Jongman Hong

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.

Detection of Airgap Eccentricity for Permanent Magnet Synchronous Motors Based on the d-Axis Inductance

The majority of the work performed for detecting eccentricity faults for permanent magnet synchronous motors (PMSM) focus on motor current signature analysis (MCSA), as it provides continuous on-line monitoring with existing current sensors. However, MCSA cannot be applied under nonstationary conditions and cannot distinguish faults with load torque oscillations, which are limitations for many PMSM drive applications. In this paper, it is shown that the d-axis inductance, Ld, decreases with increase in the severity of eccentricity due to the change in the degree of magnetic saturation, and it is proposed as a new fault indicator. The inverter can be used to perform a standstill test automatically whenever the motor is stopped, to measure Ld for eccentricity testing independent of load variations or oscillations, which is not possible with MCSA. An FE and experimental study on a 10hp PMSM verifies that eccentricity can be detected independent of the load with high sensitivity and reliability.

Detection of Airgap Eccentricity for Induction Motors Using the Single-Phase Rotation Test

The single-phase rotation test (SPRT) is a simple and reliable offline test frequently used for detecting problems in the rotor cage of induction motors without motor disassembly. Airgap eccentricity due to bearing degradation, shaft flexing, etc., is another problem that is difficult to detect, which can cause catastrophic motor failure. In this paper, the feasibility of using the SPRT for detecting eccentricity, which has not been reported before, is investigated. The variation in the inductive component as a function of rotor position and time under static, dynamic, and mixed eccentricity conditions is analyzed. It is shown that airgap eccentricity can be detected in addition to rotor cage problems with the SPRT. An experimental study on a 7.5-Hp induction motor under controlled eccentricity and broken bar conditions shows that airgap eccentricity can be reliably detected with high sensitivity and distinguished from broken rotor bars.

Automated Monitoring of Magnet Quality for Permanent-Magnet Synchronous Motors at Standstill

Demagnetization of permanent magnets (PMs) used in synchronous motors can occur due to a combination of thermal, electrical, and/or environmental operating stresses. Since PM demagnetization results in degradation of motor performance, efficiency, and reliability, it is important to monitor the quality of PMs regularly. However, there are many limitations to the offline and online methods currently used for magnet quality assessment. In this paper, a new inverter-embedded technique for automated monitoring of magnet quality for permanent-magnet synchronous motors (PMSMs), which overcomes the limitations of existing techniques, is proposed. The main concept is to use the inverter to perform a standstill test whenever the motor is stopped to detect local or uniform PM demagnetization. The machine is excited with a pulsating field at different angular positions, and the change in the current peaks caused by the change in the degree of magnetic saturation due to demagnetization is observed. An experimental study on a 10-hp PMSM verifies that local and uniform PM demagnetizations can be detected with high sensitivity.

A New Strategy for Condition Monitoring of Adjustable Speed Induction Machine Drive Systems

A new strategy for monitoring the condition of adjustable speed induction machine drive systems is presented in this paper. The main concept of the new method is to use the inverter to perform off-line tests for quality assessment of the vulnerable components in the inverter, cable, and induction motor automatically whenever the motor is stopped. This new approach is suitable for monitoring system components that deteriorate gradually to failure, if the degradation is observable. Off-line test methods for monitoring the quality of the 1) dc-link aluminum electrolytic capacitor; 2) electrical connections; 3) cable and stator winding insulation; 4) stator core; and 5) rotor bar are presented along with a summary of the failure mechanism and existing test methods. It will be shown that the new strategy can be implemented with minimal hardware requirements, and that it has many advantages over existing off-line and on-line tests. An experimental study performed under simulated fault conditions for each component shows that the method provides a reliable and sensitive indication of drive system problems for reliable, efficient, and safe operation.

A Stator Core Quality Assessment Technique for Inverter-Fed Induction Machines

Localized heating caused by damage in the laminations or inter-laminar insulation of the stator core increases the core losses and can lead to machine failure. Therefore, it is important to monitor the quality of the stator core for reliable and efficient operation of the machine. The methods currently available for core quality assessment are inconvenient since they require an outage and the machine to be disassembled or operated under no load conditions. In this paper, a new method for monitoring the quality of the stator core for inverter-fed machines without motor disassembly or operation is proposed. The main concept is to use the inverter to apply a set of test signals to assess the quality of the core whenever the motor is at standstill. A set of high frequency pulsating magnetic fields are produced using the inverter, and the power loss is observed as a function of field circumferential position to detect core problems. An experimental study on 10 hp induction motor verifies that local inter-laminar core faults can be detected with high sensitivity. The proposed technique is expected to provide a simple solution for frequent stator core quality assessment without motor disassembly, motor operation, or additional hardware for reliable and efficient operation of inverter-fed induction machines.

Quality Assurance Testing for Magnetization Quality Assessment of BLDC Motors Used in Compressors

Quality assurance (QA) testing of mass-produced electrical appliances is critical for the reputation of manufacturer since defective units will have a negative impact on safety, reliability, efficiency, and performance of the end product. It has been observed at a brushless dc (BLDC) compressor motor manufacturing facility that improper magnetization of the rotor permanent magnet is one of the leading causes of motor defects. A new technique for postmanufacturing assessment of the magnetization quality of concentrated winding BLDC compressor motors for QA is proposed in this paper. The new method evaluates the data acquired during the test runs performed after motor assembly to observe anomalies in the zero-crossing pattern of the back-EMF voltages for screening motor units with defective magnetization. An experimental study on healthy and defective 250-W BLDC compressor motor units shows that the proposed technique provides sensitive detection of magnetization defects that existing tests were not capable of finding. The proposed algorithm does not require additional hardware for implementation since it can be added to the existing test-run inverter of the QA system as a software algorithm.

A Stator-Core Quality-Assessment Technique for Inverter-Fed Induction Machines

Localized heating caused by damage in the laminations or interlaminar insulation of the stator core increases the core losses and can lead to machine failure. Therefore, it is important to monitor the quality of the stator core for reliable and efficient operation of the machine. The methods currently available for core quality assessment are inconvenient since they require an outage and the machine to be disassembled or operated under no-load conditions. In this paper, a new method for monitoring the quality of the stator core for inverter-fed machines without motor disassembly or operation is proposed. The main concept is to use the inverter to apply a set of test signals to assess the quality of the core whenever the motor is at standstill. A set of high-frequency pulsating magnetic fields is produced using the inverter, and the power loss is observed as a function of field circumferential position to detect core problems. An experimental study on a 10-hp induction motor verifies that local interlaminar core faults can be detected with high sensitivity. The proposed technique is expected to provide a simple solution for frequent stator-core quality assessment without motor disassembly, motor operation, or additional hardware for reliable and efficient operation of inverter-fed induction machines.

A new strategy for condition monitoring of adjustable speed induction machine drive systems

A new strategy for monitoring the condition of adjustable speed induction machine drive systems is presented in this paper. The main concept of the new method is to use the inverter to perform off-line tests for quality assessment of the vulnerable components in the inverter, cable, and induction motor automatically whenever the motor is stopped. This new approach is suitable for monitoring system components that deteriorate gradually to failure, if the degradation is observable. Off-line test methods for monitoring the quality of the 1) dc-link aluminum electrolytic capacitor; 2) electrical connections; 3) cable and stator winding insulation; 4) stator core; and 5) rotor bar are presented along with a summary of the failure mechanism and existing test methods. It will be shown that the new strategy can be implemented with minimal hardware requirements, and that it has many advantages over existing off-line and on-line tests. An experimental study performed under simulated fault conditions for each component shows that the method provides a reliable and sensitive indication of drive system problems for reliable, efficient, and safe operation.

Detection of Stator-Slot Magnetic Wedge Failures for Induction Motors Without Disassembly

The recent trend in large ac machines is to employ magnetic stator-slot wedges for improving the motor efficiency, power factor, and power density. The mechanical strength of magnetic wedges is weak compared to the epoxy glass wedges, and many cases of loose and missing wedges have recently been increasingly reported. Magnetic wedge failure deteriorates the performance and reliability of the motor, but there is no method available for testing the wedge quality other than visual inspection after rotor removal. Monitoring of the overall wedge condition without motor disassembly can help reduce the cost of maintenance and risk of degradation in performance. In this paper, a new offline standstill test method for detecting magnetic wedge problems for ac machines without motor disassembly is proposed. An experimental study on 380-V 5.5-kW and 6.6-kV 3.4-MW motors with magnetic wedges is performed to verify the effectiveness of the new test method. It is shown that the new method can provide reliable monitoring of magnetic wedge problems over time, independent of other faults or motor design.