Partial Demagnetisation Research Articles

Autonomous fault detection and diagnosis for permanent magnet synchronous motors using combined variational mode decomposition, the Hilbert-Huang transform, and a convolutional neural network

The continuous and online monitoring of the condition of electrical machines is key to their safe operation. This study introduces a novel fault detection and diagnosis technique for continuous monitoring of faults in permanent magnet synchronous motors (PMSM). The proposed method relies solely on built-in sensors (stator phase currents only) to detect three types of faults: inter-turn short circuit, partial demagnetisation, and static eccentricity. Our fault detection and diagnosis strategy was developed by combining variational mode decomposition (VMD), the Hilbert-Huang transform (HHT) and a convolutional neural network (CNN). The VMD is first applied to the stator phase current signals to analyse the characteristic behaviour of the current signals by decomposing the current signals into several intrinsic mode functions. The intrinsic mode functions of the healthy and faulty signals are compared, and that with the frequency shift characteristics is selected. HHT is then applied to extract the fault feature by calculating the instantaneous frequency. Finally, the instantaneous frequency feature is fed into the CNN, which is designed to detect and classify motor faults. Experimental results clearly show that the variation of the instantaneous frequency of the PMSM, working at different operating states, can be utilised for condition monitoring and fault detection.

Mixed Fault Classification of Sensorless PMSM Drive in Dynamic Operations Based on External Stray Flux Sensors.

This paper aims to classify local demagnetisation and inter-turn short-circuit (ITSC) on position sensorless permanent magnet synchronous motors (PMSM) in transient states based on external stray flux and learning classifier. Within the framework, four supervised machine learning tools were tested: ensemble decision tree (EDT), k-nearest neighbours (KNN), support vector machine (SVM), and feedforward neural network (FNN). All algorithms are trained on datasets from one operational profile but tested on other different operation profiles. Their input features or spectrograms are computed from resampled time-series data based on the estimated position of the rotor from one stray flux sensor through an optimisation problem. This eliminates the need for the position sensors, allowing for the fault classification of sensorless PMSM drives using only two external stray flux sensors alone. Both SVM and FNN algorithms could identify a single fault of the magnet defect with an accuracy higher than 95% in transient states. For mixed faults, the FNN-based algorithm could identify ITSC in parallel-strands stator winding and local partial demagnetisation with an accuracy of 87.1%.

Failure Detection by Signal Similarity Measurement of Brushless DC Motors

In recent years, Brushless DC (BLDC) motors have been gaining popularity as a solution for providing mechanical power, starting from low cost mobility solutions like the electric bikes, to high performance and high reliability aeronautical Electro-Mechanical Actuator (EMA). In this framework, the availability of fault detection tools suited to these types of machines appears necessary. There is already a vast literature on this topic, but only a small percentage of the proposed techniques have been developed to a sufficiently high Technology Readiness Level (TRL) to be implementable in industrial applications. The investigation on the state of the art carried out during the first phase of the present work, tried to collect the techniques which are closest to possible implementation. To fill a gap identified in the current techniques, a partial demagnetisation detection method is proposed in this paper. This technique takes advantage of the asymmetries generated in the current by the missing magnetic flux to detect the failure. Simulations and laboratory experiments have been carried out to validate the idea, showing the potential and the easy implementation of the method. The results have been examined in detail and satisfactory conclusions have been drawn.

Continuous demagnetisation assessment for triple redundant nine‐phase fault‐tolerant permanent magnet machine

In this study, the risk of partial irreversible demagnetisation of a triple redundant, nine-phase fault tolerant machine based on permanent-magnet-assisted synchronous reluctance machine topology has been comprehensively assessed under various faults, including the worst case scenario when the applied voltage vector is in the opposite direction of the back electromotive force due to sensing and inverter control failure, by employing a continuous demagnetisation model. The dynamic response during fault transients and the post-demagnetisation performance, such as the demagnetisation distribution, reduction in the back EMF and torque, will also be analysed and compared.

Diagnosis of air‐gap eccentricity and partial demagnetisation of an interior permanent magnet synchronous motor based on inverse transient complex inductance vector theory

On the basis of predecessors, a theory named inverse transient complex inductance vector is proposed for air-gap eccentricity fault diagnosis of interior permanent magnet synchronous motors, and a specific voltage sequence injection method with simple implementation is proposed here. It is a real-time visualised diagnostic approach. A unique fault index is proposed to diagnose eccentricities. The absolute value of the fault index is positively correlated with the total eccentricity degree. Three types of artificial eccentricities and one type of artificial partial demagnetisation are tested. Both simulations and experiments match the proposed theory and diagnostic approach. The approach gives a clear distinction in detecting air-gap eccentricity and partial magnet demagnetisation. It performs best in the low-speed operating condition.

Diagnosing and distinguishing rotor eccentricity from partial demagnetisation of interior PMSM based on fluctuation of high‐frequency d ‐axis inductance and rotor flux

An appropriate and complete mathematical modelling approach is developed for an interior permanent magnet synchronous motor (PMSM) with general eccentricity. The relationships between air-gap eccentricity and simulated inductances in different coordinate frames are analysed in depth. Then, an analytical high-frequency inductance model of the interior PMSM with rotor eccentricity is developed. Based on this model, an online diagnostic approach is proposed, which includes estimation of the high-frequency d-axis inductance and permanent magnet (PM) flux. Thus the rotor eccentricity and PM partial demagnetisation can be detected and distinguished. Experimental results validate the proposed theory and fault diagnosis approach. Finally, an effective and reliable eccentricity indicator is presented. It is shown that analysis of the essential sources of fault characteristics facilitates motor fault diagnosis.

No‐load losses based method to detect demagnetisation fault in permanent magnet synchronous motors with parallel branches

For permanent magnet synchronous motors with parallel branches, circulating currents are induced by a partial demagnetisation fault. The induced currents increase the losses of the motor. Therefore, detecting such a fault becomes possible by analysing the motor losses. This study analyses the effects of the partial demagnetisation fault and defines an index, which is the ratio of increased losses and total losses when the motor is under no-load conditions, to monitor the condition of the rotor magnets. The method is easily implemented and sensitive to the fault. The experimental results validate the effectiveness of the proposed method.

Demagnetisation fault detection in PMSM using zero sequence current components

A demagnetisation fault detection method for a delta-connected permanent magnet synchronous machine (PMSM) based on zero sequence current components (ZSCC) is proposed. The model of the delta-connected PMSM with the demagnetisation fault is first established. Then the expression of the ZSCC is presented for the delta-connected PMSM, and an equivalent fault severity index which can quantify the demagnetisation degree is obtained, being not limited by the partial or uniform demagnetisation. Finally, the simulation results show that the potential of the proposed fault severity index for the demagnetisation fault detection.

Temperature dependence of a magnetically levitated electromagnetic vibration energy harvester

Electromagnetic vibration energy harvesters including magnetically levitated devices where opposing magnets are used to form the spring have been well documented. The strength of the magnets naturally has a large influence on the dynamic characteristics and output power of such harvesters. However, it can be affected by ambient temperatures which vary from applications to applications. This paper presents investigation into the performance of a magnetically levitated electromagnetic energy harvester under various ambient temperatures. Parameters investigated include magnetic flux density, resonant frequency, damping ratio, open circuit output voltage, velocity of the relative motion and the load resistance. Both simulation and experimental results show that these properties vary with ambient temperatures. The magnetic flux density reduces as the temperature increases which results in lower resonant frequency, lower relative velocity, lower open circuit output voltage and higher damping ratio. Varying resonant frequencies with temperature can lead to harvesters being de-tuned from the target vibration frequency. Decreasing magnetic field strength and increased damping ratios will also reduce output power even if the harvester’s resonant frequency still matches the environmental vibration frequency. The power transferred to the electrical load will be reduced due to the variation in the optimal load resistance with temperature. This means the harvester is no longer matched to achieve the maximum harvested power. The specified maximum operating temperature of the magnets was found to lead to partial demagnetisation. When cycling from room to the maximum specified temperature, the magnetic field was initially found to fall but remained constant thereafter. Harvesters were found to operate beyond the specified maximum operating temperature of the magnet, but suffer from a reduced magnetic strength.

Determining demagnetisation risk for two PM wind power generators with different PM material and identical stators

Ways to utilise ferrite permanent magnets (PMs), in a better way has been in focus the last couple of years since the use of neodymium-iron-boron (NdFeB) PMs has been debated. While ferrite PMs offer a low-cost alternative to rare-earth PMs, it is a trade-off for lower energy density. Depending on the type of PM and if the PMs are surface mounted or buried, the risk of demagnetisation during a fault condition can vary significantly between machines. In this study, the demagnetisation risk of two electrically similar generators with identical stators has been studied during several short-circuit faults at different temperatures. The study is simulation-based, and the results show that the generator with the ferrite rotor will suffer from a small but not significant amount of demagnetisation in the worst, three-phase-neutral, short-circuit case at a temperature of 5°C, whereas the NdFeB PMs will suffer from partial demagnetisation if a fault occurs at 120°C. For operational temperatures between 20 and 60°C both generators will sustain a short-circuit event.

First-order symmetry of weak-field partial thermoremanence in multi-domain ferromagnetic grains. 1. Experimental evidence and physical implications

We present experimental evidence that partial thermal demagnetisation and remagnetisation treatments produce effects to the measured thermoremanent magnetisation (TRM) of an assemblage of multidomain (MD) ferromagnetic grains that are symmetrical to first order. Just as for assemblages of single domain (SD) grains, measured TRM is affected by a partial thermomagnetic treatment to an extent dependent only on the difference of the magnetic field vectors between that used to impart the TRM and that used in the partial treatment and is insensitive to the absolute field intensity. The only difference between MD and SD TRM is that the effect of a partial treatment in the former partly comprises a non-reciprocal component which cannot easily be erased. The results suggest a new paradigm for MD TRM whereby a full TRM is regarded as a background state rather than a string of partial TRMs and thermal demagnetisation treatments may be considered as overprinting the TRM rather than erasing it. The symmetrical behaviour can be explained through a simple kinematic model incorporating a temperature-dependent domain structure. This model, although greatly oversimplified, also exhibits numerous other observed traits of MD TRM behaviour including the additivity of pTRMs, the exponentially decreasing progressive effects of iterated treatments, and the concave-up Arai plots produced by a Thellier palaeointensity experiment. We argue that the conventional domain wall-pinning process is only dominant in highly stressed grains and may be responsible for a preference for MD grains to be demagnetised and consequently for a loss of symmetry which is barely discernable in this study.

Interpretation and Modelling, Based on Petrophysical Measurements, of the Wirrda Well Potential Field Anomaly, South Australia

The Wirrda Well Prospect is located approximately 20 km south-southeast of Roxby Downs and 500 km north-northwest of Adelaide, South Australia. The Wirrda Well deposit consists of a massive granite breccia. Within the breccia, hematite and magnetite are pervasive, and copper, uranium and gold are associated. Evidence suggests this distinctive deposit may be an analogue of the Olympic Dam orebody.A variety of petrophysical measurements have been made on drill core samples from the Wirrda Well deposit, including density, gamma activity, compressional velocity and thermal and electrical conductivity. Detailed magnetic studies were also undertaken, comprising magnetic susceptibility, natural remanent magnetisation, partial demagnetisation, anisotropy and saturation magnetisation measurements.Results of these studies enabled comparisons to be made between the physical properties of the host rocks, the cover sequences and the surrounding country rocks. Generally, the host rocks are of greater density, have a higher magnetic susceptibility and gross gamma ray activity, and are better conductors of thermal and electrical energy than nearby unmineralised rocks. The main carrier of magnetism was found to be multi-domain, coarse-grained magnetite and remanence was a significant component of the total magnetisation.The Wirrda Well potential field anomaly consists of near-coincident magnetic and gravity highs. Modelling of the ground magnetic data, constrained by geological logs and petrophysical measurements, could not account for the observed anomaly, suggesting the main causative body of this anomaly has not yet been intersected by drilling.From modelling of the gravity anomaly, it was demonstrated that only half of the magnitude of the anomaly can be resolved, utilising bodies constrained by drillhole data. It has been concluded that the remainder of the anomaly may be explained by the presence of a central vertical pipe with three apophyses radiating to the northwest, southeast and southwest, that are largely untested by drilling.

Resonant torque magnetometry: a new in-situ technique for determining the magnetic properties of thin film MFM tips

A new high sensitivity Resonant Torque Magnetometry (RTM) technique has been developed enabling the determination of the magnetic properties of Magnetic Force Microscope (MFM) tips in-situ. A sensitivity of better than 10/sup -12/ emu (10/sup -15/ Am/sup 2/) has been achieved at ambient temperature and pressure. We have studied the micromagnetic properties of a variety of tip coatings including hard CoCr and CoPt films and soft permalloy. In-situ determination of the magnetic properties of MFM tips is vital if quantitative interpretation of MFM images is to be achieved especially when images are made in the presence of bias fields. The m(/spl mu//sub 0/H) loops obtained for CoPt tips have a shape similar to a standard magnetisation loop when the field is applied parallel to the tip axis. With the field applied perpendicular to the tip both CoPt and CoCr demonstrate behaviour consistent with partial demagnetisation of the axial moment, to which the RTM is sensitive.

A geophysical reconnaissance survey of Great Barrier Island, North Island, New Zealand

Abstract Results of a gravity and an airborne magnetic survey of Great Barrier Island are described. The interpretation of residual Bouguer gravity anomalies shows that a sequence of Miocene andesites up to 2 km thick (Coromandel Group volcanics) has been deposited in the elongated, northwest-trending Great Barrier Depression which extends offshore to the west and northwest of the island. Late Miocene rhyolites (Whitianga Group volcanics) erupted from the Mt Hobson caldera, a 2 km deep depression within the Miocene volcanics, and at Rakitu Island and Okupu. Contemporaneous geothermal activity produced widespread alteration of near-surface volcanics, mineral deposition, and partial demagnetisation which is reflected in the low-amplitude pattern of magnetic anomalies over the central part of the island.

Note on a magnetic detector of electric waves, which can be employed as a receiver for space telegraphy

The present note bears upon the special manner in which a core or rod of iron or steel placed in a varying magnetic field is affected by high-frequency oscillations transmitted from considerable distances. The magnetisation and demagnetisation of steel needles by the effect of electrical oscillations has long been known, and has been noted especially by Professor J. Henry, Abria, Lord Rayleigh, and others. Mr. E. Rutherford also has described a magnetic detector of electric waves, based on the partial demagnetisation of a small core composed of fine steel needles, previously magnetised to saturation, and placed in a solenoid of fine copper wire connected to exposed plates.