Rotor Anisotropy Research Articles

Analysis of Rotor Anisotropy and Position Tracking Error Prediction for Surface-Mounted PMSM Under High Frequency Signal Injection

Analysis of Rotor Anisotropy and Position Tracking Error Prediction for Surface-Mounted PMSM Under High Frequency Signal Injection

Sensorless Control With Switching Frequency Square Wave Voltage Injection for SPMSM With Low Rotor Magnetic Anisotropy

High-frequency signal injection sensorless algorithms are widely studied and used for rotor angle estimation in PMSM at low speed or standstill. One of the main drawbacks of such methods is the acoustic noise connected to the voltage injection. In order to minimize this problem, it is advisable to increase the frequency of the injected signal. Thus, many studies focus on square-wave injection at the switching frequency, which is the maximum theoretical frequency. Since these methods exploit the rotor magnetic anisotropy, it is relatively easy to use them in interior PMSMs, where the rotor anisotropy is high. On the contrary, it is hard to exploit them in surface PMSMs, which have an almost symmetric rotor, although a low rotor magnetic anisotropy is still present. In this paper, a sensorless algorithm with switching frequency square-wave injection is developed for surface PMSMs. To increase the signal-to-noise ratio, current oversampling is exploited. The benefits of such a technique are demonstrated with experimental results on a 2 Nm SPMSM.

Modal-Based Anisotropy Early Warning in Wind Turbine Rotor

Subspace-based fault detection methods are widely used for linear time-invariant systems. For linear time-periodic systems, those methods cannot be theoretically used, due to the intrinsic assumptions associated with those methods in the context of linear time-invariant models. Based on the approximation of time-periodic systems as time-invariant ones, those methods can still be applied and adapted to perform change detection for time-periodic systems, through a Gaussian residual built upon the identified modal parameters and their estimated variances. The proposed method is tested and validated on a small numerical model of a rotating wind turbine, with detection and isolation of a blade stiffness reduction leading to rotor anisotropy.

Experimental Low-Speed Performance Evaluation and Comparison of Sensorless Passive Algorithms for SPMSM

Sensorless algorithms for PMSM have achieved an increasing interest in the technical literature. They can be divided into active methods and passive methods. Active methods exploit rotor anisotropy by injecting high voltage frequency signals, whereas passive methods are based on observers. Even if passive methods do not have the drawbacks of active methods – acoustic noise, torque ripple and harmonic distortion - their effectiveness is reduced in the low speed region. For this reason, active methods are usually used at low speed and the motor control switches to passive methods in the high-speed region. Since high frequency injection is associated to many drawbacks, the aim of passive methods is to work at lower speed as possible or, ideally, to be able to start directly the motor. That is particularly true for Surface-Mounted PMSM: since they have a low anisotropy, high frequency injection should be particularly elevate to achieve sufficient signal-to-noise ratio. It is difficult to compare the performance of passive algorithms proposed in the technical literature since experimental tests have been done with different conditions: motors, inverter features, measurements, type of tests, etc. In this paper, five different sensorless passive algorithms for Surface-Mounted PMSM, selected among the most promising algorithms available in the technical literature, are compared, performing the same tests on the same experimental setup, to achieve a fair comparison and establishing the most performing one.

State-Space Approach for SPMSM Sensorless Passive Algorithm Tuning

Sensorless algorithms for Permanent Magnet Synchronous Motors (PMSM) have achieved increasing interest in the technical literature over the last few years. They can be divided into active methods and passive methods: the first inject high-frequency signals exploiting rotor anisotropy, whereas the second are based on observers. Recently, a sensorless control based on a rotor flux observer has been presented in the technical literature, which gives very accurate results in terms of rotor position estimation and robustness. In this paper, the aforementioned observer is considered and a procedure for choosing stabilizing gains of the observer is proposed. The contribution of the paper is three-fold: the mathematical modelling of the rotor flux observer, the methodology for the definition of the observer gains, and the presentation of the experimental results.

Time-Resolved Fluorescence Anisotropy of a Molecular Rotor Resolves Microscopic Viscosity Parameters in Complex Environments.

Understanding viscosity in complex environments remains a largely unanswered question despite its importance in determining reaction rates in vivo. Here, time-resolved fluorescence anisotropy imaging (TR-FAIM) is combined with fluorescent molecular rotors (FMRs) to simultaneously determine two non-equivalent viscosity-related parameters in complex heterogeneous environments. The parameters, FMR rotational correlation time and lifetime, are extracted from fluorescence anisotropy decays, which in heterogeneous environments show dip-and-rise behavior due to multiple dye populations. Decays of this kind are found both in artificially constructed adiposomes and in live cell lipid droplet organelles. Molecular dynamics simulations are used to assign each population to nano-environments within the lipid systems. The less viscous population corresponds to the state showing an average 25° tilt to the lipid membrane normal, and the more viscous population to the state showing an average 55° tilt. This combined experimental and simulation approach enables a comprehensive description of the FMR probe behavior within viscous nano-environments in complex, biological systems.

Performance improvement in a sensorless surface-mounted PMSM drive based on rotor flux observer

Permanent magnet synchronous motor sensorless algorithms for low speed region are usually based on rotor anisotropy. Although many studies showed a good behavior of this type of control, high frequency voltage injection is required, which causes loud acoustic noise in addition to electrical losses. On the other hand, methods based on back electromotive force or rotor flux observer, which guarantee high performance in the medium–high speed region, do not required voltage injection. In this paper, a rotor flux observer based algorithm performance is analyzed, in order to extend its use also in the low speed region. Behavior both with no-load, linear torque, quadratic torque, constant torque and compressor-load torque is tested, especially focusing on motor starting. Improvements to rotor flux observer algorithms available in the scientific literature are discussed and their effectiveness is verified by experimental results.

Influence of rotor cage resistance in torque ripple reduction for line start synchronous machines

The interaction of spatial harmonics of electrical loading and the rotor anisotropy causes torque ripple in line start synchronous reluctance motor (a type of synchronous motor). A novel design technique for torque ripple reduction is presented in this study. Initially, synchronous reluctance motor filled with conducting material inside the flux barrier (called cage barrier) is optimised using a genetic algorithm (GA) technique. The optimised design is further verified using finite element analysis (FEA) and tested. To reduce the ripple content, the rotor has been introduced with an additional d-axis cage. Six rotor structures with an additional cage have been designed and optimised using FEA. The numerical analysis results are compared for all rotor structures. The optimised rotor structures with and without d-axis cage are fabricated and tested. Experimental results reveal that the fabricated rotor with an additional cage has reduced torque ripple and high efficiency. Finally, the acoustics test is carried out for both the fabricated structures and results are compared.

Design and Evaluation of a Short-Circuit Rotor-Ring for Enhanced Self-Sensing Capability in a Slotless PM Motor

This paper deals with the design and evaluation of a short-circuit rotor-ring for enhanced self-sensing capability in a two-pole slotless permanent-magnet motor. High-frequency rotor anisotropy is enabled by adding a short-circuited rotor-ring. Optimal ring-dimensions are determined by parametric numerical studies. An effective high-frequency saliency of up to 1.8 is demonstrated. The steady-state estimation error is within $\pm$ 2 $^\circ$ , practically unaffected by current level (in contrast to slotted self-sensing electrical machinery) due to the large effective air-gap. The proposed observer performs properly also during dynamic load changes. Fall-times from 3000 to 0 r/min in less than 50 ms with negligible overshoot are demonstrated. Also parasitic effects, such as power losses, heating, and torque ripple are analyzed.

Analytical Power Limits Curves of High-Speed Synchronous Reluctance Machines

The aim of this paper is to tackle the design of synchronous reluctance machines with transverse laminated rotor for high-speed applications by means of an analytical model. In high-speed applications, the mechanical design greatly affects the motor performance in terms of torque density and power factor. In fact, as the rated speed increases, the radial ribs thicknesses increase as well to ensure the rotor structural integrity against the centrifugal force. This increases the quadrature flux and, therefore, the rotor anisotropy gets worse. Hence, the optimal rotor design is the result of a compromise between mechanical and magnetic aspects. The purpose of this paper is twofold: First, to develop an analytical model that links the maximum average torque to the rated speed, and second, to determine the maximum power that can be generated by a specific rotor geometry. The analytical approach allows a fast motor design and analysis so that a rapid extrapolation of the results to different machine size is possible. Finite element analysis is used only to validate the final results.

Low-dimensional representations and anisotropy of model rotor versus porous disk wind turbine arrays

We experimentally compare wakes within three-bladed rotor and porous disk wind turbine arrays using PIV measurements and proper orthogonal decomposition. Disparities between rotors and disks with important implications for wind turbine simulations is found.

Synchronous Reluctance Motor Iron Losses: Considering Machine Nonlinearity at MTPA, FW, and MTPV Operating Conditions

Synchronous reluctance machine has high flux density fluctuations in the iron due to the high harmonics results from the rotor anisotropy. Thus, an accurate computation of the iron losses is of paramount importance, especially during the design stage. In this paper, a nonlinear analytical model considering the magnetic iron saturation and the slotting effect is proposed. The model estimates accurately the iron losses at a wide range of operating speed. In addition, the accuracy of the nonlinear model when the machine is highly saturated, i.e., when it works along the maximum torque per ampere (MTPA) trajectory, is presented and verified. The model presented is general and can be applied to other configurations. A 36-slot four-pole machine, with three flux-barriers per pole is considered as a case study. Finite element analysis is used to validate the results achieved by means of the nonlinear analytical model. Furthermore, an experimental setup is built to validate the simulation results.

Electric Vehicle Traction Based on Synchronous Reluctance Motors

It was recently demonstrated that the synchronous reluctance motor is well suited for electric as well as for hybrid electric vehicles. This paper deeply investigates the capabilities of a synchronous reluctance motor and compares them with those of a permanent-magnet-assisted synchronous reluctance motor, according to the typical requirement of a traction application. A proper rotor design is necessary. The average torque is due to the rotor anisotropy. The permeance difference between the direct- and the quadrature-axis is achieved by means of a high number of flux barriers. The position of the flux barrier ends and proper rotor asymmetries are chosen so as to reduce the torque ripple, mainly due to the slot harmonics. The impact of the rotor design on the motor performance is presented deeply, showing several simulation and experimental results, carried out on synchronous reluctance motors with different rotor geometries. Permanent magnets can be inset in the flux barriers to assist the synchronous reluctance motor improving its capabilities, but avoiding to use rare-earth permanent magnets. The main advantages of the permanent magnet assistance is an increase of the main torque density and of the power factor. They are evaluated experimentally. However, the drawback of adopting permanent magnets is the possible demagnetization of the magnets themselves. This can greatly limit the maximum overload capability of the motor, which is a salient requirement of a traction motor.

Design and Tests of a Four-Layer Fractional-Slot Interior Permanent-Magnet Motor

This paper deals with the design and testing of an interior permanent-magnet (IPM) machine that exhibits advanced features. It adopts fractional-slot (FS) nonoverlapped coil windings that reduces the end winding length, i.e., the Joule losses and machine cost. As a consequence, higher rated current can be supplied so that higher torque density and higher efficiency in a wide operating region can be reached. The rotor saliency increases the performance in flux-weakening operating region and the fault-tolerance capability. In order to decrease the torque ripple, it is important to minimize the interaction between the magnetomotive force (MMF) harmonics due to the stator currents and the rotor anisotropy. To do that, the winding has been optimized so as to minimize the MMF harmonics, taking advantage from the higher degree of freedom of the four-layer winding. However, the so-called slot-harmonics cannot be minimized by rearranging the winding coils. Hence, the rotor geometry has been designed so as to minimize the impact of such slot-harmonics. A 12-slot 10-pole IPM FS machine has been designed and manufactured. The experimental tests confirm the expectations.

Rotor anisotropy as a blade damage indicator for wind turbine structural health monitoring systems

Structural damage of a rotor blade causes structural anisotropy of the rotor. In rotor dynamic, the anisotropy affects the symmetry of the rotor mode shapes, and the latter can be utilized to detect the blade damage. The mode shape symmetry can be characterized by relative blades’ magnitude and phase. The study examines the potential use of these parameters as rotor damage indicators.Firstly the indicators are studied analytically using a simple 6 degrees-of-freedom model of a rotating rotor. Floquet analysis is used due to the time periodic nature of the considered system. Floquet analysis allows one to perform analytical modal decomposition of the system and study the sensitivity of the damage indicators to the amount of damage. Secondly, operational modal analysis (OMA) is involved to extract the same damage indicators from simulated experimental data, which was synthesized via numerical simulations.Finally, the same procedure was applied to operating Vestas V27 wind turbine, first using the simulated experimental data obtained by using aeroelastic simulation code HAWC2 and then using the data acquired during the measurement campaign on a real wind turbine.The study demonstrates that the proposed damage indicators are significantly more sensitive than the commonly used changes in natural frequency, and in contrast to the latter, can also pinpoint the faulty blade. It is also demonstrated that these indicators can be derived from blades vibration data obtained from real life experiment.

Power Spectrum-Based Detection of Induction Motor Rotor Faults for Immunity to False Alarms

It has recently been shown that spectrum analysis of instantaneous power can provide sensitive online detection of rotor faults in induction motors compared with current, torque, speed, or vibration spectrum analysis. However, it was reported that monitoring of the twice slip frequency, $2sf_{s}$ , components induced by the fundamental component can produce false rotor fault alarms due to asymmetry in the rotor or low frequency load oscillations. In this paper, the rotor fault components induced in the power spectrum by the stator fifth and seventh space harmonics are derived to evaluate their immunity to false alarms. It is shown that the $(6 - 8s)f_{s}$ component can provide reliable detection of rotor faults under cases where existing methods produce false alarms. An experimental study performed on custom built rotor samples shows that the new components are capable of detecting rotor faults immune to false alarms produced by rotor axial air ducts, rotor anisotropy, and low frequency load oscillations for cases where existing methods fail. The components derived in this paper can also be applied to vibration, speed, torque, or acoustic monitoring for reliable detection of rotor faults.

Evaluation of the Influence of Rotor Magnetic Anisotropy on Condition Monitoring of Two-Pole Induction Motors

If nongrain-oriented rotor laminations are punched and stacked in a uniform direction, magnetic asymmetry is present in the rotor core due to the nonideal magnetic anisotropy. In this paper, it is shown that the influence of rotor core anisotropy can be misinterpreted as rotor faults in two-pole induction motors when performing motor current signature analysis. A detailed analysis of the influence of rotor core anisotropy shows that components produced by the rotor fault and rotor anisotropy interact, making reliable fault detection difficult. It is also shown that offline testing is currently the only available means of detecting the fault, and online monitoring of the space harmonics-induced current components is proposed as a viable solution for providing reliable rotor fault detection for two-pole motors with rotor anisotropy. The analysis and conclusions presented in this paper are verified through testing on custom-built lab motors and on 3.3-kV motors.

High-Power-Factor Vernier Permanent-Magnet Machines

Vernier permanent-magnet (VPM) machines are well known for high torque density but low power factor. This paper deals with the low power factor of VPM machines. The goal is not obtained by reducing the electrical loading or adjusting current advance angle but by proposing a novel vernier topology, i.e., a dual-stator spoke-array (DSSA) VPM topology. In this paper, the characteristics of the DSSA VPM topology, such as active part, auxiliary mechanical structure, and rotor anisotropy, are analyzed in detail. Performances are evaluated based on finite-element analysis, including power factor, torque density, and cogging torque. The results show that the DSSA VPM topology exhibits high power factor, viz., ~0.9, and significantly high torque capability. The verification of the mechanical structure scheme is also done in this paper. Finally, theoretical analyses are validated by the experimental results by a 44-rotor pole 24-slot DSSA VPM prototype.

Design of a Low-Torque-Ripple Fractional-Slot Interior Permanent-Magnet Motor

Despite a strong nonlinear behavior and a complex design, the interior permanent-magnet (IPM) machine is proposed as a good candidate among the PM machines owing to its interesting peculiarities, i.e., higher torque in flux-weakening operation, higher fault tolerance, and ability to adopt low-cost PMs. A second trend in designing PM machines concerns the adoption of fractional-slot (FS) nonoverlapped coil windings, which reduce the end winding length and consequently the Joule losses and the cost. Therefore, the adoption of an IPM machine with an FS winding aims to combine both advantages: high torque and efficiency in a wide operating region. However, the combination of an anisotropic rotor and an FS winding stator causes some problems. The interaction between the magnetomotive force harmonics due to the stator current and the rotor anisotropy causes a very high torque ripple. This paper illustrates a procedure in designing an IPM motor with the FS winding exhibiting a low torque ripple. The design strategy is based on two consecutive steps: at first, the winding is optimized by taking a multilayer structure, and then, the rotor geometry is optimized by adopting a nonsymmetric structure. As an example, a 12-slot 10-pole IPM machine is considered, achieving a torque ripple lower than 1.5% at full load.

Rotor Flux-Barrier Geometry Design to Reduce Stator Iron Losses in Synchronous IPM Motors Under FW Operations

The interior permanent-magnet (IPM) synchronous motor is characterized by a high rotor anisotropy. Such an anisotropy is the cause of a high harmonic content of the air-gap flux density distribution, almost independent of the main flux. As a consequence, there are fluctuations of the flux density in the stator iron and, consequently, eddy-current iron losses. This aspect is prominently evident during flux-weakening operations, when the armature current weakens the permanent magnet flux and the motor runs above the base speed. This paper presents a complete study of such a phenomenon, including an analytical model and a finite element validation, as well as an experimental confirmation of the predicted stator tooth flux waveforms. Finally, some suggestions are given in order to design an IPM motor exhibiting reduced iron losses during all operating conditions.