a 2) custom-built motor driven air compressor with speed reduction belt-pulleys under controlled fault conditions. It is shown that the proposed method can provide automated, remote, and safe detection of belt-pulley defects based on existing current measurements for improving system reliability and efficiency.

Detection of demagnetization in permanent magnet synchronous machines using hall-effect sensors

Torque production capability of permanent magnet synchronous machines (PMSMs) depends on the magnetization state of the permanent magnets (PMs). Electrical stress, thermal stress, or the combined effect of both can produce demagnetization of the PMs, which eventually can result in several adverse effects including decrease in the motor torque and efficiency and increase of the torque ripple and vibration, eventually degrading the performance and reliability of the motor and drive system. A number of approaches have been proposed for detecting PM demagnetization using model-based flux estimation, signal injection, and spectrum analysis of current/back electromotive force; however, all these methods show limitations in terms of invasiveness, implementation cost, and/or reliability of the diagnosis. In this paper, accurate PM demagnetization detection based on measurements from Hall-effect sensors is proposed. Such sensors are often mounted in commercial PMSMs; the proposed method can, therefore, be implemented at practically no cost.

On-line detection of rotor eccentricity for PMSMs based on hall-effect field sensor measurements

Rotor eccentricity in permanent magnet synchronous motors (PMSMs) increases unbalanced magnetic pull and motor vibration resulting in accelerated aging of motor components. If eccentricity remains undetected, it can increase in severity, and increase the risk of stator-rotor contact, which causes forced outage of the motor and driven process. Detection of eccentricity currently relies on off-line testing and on-line vibration/current spectrum analysis, which are inconvenient or cannot provide reliable detection as they are influenced by other mechanical non-idealities in the motor or load. In this paper, the feasibility of using the signals from analog Hall-effect field sensors for detecting eccentricity is investigated. It is shown that Hall sensors already present in machines for motion control can be used for measuring the variation in the flux inside the motor due to eccentricity. 3-dimensional (3D) finite element analysis (FEA) and experimental results performed on an interior PMSM (IPMSM) show that the proposed method can provide sensitive and reliable detection of dynamic and mixed eccentricity with minimal hardware modifications.

Off-line flux injection test probe for screening defective rotors in squirrel cage induction machines

The recent trend in squirrel cage induction motor manufacturing is to replace fabricated copper rotors with aluminum die-cast rotors to reduce manufacturing cost to stay competitive in the global market. Porosity in aluminum die-cast squirrel cage rotors is inevitably introduced during the die cast process. Porosity can cause degradation in motor performance and can lead to a forced outage causing irreversible damage if it is extreme. Many off-line and on-line quality assurance test methods have been developed and applied for assessment of rotor quality. However, years of experience with the existing test methods revealed that they are not suitable for quality testing or capable of providing a quantitative assessment of rotor condition with sufficient sensitivity. In this paper, a new off-line test method capable of providing sensitive assessment of rotor porosity is proposed. It is shown that rotors with minor and distributed porosity that are difficult to detect with other tests can be screened out during manufacturing. The proposed method is verified through a 3 dimensional finite element analysis and experimental testing on closed and semi-open slot aluminum die cast rotors of 5.5 kW induction motors with porosity.