Tae June Kang

Detection and Classification of Rotor Demagnetization and Eccentricity Faults for PM Synchronous Motors

Condition monitoring of rotor problems such as demagnetization and eccentricity in permanent magnet synchronous motors (PMSM) is essential for guaranteeing high motor performance, efficiency, and reliability. However, there are many limitations to the off-line and on-line methods currently used for PMSM rotor quality assessment. In this paper, an inverter-embedded technique for automated detection and classification of PMSM rotor faults is proposed as an alternative. The main concept is to use the inverter to perform a test whenever the motor is stopped, to detect rotor faults independent of operating conditions or load torque oscillations, which is not possible with motor current signature analysis (MCSA). The d-axis is excited with a dc+ac signal, and the variation in the inductance pattern due to the change in the degree of magnetic saturation caused by demagnetization or eccentricity is observed for fault detection. An experimental study on a 7.5kW PMSM verifies that demagnetization and eccentricity can be detected and classified independent of the load with high sensitivity.

Advanced Induction Motor Rotor Fault Diagnosis Via Continuous and Discrete Time–Frequency Tools

Transient-based fault diagnosis in induction motors has gained increasing attention over the recent years. This is due to its ability to avoid eventual wrong diagnostics of the conventional motor current signature analysis in certain industrial situations (presence of load toque oscillations, light loading conditions, and so on). However, the application of these transient methodologies requires the use of advanced signal processing tools. This paper presents a detailed comparison between the two main groups of transforms that are employed in transient analysis: discrete and continuous. This paper does not focus on trivial fault cases but on difficult real situations where the application of the conventional methods often leads to false diagnostics (outer bar breakages in double-cage motors, motors with rotor axial duct influence, and combined faults). Indeed, it is the first time that continuous tools are applied to some of these controversial situations. The results in this paper prove the special advantages of the continuous transforms, tearing down some false myths about their use.

Reliable Detection of Induction Motor Rotor Faults Under the Rotor Axial Air Duct Influence

Axial cooling air ducts in the rotor of large induction motors are known to produce magnetic asymmetry and can cause steady-state current or vibration spectrum analysis based fault detection techniques to fail. If the number of axial air ducts and that of poles are identical, frequency components that overlap with that of rotor faults can be produced for healthy motors. False positive rotor fault indication due to axial ducts is a common problem in the field that results in unnecessary maintenance cost. However, there is currently no known test method available for distinguishing rotor faults and false indications due to axial ducts other than offline rotor inspection or testing. Considering that there is no magnetic asymmetry under high slip conditions due to limited flux penetration into the rotor yoke, the detection of broken bars under the start-up transient is investigated in this paper. A wavelet-based detection method is proposed and verified on custom-built lab motors and 6.6-kV motors misdiagnosed with broken bars via steady-state spectrum analysis. It is shown that the proposed method provides the reliable detection of broken bars under the start-up transient independent of axial duct influence.

Screening of False Induction Motor Fault Alarms Produced by Axial Air Ducts Based on the Space-Harmonic-Induced Current Components

Motor current signature analysis (MCSA) based on the 50/60-Hz sidebands has become a common test in industry for monitoring the condition of the induction motor rotor cage. However, many cases of unnecessary motor inspection or outage due to false alarms produced by rotor axial duct interference have been reported. If the number of axial ducts and poles is identical, this can produce 50/60-Hz sideband frequency components in MCSA that overlap with that of rotor faults, resulting in false alarms. However, there currently is no practical test method available for distinguishing rotor faults and false indications other than testing the rotor offline or under the startup transient. In this paper, the feasibility of using the rotor fault frequency component produced by the space harmonic waves is evaluated as a solution for the first time. Since the fifth or seventh space harmonics have a spatial distribution of flux that does not penetrate in the rotor yoke to reach the axial ducts, they do not produce false alarms. The proposed method is verified on 6.6-kV motors misdiagnosed with broken bars via the 50/60-Hz sidebands of MCSA. It is shown that it provides reliable online indication of rotor faults independent of axial duct influence and can be used for screening out false alarms.

Experimental evaluation of using the surge PD test as a predictive maintenance tool for monitoring turn insulation quality in random wound AC motor stator windings

Turn insulation degradation is one of the major root causes of stator insulation failure leading to motor breakdown. The surge test is the only test available for testing the integrity of turn insulation; however, it is a high voltage pass/fail test that provides an indication only if an arc is instigated between the turns of weakened turn insulation, and therefore does not provide information regarding remaining lifetime. The surge PD test measures the partial discharge (PD) activity under surge excitation, and is used to date for assuring that voltage source PWM inverter-fed motors (IEC 60034-18-41 type I) are PDfree. In this paper, the potential of using the surge PD test as a predictive maintenance tool for turn insulation quality assessment is evaluated. Under the expectation that increasing PD activity in the voids formed by insulation degradation may be detectable before turn insulation failure, the test is performed periodically under accelerated thermal degradation on 6 windings. It is shown that change in the PD inception voltage under the surge PD test can be clearly observed before any other insulation test indicator. The results suggest that the surge PD test can be used for monitoring the condition of turn insulation for providing an early indication of stator insulation problems without the risk of puncturing turn insulation.

Experimental Evaluation of Low-Voltage Offline Testing for Induction Motor Rotor Fault Diagnostics

The offline single-phase rotor rotation test is frequently used in the field for assessing the integrity of the rotor cage without motor disassembly. However, the inconvenience of the large variable voltage supply requirement has triggered the advent of a portable low-voltage version of the test also known as the rotor influence check. This test has recently become common in the pulp and paper industry as it is convenient to use in an industrial environment. However, concerns on the reliability of the test have been raised due to frequent false test indications. The objective of this work is to provide an analysis and experimental evaluation of low-voltage rotor tests under controlled rotor fault conditions on 380-V and 6.6-kV motors. The results of the low-voltage test reveal that it cannot provide reliable diagnosis due to the low excitation flux level, particularly for motors with the closed rotor slot design. The single-phase rotation test and online current spectrum analysis are also performed under identical fault conditions to show that the fault conditions can be detected with standard tests.

Detection and classification of rotor demagnetization and eccentricity faults for PM synchronous motors

Condition monitoring of rotor problems such as demagnetization and eccentricity in permanent magnet synchronous motors (PMSM) is essential for guaranteeing high motor performance, efficiency, and reliability. However, there are many limitations to the off-line and on-line methods currently used for PMSM rotor quality assessment. In this paper, an inverter-embedded technique for automated detection and classification of PMSM rotor faults is proposed as an alternative. The main concept is to use the inverter to perform a test whenever the motor is stopped, to detect rotor faults independent of operating conditions or load torque oscillations, which is not possible with motor current signature analysis (MCSA). The d-axis is excited with a dc+ac signal, and the variation in the inductance pattern due to the change in the degree of magnetic saturation caused by demagnetization or eccentricity is observed for fault detection. An experimental study on a 7.5kW PMSM verifies that demagnetization and eccentricity can be detected and classified independent of the load with high sensitivity.

The influence of the rotor on surge pd testing of low voltage AC motor stator windings

Partial discharge (PD) has been identified as one the main root causes of stator winding failures in low voltage (LV) random wound AC motors fed by PWM inverters. To ensure that PWM inverter-fed motors (type I) are PD-free under impulse excitation, the test procedure for acceptance testing of the stator insulation system was developed (IEC TS 60034-18-41). Although many successful cases of applying the TS have been reported, some concerns regarding the test procedure have been raised, and the TS is currently under revision. It is implied in the TS that the rotor has no impact on the surge PD test result, and testing is often performed without the rotor; however, it was observed that the surge PD test results can change with rotor insertion for some types of machines. The off-line surge PD test must be performed under conditions representative of actual operation (with the motor assembled) to guarantee absence of PD. Therefore, it is important to investigate the influence of the rotor, and to incorporate the findings in the test procedure, if necessary. In this paper, the influence of the rotor on the off-line surge PD test is investigated for AC machines, and it is shown that the PD inception voltage (PDIV) can change significantly depending on the rotor design. A detailed analysis is given along with experimental test results on a number of induction and permanent magnet (PM) synchronous motor samples with different rotor designs. Based on the observations made in this work, suggestions on the TS that can provide surge PDIV measurements under conditions closer to that of the actual operation with the rotor are given to help reduce the likelihood of false indications with off-line surge PD testing.

Case studies of stator winding turn insulation failures in medium voltage motors

Breakdown of the stator winding turn insulation is a frequent cause of stator insulation failure leading to forced outage of medium voltage (MV) motors. Case studies of 15 turn insulation failures in mains-fed MV motors that occurred in Korea between 2011–2015 in the pulp and paper, power generation, and petrochemical industries are presented in this paper. The turn insulation failures are analyzed based on the insulation system design, motor operating conditions, fault location and pattern of the motor failures. Stator insulation inspection and test records including the insulation resistance (IR), polarization index (PI), leakage current, dissipation factor, and partial discharge (PD) tests for the failed motors are also shown. An insight into how stator winding turn insulation failures can be prevented is provided based on the investigation of the failures.

Use of discrete and optimized continuous TFD tools for transient-based diagnosis in controversial fault cases

Transient-based diagnosis of electromechanical failures in induction motors has gained an increasing attention over recent years. The diagnostic in some specific situations (presence of load toque oscillations, light loading conditions) or of specific failures may be difficult when using the classical MCSA approach. In this context, the transient-based methodologies have been proven to become valuable informational sources for the diagnosis, either confirming the MCSA results or avoiding its possible false positives. The application of these transient methodologies requires the use of modern signal processing tools that are in continuous evolution. This work proposes the application of an advanced tool; the recently developed Adaptive Slope Transform. The paper compares the performance of this continuous transform and that of a discrete counterpart, the Discrete Wavelet Transform, when applied to different controversial fault cases in which the classical MCSA may not lead to correct results: outer bar breakages in double cage motors and motors with rotor axial duct influence. The results show the potential of the continuous transforms for the transient tracking of high-order fault-related components as well as for the improved discrimination between fault components.