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On-line detection of rotor and load faults in permanent magnet synchronous motors (PMSM) based on spectrum analysis of current or vibration is not capable of identifying the root cause of the fault, since all faults produce identical fault signatures. The sensitivity of fault detection also depends on the motor and controller design making fault detection unreliable. In addition, operation under variable frequency and load limits the effectiveness of spectrum analysis based methods. In this paper, airgap flux monitoring is investigated as an alternative for providing reliable identification of rotor and load faults in PMSMs. Based on the analysis of airgap flux under partial and uniform demagnetization, dynamic eccentricity and load unbalance, an airgap search coil voltage based method for detection and classification of the faults is proposed. The claims made in the paper are verified through experimental testing on an IPMSM under emulated fault conditions along with a comparison to vibration and current spectra-based detection. It is shown that the proposed method provides reliable on-line identification of the faults for cases where conventional spectrum analysis based methods fail.

There has been active research in fault diagnosis of permanent magnet synchronous motors (PMSM) with the recent increase in reliability-critical PMSM applications. Most of the research effort is focused on motor current signature analysis (MCSA), as it allows remote, on-line detection of rotor and load defects using the current measurement available in the drive. A critical limitation of MCSA or vibration analysis is their inability to distinguish the type of rotor or load defect. It is crucial for the maintenance engineer to identify the type of defect, since the course of action for continued operation differs depending on the fault. In this work, a simple automated test method for identifying mass unbalance using vibration measurement commonly available in motor drive systems, is proposed. The effectiveness of the proposed method is verified on a PMSM under emulated PM demagnetization, dynamic eccentricity, and mass unbalance conditions.

Motor current signature analysis (MCSA) has become the most widespread test for detecting rotor cage faults as it can provide remote, on-line monitoring. Although MCSA has been successful, many cases of false indications have been reported in the field. False negative indications, where MCSA fails to detect the fault due to load variation, low slip operation, error in speed estimate, and non-adjacent broken bars, is considered serious as it can lead to unexpected motor failure. This has triggered research on alternative monitoring techniques for reliable detection of rotor faults with flux monitoring being one of them. In this paper, a new test method based on time-frequency analysis of the airgap flux is proposed and verified. Experimental test results are given under load variation, low slip, and non-adjacent broken bar conditions to support the claims on the reliability of the proposed method for cases where MCSA produces false negative indications.

Motor current signature analysis (MCSA) is widespread in the field as a means of providing remote, online monitoring of industrial induction motor rotor faults. However, numerous cases of false indications with MCSA produced due to asymmetries in the rotor structure or load leading to unnecessary inspection or forced outages have been reported. Although alternative test methods under motor standstill or starting transient immune to the false indications have been proposed, a reliable method capable of “online” monitoring is desirable. In this article, a new fault indicator in the airgap or stray flux capable of reliable online detection is proposed. It is shown that the fifth sideband of the rotor rotational speed frequency can provide detection of rotor faults immune to most of the false MCSA indications. Experimental results under the most common false indications are provided to verify the claims made for cases where online MCSA produces false indications. The proposed fault indicator is also shown to be immune to eccentricity and common load defects such as load unbalance and misalignment.

Damper and field windings are critical components of wound field synchronous motors (WFSM), where failure can lead to forced outage of the motor and industrial process. Due to the relatively rare presence of WFSMs in industry compared to induction motors, research on fault detection techniques for WFSMs is not as active. Rotor fault detection in WFSMs is also difficult, as synchronous operation leaves damper bars inactive during steady state operation while a small number of shorted field windings do not cause noticeable asymmetry. Therefore, most of the field-wide accepted tests rely on off-line testing or visual inspection of the rotor, or require airgap flux measurements, which necessitate motor disassembly and/or installation of sensors. Hence non-intrusive test methods which do not require motor disassembly are sorely needed to guarantee continued operation of the motor and industrial process. In this article, novel test methods based on the use of the leakage flux measured via search coils placed externally on the motor frame are proposed. A non-intrusive method for detecting and distinguishing damper and field winding faults during motor starting is devised from the time and frequency domain analysis of leakage flux and current. The methods are verified via experimental testing on a 30-kW salient pole WFSM.

Online monitoring of various faults that are associated with the permanent magnet synchronous motor (PMSM) is receiving much attention with the increasing demand in PMSM applications. Mechanical load imbalance is one of the mechanical defects that causes increase in the torque ripple, motor vibration, and can lead to accelerated wear and unstable drive operation, if the fault is undetected. Unlike existing methods that are limited to detection of rotor defects, in this article, a novel composite-control scheme is designed to estimate and compensate for the load torque disturbance (produced due to load imbalance) in the feedforward compensation design. It is shown that the proposed design with the feedback control law provides accurate compensation of load torque disturbance and achieves high-precision speed control (improved tracking performance up to 90% and smaller convergence time of up to 70%). Simulations as well as experimental testing validate the effectiveness and reliability of the proposed scheme.

Motor current signature analysis (MCSA) has been accepted in the field as a reliable means of detecting faults in the rotor cage of induction motors. However, there are many limitations to MCSA-based detection for other types of faults including rotor eccentricity and load defects. Recently, there has been increasing interest in airgap or leakage flux monitoring as an alternative to replace or complement MCSA. Flux monitoring can provide a reliable means of detecting rotor faults since anomalies in the rotor magnetomotive force or airgap can be directly observed. In this article, a new method based on monitoring the attenuation of rotor rotational frequency sidebands in the flux spectra is proposed for reliable detection and classification of rotor cage and eccentricity faults. It is also shown that flux monitoring can provide detection of rotor faults that is insensitive to the load defects. An experimental study under controlled broken bar, eccentricity, misalignment, and load unbalance conditions are given to support the claims. The results show that rotor cage, eccentricity, and load defects can be detected and distinguished for cases where MCSA alone is ineffective. A comparative evaluation between the proposed flux monitoring method and existing methods (MCSA, vibration analysis) is also given.

Recently, bioaerosols, including the 2019 novel coronavirus, pose a serious threat to global public health. Herein, we introduce a visible-light-activated (VLA) antimicrobial air filter functionalized with titanium dioxide (TiO2)-crystal violet (CV) nanocomposites facilitating abandoned visible light from sunlight or indoor lights. The TiO2-CV based VLA antimicrobial air filters exhibit a potent inactivation rate of ∼99.98% and filtration efficiency of ∼99.9% against various bioaerosols. Under visible-light, the CV is involved in overall inactivation by inducing reactive oxygen species production both directly (CV itself) and indirectly (in combination with TiO2). Moreover, the susceptibility of the CV to humidity was significantly improved by forming a hydrophobic molecular layer on the TiO2 surface, highlighting its potential applicability in real environments such as exhaled or humid air. We believe this work can open a new avenue for designing and realizing practical antimicrobial technology using ubiquitous visible-light energy against the threat of infectious bioaerosols.

There has been active research on motor current signature analysis (MCSA) for over the last 30+ years, as it can provide remote monitoring of rotor and load defects using the current sensor available in the motor control center. However, many years of experience has shown that 1) rotor eccentricity and load defects cannot be distinguished and 2) false rotor cage fault indications are common in the field. There is a recent trend toward integrating “smart” self-diagnostics in electric machines through embedded sensors, especially in applications such as submersible pumps or nuclear plants where motor inspection is difficult. The objective of this article is to evaluate airgap flux measurement as an option for providing sensitive and reliable monitoring of motor and load defects. A new method based on the analysis of the airgap flux search coil voltage during motor starting is proposed for reliable detection and classification of rotor cage, eccentricity, and mechanical defects in the load. Experimental testing is provided to verify the claims made on the proposed method, for cases where MCSA fails.

Detection of rotor cage faults in induction motors based on motor current signature analysis (MCSA) is being extensively applied in the field for preventing forced outage of the motor and industrial process. Although MCSA is very effective for detecting broken bars that are adjacent to each other, it can fail if the broken bars are nonadjacent, which is common for applications with frequent starts. If multiple broken bars are spread out at locations where the rotor “electrical” asymmetry is canceled, the presence of broken bars is difficult to detect with MCSA. A false indication can lead to a catastrophic-forced outage, but the only known means of detecting this type of fault in the field is through rotor visual inspection. In this article, the feasibility of detecting nonadjacent broken rotor bars from the rotor rotational frequency sideband components in the internal and external search coil measurements during steady state and motor starting is evaluated. Experimental testing on a 7.5-hp induction motor shows that nonadjacent broken bars can be reliably detected from the analysis of flux measurements for cases where MCSA and all other electrical tests fail.

There are numerous issues associated with bacteria, particularly biofilms, which exhibit a strong resistance to antibiotics. This is currently considered an urgent global issue owing to the lack of effective treatments. Graphene oxide (GO) nanosheets are two-dimensional carbon materials that are available as a substrate for metal nanoparticles and have a lower release rate of metal ions than free metal nanoparticles by regulating the oxidation of metal nanoparticles, which is known to reduce the cytotoxicity caused by the free metal nanoparticles. Over centuries, metal particles, including Ag and Cu, have been considered as antibacterial agents. In this study, Ag and Cu bimetallic nanoparticles on a GO surface (Ag/Cu/GO) were synthesized using a chemical reduction method, and their antimicrobial effects against several bacterial species were demonstrated. Ag/Cu/GO nanocomposites were characterized by transmission electron microscopy and energy-dispersive X-ray spectroscopy. The in vitro cytotoxicity of an Ag/Cu/GO nanocomposite was evaluated in human dermal fibroblasts, and its antibacterial activity against Methylobacterium spp., Sphingomonas spp., and Pseudomonas aeruginosa (P. aeruginosa) was also tested. The synthesized Ag/Cu/GO nanocomposite was able to eradicate all three bacterial species at a concentration that was harmless to human cells. In addition, Ag/Cu/GO successfully removed a biofilm originated from the culturing of P. aeruginosa in a microchannel with a dynamic flow. In a small-animal model, a biofilm-infected skin wound was healed quickly and efficiently by the topical application of Ag/Cu/GO. The Ag/Cu/GO nanocomposites reported in this study could be used to effectively remove antibiotic-resistant bacteria and treat diseases in the skin or wound due to bacterial infections and biofilm formation.

Partial discharge (PD) has been identified as one of the root causes of stator insulation failure even for low voltage (LV) ac motors. For reliable operation of variable frequency drive motors, a standard acceptance test procedure (IEC STD 60034-18-41) to ensure PD-free operation of LV motors was developed. Although the test can verify whether PD exists in the stator for a surge with predetermined voltage level and risetime, the information on where PD activity is present in the stator insulation is not provided. In this article, a test method based on a new variable risetime surge generator is proposed for identifying the component of stator insulation where PD occurs in LV random wound stators. The proposed test allows the user to locate whether PD is in the turn, phase, or ground insulation. This information is critical for lowering the PD inception voltage for improving the motor reliability through insulation system design/manufacturing or motor/drive system reconfiguration. The test is implemented with a 4.5 kV variable risetime surge generator prototype, and seven LV ac motors are tested to show where PD is likely to occur in commercial LV motors.

The damper (or amortisseur) winding of synchronous motors (SMs) is a critical component required for starting the motor. Several cases of the forced outage of industrial processes due to the starting failure of salient-pole SMs caused by damper bar failure have recently been reported. The detection of broken damper bars is difficult since they are active only during rotor acceleration, and offline visual inspection is the only means of testing available in the field. It was recently shown that the broken damper bars can be detected from the airgap flux, but this requires the installation of airgap flux sensors inside the motor. In this article, a method based on the analysis of the stator starting current is proposed for detecting broken damper bars. In addition, an electrical detection method based on signal injection from the motor terminals is proposed. The new methods have been devised to provide remote testing from the motor control center without motor disassembly required in existing tests. Finite-element analysis (1 MW) and experimental testing (30 kW) performed on salient-pole SMs with emulated broken damper bars show that the proposed methods can provide the sensitive and reliable detection of damper bar failures.

A difficult problem of a high-resistance (HR) connection for permanent magnet synchronous motor (PMSM) drives is investigated in this article. The presence of HR affects stator currents in such a way to deviate from their reference currents in the vector-control structure of the drive, which degrades the performance, reliability, and efficiency of the system. Unlike existing schemes, which are limited to detection of the HR connection, in this article, a current-control scheme based on a higher order sliding mode is designed to not only detect, but also provide fault-tolerant control simultaneously. The HR connection control is obtained by canceling extra dynamics (which are generated due to the HR connection) in the considered PMSM current model, which provides accurate tracking of actual current dynamics $(d,q)$ to the reference currents. Moreover, the HR is also estimated from the robust sliding-mode control terms to monitor the asymmetry in the faulted phase. Simulation and experimental results on a three-phase laboratory test PMSM confirm the validity of the proposed scheme for various cases of stator phase resistances.

The failure of Medium Voltage (MV) induction motors is a major concern in the steel industry since it can cause an unplanned outage resulting in significant economic consequences. Predictive maintenance based on offline and online testing of the induction motors is critical for maintaining reliable, efficient, and safe plant operations. Many years of experience has indicated that the MV induction motor failure rate can be reduced with periodic motor testing, resulting in improved plant reliability and productivity. It has been observed that false fault indications are commonly produced by commercially available test equipment. Considering that the application of multiple types of tests can improve outcome reliability, the effectiveness of a new test approach based on the analysis of the motor starting current was investigated for detecting defects in the rotor cage and reactor starting. An analysis of the starting data obtained from 614-MV induction motors operating at a steel mill over the past 10+ years showed that these two types of faults were present in 45-MV motors. Five successful case studies of rotor cage and four cases of reactor starting defects are presented in this article to demonstrate the advantages of starting current analysis for induction motor monitoring.

Many brands of filtering facepiece respirators are used against air pollution, including bioaerosols; however, studies have explored exposure to bioaerosols from the inside surfaces of respirators. We evaluated the filtration efficiencies and microbial recovery rates of commercial filtering facepiece respirators against bioaerosols. Eight filtering facepiece respirators and one surgical mask were selected, all with high market shares in the Republic of Korea and certified by national or international standards. The tested filtering facepiece respirators were installed on the head of a mannequin under various airflow velocity and relative humidity (RH) conditions. The filtration efficiency against Staphylococcus epidermidis and Escherichia coli bioaerosols, the pressure drop of the filter, and the relative recovery rates for the bacteria were evaluated. The filtration efficiency of each filtering facepiece respirator ranged from 82% to 99%, depending on the filtration grade. The pressure drop was significantly affected by variations in the surrounding RH. The mean relative recovery rates of all filtering facepiece respirators were 14 ± 4.8% and 9 ± 4.7% for S. epidermidis and E. coli, respectively. These results indicate that airborne microorganisms can survive and accumulate on the surfaces of filtering facepiece respirators, which may lead to harmful health outcomes. Our findings will be useful as background information for the development of commercial filtering facepiece respirators while considering their biological properties and reliable guidance to users.

As a social insect, termites have different castes and division of labor in a colony. Investigating the social behavior of subterranean termites is a challenge due to the cryptic nature and large colony size. Planar arenas are commonly used to study these termites under laboratory conditions, and have provided several advantages. However, there is no means to designate areas such as a royal chamber or central nest from foraging sites because reproductives can move freely across arenas. In this study, we examined the minimum passing size of different castes of Coptotermes formosanus Shiraki (Blattodea: Rhinotermitidae), in order to develop a reproductive excluder and correlated minimum passing size with head widths and heights. We found that workers and soldiers of C. formosanus were able to pass through a gap greater than or equal to 0.7 mm. Our results showed that there are significant differences in the head width and height based on castes and head height was more critical than head width to determine passing size. We further confirmed feasibilities of the reproductive excluders using incipient colonies of C. formosanus. Confining reproductives using the excluder in laboratory experiments will provide more chances to study the royal chamber and central nest independently of foraging sites.

IEEE Energy Conversion Congress and Exposition (ECCE) (2018, Portland), This work was supported in part by the Research, Technological Development and Innovation of the Spanish Ministry Economy and Competitiveness, under grant MINECO-17-ENE2016-80047-R and by the Economic Development Agency of the Principality of Asturias (IDEPA) & ThyssenKrupp under grant SV-PA-17-RIS3-2.

IEEE Energy Conversion Congress and Exposition, ECCE (13th. 2021. Virtual), This work was supported in part by the Research, Technological Development and Innovation Programs of the Spanish Ministry of Science and Innovation, under grant PID2019-106057RB-I00.

Air pollution causes millions of deaths every year. The aerosols, especially airborne nanoparticles generated by combustion, have detrimental effect on health. To protect public health against harmful aerosols, efforts to develop effective air cleaning technology have continued over the past several decades. However, the aerosol generation method used in air cleaning performance tests still rely largely on the traditional methods such as burning cigarettes, paper, and incense. Since the traditional method is inaccurate and unsteady, a more precisely controlled aerosol generation method should be developed. Here, we present a simple and inexpensive aerosol generation method that can easily and consistently produce submicron aerosols through laser ablation. This device constitutes an integrated system with a high-quality mini laser for rapid aerosol generation and a two-axis moving stage for continuous aerosol generation. We demonstrate that the concentration of generated aerosols could be easily controlled by selecting the laser irradiation time and power, resulting in the generation of ~104 particles/cm3 within a few seconds. In addition, the shape and size of generated aerosols can be controlled by changing the target material. This submicron aerosol generation process can be stably maintained for up to 1 h using small-sized (3 cm × 8 cm) affordable and accessible materials, such as wood and leather, highlighting the advantages of this inexpensive and easy-to-use combustion airborne submicron particle generation method.

Indoor air quality (IAQ) directly affects the health of occupants. Household manufacturing equipment (HME) used for hobbies or educational purposes is a new and unexplored source of air pollution. In this study, we evaluated the characteristics of particulate and gaseous pollutants produced by a household laser processing equipment (HLPE). Various target materials were tested using a commercial HLPE under various operating conditions of laser power and sheath air flow rate. The mode diameters of the emitted particles gradually decreased as laser power increased, while the particle number concentration (PNC) and particle emission rate (PER) increased. In addition, as the sheath air flow rate quadrupled from 10 to 40 L/min, the mode diameter of the emitted particles decreased by nearly 25%, but the effect on the PNC was insignificant. When the laser induced the target materials at 53 mW, the mode diameters of particles were 2.0 × 104 particles/cm3. Particularly, analyses of sampled aerosols indicated that harmful substances such as sulfur and barium were present in particles emitted from leather. The carcinogenic gaseous pollutants such as acrylonitrile, acetaldehyde, 1,3-butadiene, benzene, and C8 aromatics (ethylbenzene) were emitted from all target materials. In an actual indoor environment, the PNC of inhalable ultrafine particles (UFPs) was >5 × 104 particles/cm3 during 30 min of HLPE operation. Our results suggest that more meticulous control methods are needed, including the use of less harmful target materials along with filters or adsorbents that prevent emission of pollutants.

Lately, the monitoring and analysis of the induction motor stray flux has been a modern trend and significant research work was accomplished. Most papers have focused on the monitoring of rotor electrical faults around the fundamental stray flux signature, imitating in this way the traditional Motor Current Signature Analysis (MCSA). However, such signatures may still have some disadvantages leading to false alarms. This is the motivation behind the use of alternative signatures such as, in this particular case, the mechanical frequency. The existence of the mechanical frequency in the stator current is still the best signature for detection of the mixed rotor eccentricity fault and load imbalances. Even healthy motors present this harmonic due to some low-level inherent eccentricity and inherent load oscillations. Despite that, it will be shown for the first time in this paper that the mechanical frequency associated harmonics in the stray flux is possible to originate purely from rotor electrical faults. Those signatures present significant sensitivity to the broken bar fault severity. Moreover, they are advantageous for low load operation compared to traditional signatures. Finally, the sidebands of the mechanical frequency related harmonics are very sensitive to the broken rotor bar fault while quite immune to the number of the rotor bars.

A number of induction motor manufacturers are replacing fabricated copper rotors with the aluminum die-cast rotor design since they can be produced at lower cost and allow flexibility in the design with no restrictions in rotor bar shape. Despite the advantages, porosity is inevitably introduced during the die-casting process, and causes degradation in the starting and operating performance. The porosity level and distribution varies from rotor to rotor, and can cause motors of identical design to exhibit different characteristics. This makes it difficult for motor designers to predict the performance of motors accurately to guarantee that they meet the minimum efficiency specified in international standards; however, the influence of porosity has not been properly studied in the literature. In this paper, a method based on a combined 2-D and 3-D finite-element analysis that takes the porosity level and distribution into account is proposed for accurate prediction of motor performance. Experimental test results on a 440 V, 15 kW induction motor prototype with rotors that have 93% and 67% aluminum fill factor show that the proposed method provides reliable prediction of motor efficiency for rotors with porosity.

A number of appliance manufacturers have recently removed their copper (Cu) motor stator winding material and replaced them with aluminum (Al) to reduce product costs in the competitive global appliance market. Although a reduction in the motor cost can be achieved, it is accompanied by a noticeable increase in the failure rate due to stator insulation breakdown.

Two Korean citrus fruits, Jinjihyang and Chunggyun, were exposed to gamma irradiation at 0, 0.4 and 1 kGy as an ionizing quarantine treatment and were then subjected to evaluations of physicochemical and sensory quality attributes during storage at 4 ± 1 °C for 0, 10 and 20 days. Upon irradiation at 1 kGy, the narinrutin content of Jinjihyang significantly decreased (109.64–94.80 mg/100 g), while hesperidin content remained stable. However, both flavonoids decreased significantly during storage in both fruit varieties. Vitamin C was stable after irradiation but a significant decrease (42.97–27.21 mg/100 mL) was observed in 1 kGy-irradiated sample at the 20th day of storage. Brix, organic acids, and sensory properties of both fruits were not affected by irradiation or storage. The 1 kGy-treated fruits showed the highest titratable acidity (%) but this was not reflected in the sensory scores, even though Jinjihyang was more susceptible to irradiation than Chunggyun. Overall, irradiation at less than 1 kGy shows a potential to be used as a postharvest quarantine treatment for Korean citrus fruits.

The reliability of medium voltage (MV) motors used for driving pumps, fans, and other loads in the power generation industry is critical for continued power production. Periodic testing of stator winding insulation, which is the most vulnerable component in MV motors, is an essential part of predictive maintenance for preventing unplanned reduction in the power generation capability. Korea Electric Power Corporation (KEPCO) Research Institute's 17+ years of experience on predictive maintenance of stator insulation for MV motors is presented in this paper. KEPCO's stator insulation test program and statistics of test records are summarized, and cases of recent turn and phase insulation failures that resulted in motor forced outages are investigated. Analysis of the turn or phase insulation failures and insulation test records performed on the failed motors show that it is difficult to predict turn or phase insulation failures with the insulation tests currently performed. Turn or phase insulation test methods are summarized and evaluated based on the findings of the investigation to help target future research efforts toward industrial needs.

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 the following: 1) 6.6-kV motor-driven pulpers; and 2) a 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. It is also shown that the proposed method can be applied to vibration measurements for motors where vibration sensors are installed.

Control of permanent magnet synchronous machines (PMSMs) requires rotor position measurement/estimation, as well as the magnet polarity detection for startup of the machine. The rotor position of PMSMs is typically measured using a speed/position sensor (e.g., an encoder), while the magnet polarity is commonly measured using digital hall-effect sensors. Since simplifying the system can help improve reliability and reduce the cost associated with the measurements, the use of low cost analog hall-effect sensors for the estimation of the PM position, and polarity for control of PMSMs drives is evaluated in this paper. Experimental results performed on a 7.5-kW motor confirm that analog hall-effect sensors can replace both speed/position sensors and digital hall-effect sensors while maintaining adequate performance of the drive at a lower cost.

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 in extreme cases. Many offline and online 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 porosity with sufficient sensitivity. In this paper, a new offline 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-D finite element analysis and experimental testing on closed and semiopen slot aluminum die cast rotors of 5.5 kW induction motors with porosity.

One of the most important requirements for low-voltage induction motor designers is to meet the customer-specified starting characteristics and international regulations on efficiency. Although porosity in die-cast rotors degrade motor performance, the fill factor (FF) of the rotor bar and end ring are usually assumed to be 100 in the design stage. If the influence of FF is not properly taken into account in the design, it results in an inaccurate prediction of the starting performance. In this paper, the influence of die-cast rotor FF on the starting torque is investigated based on the FF and porosity distributions measured from actual aluminum die-cast rotors. A simplified 3-D finite-element analysis model that accounts for the porosity distribution in bars and end rings for different FFs is presented for predicting the starting characteristics. The effectiveness of the proposed method is verified on a 440 V, 15 kW induction motor prototype with rotors with FF of 93% and 67%. The results show that porosity has a significant influence on the starting torque, and must be taken into account for reliable prediction of motor performance for design optimization.

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 machine's 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.

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.

A simplified approach for predicting the starting performance of induction motors based on the equivalent circuit model and single slot pitch 2D and 3D FEA is proposed in this paper. The influence of the radial cooling ducts, rotor bar extension/end ring, as well as deep bar or skin effects, magnetic saturation are taken into account for prediction of starting performance with high precision. The results of the proposed method are compared to conventional methods, and verified through experimental testing on 6.6 kV, 522 kW induction motors. The results show that the proposed method can provide fast prediction of the starting characteristics for the given rotor design without sacrificing accuracy.

Motor current signature analysis (MCSA) based detection of rotor cage faults (RFs) in medium-high voltage induction motors has become common in the field due to its low-cost remote monitoring capability. However, low-frequency load oscillations (LOs) in applications that employ speed reduction couplings can induce frequency components in the vicinity of that of RFs. It has been reported that false-positive or -negative RF indications can be produced with the MCSA, if the frequency components produced by RF and LO overlap. There, recently, have been studies on discriminating RF and LO; however, the case where two harmonic components overlap at the same frequency, which is the most difficult and serious case, has not been investigated. In this paper, a spectrum analysis of instantaneous active and reactive power is applied, and it is shown that RF and LO can be separated for the case where the MCSA fails due to the overlap in the two spectral components. An experimental study performed on a 380 V motor with a speed reduction gear shows that RF and LO can be reliably detected and separated for avoiding false RF indications.

Lately, the zero-sequence current has been proposed for reliable detection of faults in induction motors. The zero-sequence current offers some distinct advantages compared to traditional methods, one of which is that it is able to detect a variety of faults with a single analysis. Despite its numerous advantages, the zero-sequence current has one significant drawback; it can only be applied to delta-connected induction motors or star-connected with the stator neutral connected to the supply. However, in many industrial cases, the large induction motors are connected in star without the neutral connected, to avoid extra losses. In those cases, the zero-sequence current method cannot be applied as the zero-sequence current simply does not exist. To overcome this drawback, this paper proposes for the first time the use of the zero-sequence stray flux for the detection of induction motor rotor electrical faults. It will be shown that the zero-sequence flux carries all the advantages of the zero-sequence current, while it can be applied to any 3-phase electrical machine independently from the stator winding configuration.

Monitoring of induction motor faults based on stray flux measurement has been investigated by many researchers due to its potential benefits in cost and simplicity. Although it was shown that flux-based monitoring can provide sensitive fault detection comparable to that of motor current signature analysis, the lack of “remote” monitoring capability has limited its practical use. The performance and reliability of stray flux-based detection of induction motor rotor cage faults are evaluated in this paper. It is shown for the first time in this work that the spectrum analysis of the radial stray flux can provide reliable detection of rotor faults immune to the influence of rotor axial air ducts, which is the most common cause of false rotor fault alarms. The reliability and sensitivity of stray flux-based rotor fault detection are demonstrated through experimental testing on laboratory and 6.6 kV field motors.

Recent cases of broken damper bars and shorted field winding turns in synchronous motors (SM) have shown that rotor faults can degrade motor performance and lead to a forced outage of the motor and driven process. However, detection of SM rotor faults has not received much attention since SMs are not as common as induction motors. Detection of SM rotor faults is difficult, as the damper bar is active only under the starting or load transients, and shorted field winding turns result only in a slight increase in rotating asymmetry. SM rotor fault testing in the field mainly relies on visual inspection or off-line tests, and new test methods that can provide reliable detection of the faults without motor disassembly is highly desirable. In this paper, new test methods for not only detecting, but also classifying the two types of SM rotor faults under motor standstill and starting are proposed. An experimental study on a 30 kW salient pole SM is provided to verify the proposed methods. It is shown that damper and field winding faults can be detected and classified with high sensitivity and reliability compared to conventional tests.

Detection of rotor cage faults in induction motors based on motor current signature analysis (MCSA) is being extensively applied in the field for preventing forced outage of the motor and industrial process. Although MCSA is very effective for detecting broken bars that are adjacent to each other, it can fail if the broken bars are non-adjacent, which is common for applications with frequent starts. If multiple broken bars are spread out at locations where the rotor "electrical" asymmetry is canceled, the presence of broken bars is difficult to detect with MCSA. A false indication can lead to a catastrophic forced outage, but the only known means of detecting this type of fault in the field is through rotor visual inspection. In this paper, the feasibility of detecting non-adjacent broken rotor bars from the rotor rotational frequency sideband components in the internal and external flux measurements during steady state and motor starting is evaluated. Experimental testing on a 7.5 hp induction motor shows that non-adjacent broken bars can be reliably detected from the analysis of flux measurements for cases where MCSA and all other electrical tests fail.

The damper (or amortisseur) winding of synchronous motors (SM) is a critical component required for starting the motor. Several cases of forced outage of industrial processes due to starting failure of salient pole SMs caused by damper bar damage have recently been reported. Detection of damper bar failure is difficult since they are active only during motor starting, and testing in the field currently relies on off-line visual inspection. It was recently shown that broken damper bars can be detected from the airgap flux, but this requires installation of airgap flux sensors inside the motor. In this paper, a method based on analysis of the stator starting current is proposed for detecting broken damper bars. In addition, an electrical detection method based on signal injection from the motor terminals is proposed. The new methods have been devised to provide remote testing from the motor control center without motor disassembly required in existing tests. Experimental testing performed on a 30 kW salient pole SM prototype with emulated broken bars shows that the proposed methods can provide sensitive and reliable detection of damper bar failures.

Partial discharge (PD) has been identified as one of the root causes of stator insulation failure even for low voltage (LV) ac motors. For reliable operation of variable frequency drive (VFD) motors, a standard acceptance test procedure (IEC STD 60034-18-41) to ensure PD-free operation of LV motors was recently developed. Although the test can verify whether PD exists in the stator for a voltage surge with predetermined voltage level and risetime, the information on where PD activity is present in the stator insulation is not clearly provided. In this paper, a test method for applying variable risetime surge voltage is proposed for identifying the component of stator insulation where PD occurs in the LV random wound motor stator. The proposed test allows the user to locate whether PD is in the turn, phase, or ground insulation. This information is critical for lowering the PD inception voltage for improving the motor reliability through insulation system design/manufacturing or motor/drive system reconfiguration. The test is implemented with a 4.5 kV variable risetime surge generator prototype, and 4 LV ac motors are tested to show where PD is likely to occur in commercial LV motors.

Lately, the monitoring and analysis of the induction motor stray flux has been a modern trend and significant research work has been accomplished. Most papers have focused on the monitoring of rotor electrical faults around the fundamental stray flux signature, imitating in this way the traditional Motor Current Signature Analysis (MCSA). However, more significant fault related harmonics exist at other frequencies and most significantly around the mechanical frequency. The existence of the mechanical frequency in the stator current is still the best signature for detection of the mixed rotor eccentricity fault. Even healthy motors present this harmonic due to some low level inherent eccentricity. Despite that, it will be shown for the first time in this paper, with extensive Finite Element Analysis (FEA) and experimental testing, that the mechanical frequency associated harmonics in the stray flux can be purely rotor electrical fault related and completely independent from any rotor eccentricity and rotor imbalance. This makes this specific harmonic unreliable for any rotor fault diagnosis although can be a good indicator of rotor electrical faults at low slip operation. Finally, the sidebands of the mechanical frequency harmonics appear to be very sensitive to the broken rotor bar fault while quite immune to the number of the rotor bars.

IEC 60034-18-41 has been developed as a standard acceptance test to ensure partial discharge (PD)-free operation for improving the reliability of low voltage (LV) variable frequency drive (VFD) motors. Although the test can verify whether PD exists in the stator for a voltage surge with predetermined magnitude and risetime, the information on where PD activity is occurring in the stator insulation is not clearly given. In this paper, a method based on variable risetime surge voltage testing is proposed for identifying the component of the stator insulation system where PD occurs in LV random wound motor stators (turn, phase, or ground insulation). This information can be used for increasing the PD inception voltage for improving the reliability of the motor through insulation system design/manufacturing. The test is implemented with a 4.5 kV variable risetime surge generator prototype. 4 LV ac motors are tested to show where PD is likely to occur in commercial LV motors.

Online monitoring and diagnostics of permanent magnet synchronous motors (PMSMs) is becoming important with the increasing demand in PMSM applications. Most of the research effort focuses on motor current signature analysis (MCSA) as it can provide remote, online monitoring at low cost. However, all types of defects that produce asymmetries in the PMSM rotor or load produce identical rotor rotational frequency components. This is a serious limitation when applying MCSA since it can produce false indications and degrade the sensitivity of fault detection. The requirement of complex time–frequency analysis techniques is another limitation of MCSA for applications with speed variations. In this paper, the feasibility of using analog Hall sensor signals to complement MCSA for detection and classification of rotor- and load-related defects is investigated. It is shown that Hall sensors installed in machines for initial rotor position estimation can be used with minimal hardware modifications to detect and classify signatures produced by the rotor and load for cases where MCSA fails, even during transient conditions. Experimental testing performed on an interior permanent magnet synchronous motor (IPMSM) under eccentricity, local demagnetization, and load unbalance conditions shows that the reliability and sensitivity of fault detection in PMSM systems can be improved at low added cost.

Rotor eccentricity and local demagnetization in permanent magnet synchronous motors (PMSMs) increase unbalanced magnetic pull and motor vibration resulting in accelerated aging of motor components. If the asymmetry in the rotor 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 PMSM rotor asymmetry currently relies on offline testing and online vibration/current spectrum analysis. However, they are inconvenient or cannot provide reliable detection of rotor faults for all PMSM designs. In this paper, the feasibility of using the signals from analog Hall-effect field sensors for detecting eccentricity and local demagnetization is investigated. It is shown that Hall sensors present in machines for motion control can be used for directly measuring the variation in the flux inside the motor due to rotor magnetic asymmetry with minimal hardware modifications. Three-dimensional finite-element analysis and experimental results performed on an interior PMSM show that the proposed method can provide sensitive and reliable detection of dynamic/mixed eccentricity and local PM demagnetization.

Stator-slot magnetic wedges are used instead of conventional epoxy glass wedges in large, high-output induction motors since the motor efficiency and power factor can be improved. However, cases of loose or missing magnetic wedges due to their weak mechanical strength have recently been increasingly reported. Although missing wedges can deteriorate motor performance and reliability, there is currently no test method accepted in the field for wedge quality assessment other than offline, disassembled visual inspection. In this paper, a new in-service method for monitoring the magnetic wedge condition in induction motors based on the negative-sequence information is proposed. The new method is developed based on the analysis of the dynamic model of the induction motor with missing magnetic slot wedges, derived in this paper. The proposed method is verified on a finite-element (FE) model of a 6.6-kV, 1850-kW induction motor. It is also verified experimentally on a 6.6-kV, 220-kW induction motor operating in the field with missing wedges (before and after rewedging), and on a 380-V, 5.5-kW induction motor with custom-built, removable magnetic wedges. It is shown that the proposed method can provide online detection of missing magnetic wedges, which can help reduce maintenance costs and risk of degradation in motor performance.

Recent cases of broken damper bars and shorted field winding turns in synchronous motors (SMs) have shown that rotor faults can degrade motor performance and lead to a forced outage of the motor and driven process. However, detection of SM rotor faults has not received much attention since SMs are not as common as induction motors. Detection of SM rotor faults is difficult, as the damper bar is active only under the starting or load transients, and shorted field winding turns result only in a slight increase in rotating asymmetry. SM rotor fault testing in the field mainly relies on visual inspection or off-line tests, and new test methods that can provide reliable detection of the faults without motor disassembly is highly desirable. In this article, new test methods for not only detecting, but also classifying the two types of SM rotor faults under motor standstill and starting are proposed. A finite element and experimental study on 1.1 MW and 30 kW salient pole SMs are provided to verify the proposed methods. It is shown that damper and field winding faults can be detected and classified with high sensitivity and reliability compared to conventional tests.

Damper bars are often used in high output salient pole synchronous motor (SM) rotors for motor starting and stabilization of performance. Several cases of broken damper bars in the rotor of SMs that lead to motor starting failure and/or forced outage have recently been reported. However, detection of damper bar failures is difficult, since the damper bars are active only under the starting or load transients. Damper bar testing in the field currently relies on off-line visual inspection, and there is no means available for detecting failures on line. In this paper, a new method for detecting damper bar failures based on the analysis of airgap search coil measurements under the starting transient is proposed. Since airgap search coils are being increasingly installed in large SMs for detecting field winding short circuits, the proposed method can be implemented without additional hardware. The effectiveness of the proposed method is verified with two-dimensional finite element analysis and experimental testing on a custom-built 30 kW salient pole SM under controlled fault conditions. It is shown that the proposed method can provide reliable and sensitive detection of broken damper bars whenever the motor is started at low additional cost. The off-line single phase rotation test is also shown to provide reliable monitoring of damper bar condition.

We developed an antimicrobial air filter using natural sea salt (NSS) particles. Airborne NSS particles were produced via an aerosol process and were continuously coated onto the surface of an air filter under various deposition times. The filtration efficiency and bactericidal performance of the NSS-coated filter against aerosolized bacterial particles (Staphylococcus epidermidis, Escherichia coli) were evaluated quantitatively. The filtration efficiency of the tested filter ranged from 95% to 99% depending on the deposition time, and the bactericidal performance demonstrated efficiencies of more than 98% against both tested bacterial bioaerosols when the NSS deposition ratio was more than 500 μg/cm2. The experimental results indicated that the NSS-coated filters have the potential to be used as effective antimicrobial air filters for decreasing environmental exposure to microbial contaminants.