Permanent Magnet Demagnetization Research Articles

Irreversible Demagnetization Analysis for Multilayer Magnets of Permanent Magnet-Assisted Synchronous Reluctance Machines Considering Current Phase Angle

This paper investigates the demagnetization problem of permanent magnets for permanent magnet-assisted synchronous reluctance motors (PMa-SynRM) and applies “demagnetization ratio” as an index to evaluate demagnetization of permanent magnet (PM). The PMa-SynRM is often designed with multilayer flux barriers to improve saliency and reluctance torque in the rotor. Weaker or less PM (than that for interior PM motors) is embedded into the rotor which could be demagnetized in high-performance operation. PM demagnetization involves temperature, current magnitude, current phase angle, designed PM operating point, or combined effect of the above factors. This work investigates the demagnetization risk for PMs in all the layers of PMa-SynRM rotor considering these factors. In particular, the current phase angle for flux weakening operation is often omitted in demagnetization analysis. First, the motor performance to satisfy the operation condition over a wide speed range is analyzed and the proportion of motor torque components, i.e., reluctance torque and electromagnetic torque, considering temperature is investigated. Then, the flux density distributions on PMs at each position/layer are studied and the PM operating points are obtained based on the variation of excitation current, current phase angle, and temperature in order to evaluate the demagnetization risk. The analysis suggests that the high current phase angle combining the effect of high temperature is the major cause of irreversible demagnetization. Finally, a method incorporating the index “demagnetization ratio” is proposed to redesign the PMa-SynRM to avoid demagnetization. Experiments are conducted to validate the simulation.

Demagnetization research on PMs in the Halbach magnetized pulsed alternator

Demagnetization behavior of Halbach magnetized compensated pulsed alternator (CPA) is studied by using finite element method (FEM) under the different demagnetization factors in this paper. The effect of armature reaction magnetic field and thermal rise on demagnetization of PMs is analyzed. This paper investigates the effect of different demagnetization factors on magnetic field distribution, load current, and no-load phase voltage. A series of dynamic demagnetization points in Halbach array permanent magnet (PM) are evaluated to search the worst working point. Partial demagnetization risk can be represented by the worst working point, and the global demagnetization of the PM is represented by the no-load phase voltage characteristics after discharge. The research results demonstrated that the compensation shield with a certain thickness can increase the discharge current and weaken the demagnetization influence of armature reaction. The demagnetization effect of armature reaction on PMs at high temperatures will be strengthened.

Comparative Study of Two Novel Double-Sided Hybrid-Excitation Flux-Reversal Linear Motors With Surface and Interior PM Arrangements

In this paper, two novel double-sided hybrid-excitation flux-reversal linear motors (DSHEFRLM) with the surface (S-) and interior (I-) permanent-magnet(PM) arrangements are proposed and comparatively investigated. The proposed motors feature an asymmetrical double-sided primary configuration which enables the armature and field excitations to be placed separately. Through 2-D finite-element method (FEM), two motor structures with different slot/pole combinations are optimized, and then, the magnetic fields and electromagnetic performances are calculated, including inductances, no-load flux linkage, thrust characteristics, and PM demagnetization capability. By comparison, it is found that S-DSHEFRLMs show larger thrust density while I-DSHEFRLMs exhibit superior flux-adjusting capability. Finally, these predicted results are validated by 3-D FEM.

Hybrid Adopted Materials in Permanent Magnet-Assisted Synchronous Reluctance Motor With Rotating Losses Computation

This paper presents an optimized design in the permanent magnet-assisted synchronous reluctance motor (PMa-SynRM) rotor structure design with hybrid adopted materials for enhancing the motor characteristics and operation. Two types of permanent magnet (PM) materials are embraced in the PMa-SynRM rotor of rare-earth and ferrite PMs. The optimized design for the PMa-SynRM rotor structure considers the motor magnetic equivalent circuit for reducing the reluctance of the hybrid PMs to improve the motor performance. Moreover, the irreversible demagnetization of the ferrite PM which occurs due to the reversing of the magnetic field of the rare-earth PM is presented. The analytical rotating losses computation with the hybrid adopted materials at the design process is introduced. The rotating losses of the PMa-SynRM are investigated as core, magnet, proximity, ohmic, and friction losses. The core loss in the PMa-SynRM occurs due to the altering of the flux densities in numerous fragments of the iron structures. The loss measurements can be accomplished by measuring the electric parameters of the PMa-SynRM at a defined operating point. The magnetic modeling of the improved 6 KW PMa-SynRM manufactured prototype structure is derived and rotational losses are computed, validated, and compared with the finite element analysis (FEA).

Mode Recognition and Fault Positioning of Permanent Magnet Demagnetization for PMSM

This paper proposes a demagnetization fault detection, mode recognition, magnetic pole positioning, and degree evaluation method for permanent magnet synchronous motors. First, the analytical model of the single-coil no-load back electromotive force (EMF) of demagnetization fault for Permanent magnet synchronous motor (PMSM) arbitrary magnetic poles is established. In the analytical model, the single-coil no-load back EMF residual of the health state and the single magnetic pole sequential demagnetization fault are calculated and normalized. Model results are used as the fault sample database. Second, the energy interval database of the single-coil no-load back EMF residual with different numbers of magnetic pole demagnetization is established. Demagnetization fault detection and degree evaluation are performed by the real-time acquired amplitudes of the single-coil no-load back EMF residual. The number of demagnetization poles is determined by comparing the energy of the single-coil no-load back EMF residual with the energy interval database. Demagnetization mode recognition and magnetic pole positioning are realized by analyzing the correlation coefficients between normalized the single-coil no-load back EMF residual and the fault sample database. Finally, results of analysis of the finite element simulation validate the feasibility and effectiveness of the proposed method.

Robust Model-Free Nonsingular Terminal Sliding Mode Control for PMSM Demagnetization Fault

In this paper, a robust model-free nonsingular terminal sliding-mode control (MFNTSMC) algorithm based on the ultra-local model is proposed to reduce the influence of permanent magnet (PM) demagnetization for PM synchronous motors (PMSMs). First, the PMSM mathematical model in normal and demagnetization is described, and the ultra-local model of speed loop and current loop is constructed based on the input and the output of the PMSM vector control system. Then, the MFNTSMC method is proposed and adopted to design the speed controller and d-q -axis current controller, and the sliding-mode observer is designed to estimate the unknown terms of the ultra-local model. Finally, compared with the PI control method and model-free control method, the results of simulations and experimentations show that the MFNTSMC method can improve the dynamic response while maintaining robustness of PMSM driven system, reduce the dependence of the design of controller on the precise PMSM model, and has fault-tolerant control function for PM demagnetization fault.

A Novel Fault Tolerant Machine With Integral Slot Non-Overlapping Concentrated Winding

A novel fault tolerant machine with integral slot non-overlapping concentrated winding (ISNCW) is proposed in this paper. With the distribution of phase windings in axial, the proposed machine realized excellent isolation capability between different phases. The performance comparisons between the proposed machine and the traditional integer slot overlapping winding (ISOW) machine are made by the analytical method and finite element analysis (FEA). Compared to the ISOW machine, the proposed machine has better performance in low speed high torque applications, and much better fault tolerant capability and flux weakening capability. Especially, the proposed machine appears excellent performance in limiting the short circuit current and preventing the irreversible demagnetization of permanent magnet under short-circuit fault. The phase independent structure makes the maintenance and replacement of the fault module convenient and saves the maintenance cost further.

Cascaded Robust Fault-Tolerant Predictive Control for PMSM Drives

This paper presents a cascaded robust fault-tolerant predictive control (CRFTPC) strategy with integral terminal sliding mode observer (IT-SMO) to achieve high performance speed loop and current loop for permanent magnet synchronous motor (PMSM) drives. The modeling of PMSM considers the disturbance caused by parameter perturbation and permanent magnet demagnetization. With this model, we can derive the optimal control law of the proposed scheme, which avoids the tuning work of the weight factor effectively. This new CRFTPC strategy has a cascaded structure, external loop and internal loop, both implemented with robust fault-tolerant predictive control. In addition, a new integral terminal sliding mode observer is designed to estimate the disturbances, and thus the robustness of the proposed method can be increased significantly. Comparative simulations and experimentations verify that the proposed CRFTPC provides fast dynamic response, static-errorless speed, and current tracking, even with the system disturbance.

Rotor Structure on Reducing Demagnetization of Magnet and Torque Ripple in a PMa-synRM With Ferrite Permanent Magnet

A rotor structure of Permanent Magnet (PM)-assisted synchronous Reluctance Machine (PMa-synRM) with ferrite PM is presented in this paper, in which the irreversible demagnetization of PM and torque ripple are reduced. The bypass rib is proposed and added to the flux barrier of the rotor. The path for the demagnetizing field of armature windings is provided and the torque ripple can also be reduced by the bypass rib. Based on the 2-D finite-element analysis (FEA), the location and dimensions of the bypass rib are investigated and improved. A PMa-synRM with the proposed rotor structure is designed and compared with that using a conventional rotor. The field distributions for different currents are studied to explain the principle of the bypass rib. The results show that the PMa-synRM with the proposed rotor achieves higher antidemagnetization ability and lower torque ripple.

Investigation of Direct & Quadrature Current Effects on Demagnetization of Flux Switching Permanent Magnet Motors

In recent years flux-switching permanent magnet machines have become popular for industrial applications and research, due to advantages such as robustness, high torque, and power density. However, as permanent magnet motors, their performance is affected by demagnetization. It is known, for example, that armature reactions and temperature increases cause permanent magnet demagnetization that leads to power and torque density reduction in motors. In this paper, partially irreversible demagnetization in an flux-switching permanent magnet motor will be investigated using the time stepping finite element method. In addition, the effects of the d and q - axis currents on this type of demagnetization are evaluated by considering several points on the magnets and comparing the flux density of these points with the flux density of the knee point. It is shown that an increase of the q-axis current component increases the machine’s demagnetization tolerance.

Multi-Sensor Fusion-Based Permanent Magnet Demagnetization Detection in Permanent Magnet Synchronous Machines

When permanent magnet (PM) demagnetization in a PM synchronous machine (PMSM) occurs, the demagnetization characteristic information is usually coupled with other characteristics of the PMSM which may demonstrate similar features. Therefore, it is difficult to accurately detect the demagnetization by monitoring only one parameter. This paper explores the use of multi-sensor information, namely, acoustic noise and torque, for demagnetization detection through the information fusion technique. The acoustic noise and torque information are processed and analyzed using wavelet transforms for filtering and extracting features. Bayesian network-based multi-sensor information fusion is then proposed to detect the demagnetization ratio from the extracted features. The proposed approach is experimentally verified on a laboratory PMSM and compared with single-sensor dection methods.

Analytical Performance Calculation of Vernier Hybrid Machine with Subdomain Method

The analytical subdomain models of vernier hybrid machines with two kinds of permanent magnet (PM) arrangements are developed, and the magnetic vector potentials are considered unknown variables to obtain the no-load and on-load magnetic field in this article. According to the electromagnetic properties and machine structure, the entire solving region is divided into five subdomains, namely the stator slot, slot opening, PM, air gap, and rotor slot. Using the general solutions of magnetic vector potentials and boundary conditions, the magnetic field distributions of all subdomains are obtained. Then the air gap flux density, flux linkage, back-EMF, cogging torque, electromagnetic torque, unbalanced magnetic pulls, and PM loss are calculated and compared for the two models. With the analytical model, the PM demagnetization withstand capacity, effects of main parameters on the torque ripple and average torque, and combinations of PM and rotor slot numbers are also studied. The 2D finite element method and experimental results of two prototype machines validate the accuracy of the analytical models.

PMSG fault identification applied to wind power

Wind farms consist in many midsize generating units. The height of equipments, their placement and the ongoing tendency to put them offshore require monitoring at distance. The technology to convert wind energy with permanent magnet generators is widely used in wind energy systems, despite the fact that this electric machine is one of most recent generators on the market. The goal of this paper is the proposal and the experimental validation of voltage and current signatures to detect incipient faults on PMSG (Permanent Magnet Synchronous Generator). The three faults discussed in this paper are the stator turn-to-turn fault, the permanent magnet demagnetization and the rotor eccentricity. The short circuit fault between turns was determined by analyzing the signature of the third harmonic voltage and its multiples. The proposed methodology is able to detect the faulty phase despite the presence of a third harmonic caused by machine asymmetries and asymmetric loads. The use of multiples of the third harmonic is essential for that. Regarding the permanent magnet demagnetization, the detection was made by measuring the voltage waveform of a stator derivation coil. The current signature method proposed can detect the static eccentricity, even in the prototype that has low inductance and large air gap, typical characteristics of midsize generating units for wind farms.

Influence of Magnetoresistance and Temperature on Permanent Magnet Condition Estimation Methods Using High-Frequency Signal Injection

Torque production capability of permanent magnet synchronous machines (PMSMs) depends on the permanent magnet (PM) magnetization state (MS). MS is a function of PM initial magnetization level, which decreases as temperature increases. Furthermore, excessive PM temperature can produce irreversible PM demagnetization. PM MS and temperature measurement/estimation is therefore important both for torque control and monitoring purposes. The injection of a high-frequency (HF) signal in the stator windings has been shown to be a viable option for temperature and MS estimation. This technique estimates the PM temperature or/and MS from the variation of the stator reflected PM HF resistance. However, since PM HF resistance is affected by both PM temperature and MS due to magnetoresistive effect, separating both effects is not trivial. This paper studies the effect of magnetoresistance and temperature on the PM resistivity, the target being twofold: to understand how temperature and MS estimation methods interfere with each other, and to further use this knowledge for the development of methods to be able to estimate the magnet temperature and MS simultaneously.

Design Optimization of a High Torque Density Spoke-Type PM Motor for a Formula E Race Drive Cycle

Performance trade offs for achieving high torque density and high drive-cycle efficiency in spoke-type permanent magnet (PM) motors are investigated. Unlike the traditional method, which relies on analytical equations, this trade-off study is conducted through a statistical analysis of the Pareto-optimal design candidates resulting from a large-scale design optimization of an example traction machine over a Le Mans-based drive cycle. In the design optimization process, both low-speed and extended-speed/field-weakening operating regions are evaluated using high-fidelity electromagnetic finite element (FE) simulations with an objective to simultaneously increase the torque density and decrease the power losses over the high energy-throughput-zones of the machine torque–speed plane. The resultant 3400 design candidates are utilized to investigate the impact of increasing power density on other important performance metrics such as power losses, PM demagnetization, and torque ripple. This analysis is supplemented by multiphysics simulation of three counterpart optimized designs and successful experimental verification of a prototype of one of those three designs which represents a record high power density motor in traction applications.

Robust Fault-Tolerant Predictive Current Control for Permanent Magnet Synchronous Motors Considering Demagnetization Fault

In this study, a robust fault-tolerant predictive current control (RFT-PCC) algorithm based on a composite observer is proposed for permanent magnet synchronous motors (PMSMs), which can eliminate the influence of motor parameter perturbation and permanent magnet demagnetization. First, the effects of the parameter perturbation and permanent magnet demagnetization on the current vector are analyzed based on the conventional predictive current control. The composite observer based on the sliding mode observer and the Luenberger observer is then designed to observe the compensation voltage and current prediction value of the next time simultaneously. Finally, an RFT-PCC method is developed. The proposed RFT-PCC is able to effectively enhance robustness against parameter perturbation and permanent magnet demagnetization by adding compensation voltage. The current prediction value of the next time is used to replace the sampled current in RFT-PCC to compensate for the influence of the one-step delay. Comparative simulations and experimentations verify that the proposed RFT-PCC can improve the robustness while maintaining quick dynamic response of the current control loops.

Detection of Demagnetization in Permanent Magnet Synchronous Machines Using Hall-Effect Sensors

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

Analysis and Design of SPM Type Variable Flux Memory Motor Considering Demagnetization Characteristic of Permanent Magnet

Permanent magnet (PM) of variable flux memory motor (VFMM) with a distributed winding is demagnetized by the opposite direction magnetic field at rear edge of PM. Moreover, average torque, performance, and power of VFMM are decreased when the PM is demagnetized. Therefore, method to prevent the demagnetization of PM is an important issue for VFMM design. In this paper, some methods are proposed to reduce the demagnetization of the PM. First method is to replace certain regions of PM that is vulnerable to demagnetization with higher grade PM material, as demagnetization is highly effected by magnetic field intensity. However, the cost of the high grade PM material and manufacturing complexity may be problematic. Therefore, other methods that are based on magnetic resistance concept are introduced, which alters different design variables to solve the issue. To verify the validity of the methods, demagnetization ratio of the base model is compared to that of the suggested method applied models under same operating condition.

Acoustic Noise-Based Uniform Permanent-Magnet Demagnetization Detection in SPMSM for High-Performance PMSM Drive

This paper explores the idea of detecting uniform permanent-magnet (PM) demagnetization by using acoustic noises in order to develop a reliable PM synchronous machine (PMSM) controller. A flux-based acoustic noise model is proposed to demonstrate that demagnetization will induce acoustic noise containing abnormal frequency. This paper will also analyze online PM demagnetization detection by using a back propagation neural network (BPNN) with acoustic noise data. First, seven objective and psychoacoustic indicators are proposed to evaluate the acoustic noise of healthy and demagnetized PMSMs under different speed and load conditions. Next, a novel BPNN-based PM demagnetization detection method is proposed. In this method, the PM demagnetization is detected by means of comparing the measured acoustic signal of PMSM with an acoustic signal library of seven acoustical indicators. The proposed PM demagnetization detection approach is experimentally evaluated. Unlike other approaches, this is a noninvasive method and is independent of internal motor parameters. The aforementioned seven indicators can process nonlinear signals and are used to comprehensively reflect noise quality.

Analysis of Dual Three-Phase Permanent-Magnet Synchronous Machines With Different Angle Displacements

In this paper, a novel nonconventional angle displacement (i.e., 15°) between two sets of three-phase windings is proposed for dual three-phase permanent-magnet (PM) machines. First, all feasible angle displacements between the two three-phase winding sets for various slot/pole number combinations of dual three-phase machines are discussed in general. Then, an in-depth investigation is carried out on a 24-slot/10-pole dual three-phase PM machine as an example covering various electromagnetic performances. The investigation shows that under healthy and three-phase open-circuit conditions, the proposed 15° configuration has a comparable electromagnetic performance to the 30° configuration, and a better performance than the 0° counterpart. Furthermore, under three-phase short-circuit (SC) condition, the proposed 15° configuration has the lowest SC current, smallest braking torque, and the best PM demagnetization withstand capability. Finally, three prototype 24-slot/10-pole surface-mounted PM machines are built and tested to verify the theoretical analyses.