Permanent Magnet Rotor Research Articles

Design of Low-Cost Synchronous Machine to Prevent Demagnetization

The request for high efficiency motor paves the way for the replacement of induction motors with permanent magnet synchronous motors. Although the efficiency is increased, for medium and high power, the current ripple causes significant additional losses in the magnet and lamination; and, high temperature can lead to demagnetization. In this paper, a new rotor topology is proposed and compared to a traditional surface permanent magnet rotor to reduce the magnet losses and protect them from demagnetization. A reference surface permanent magnet machine is compared with the proposed one in terms of performance and magnet losses. Both analytical and experimental analysis are carried out and discussed.

Optimal Design of Line-Start Permanent Magnet Synchronous Motor Based on Magnetic Equivalent Parameters

This paper used a lumped magnetic parametric approach-based analytical technique to design the 7.5 kW, three-phase line start permanent magnet (LSPM) motor. In order to enhance the efficiency and power factor (PF) of the prototype LSPM machine, an optimized slot shape of rotor permanent magnet (PM) was selected. The lumped magnetic circuit model is developed to present the magnetic characteristics, and analytical expressions are derived under the open circuit condition. The impact of the design variables on the analytical model air gap flux density distribution and magnetic flux leakage is studied for the LSPM. The design variables have a significant influence on the steady-state performance characteristics of the LSPM motor. Therefore, to verify the output results of the purposed model, the two-dimensional finite element analysis (FEA) was evaluated for the numerical solution of the analytical model.

Design Optimization of an Electric Machine for a 48-V Hybrid Vehicle With Comparison of Rotor Technologies and Pole-Slot Combinations

Through analytical equations followed by a systematic optimization study using high-fidelity finite-element (FE) models, the torque capability of three common synchronous machine technologies—namely, a permanent magnet (PM) synchronous machine (PMSM) with V-shaped rare-earth rotor magnets, a PMSM with deep V-shaped ferrite rotor magnets, and a synchronous reluctance machine (SynRM) with four layers of conforming flux barriers are compared for an application in a 48-V mild hybrid electric car. The optimization of the deep V-shaped ferrite PMSM automatically resulted in a spoke-type PM layout for maximum torque production. Yet, an optimized SynRM with conforming flux barriers was achieved that could produce a higher torque than ferrite PMSM. One machine technology is selected and six variants of it, with two rotor PM layouts each with three alternative pole-slot combinations, are subsequently subjected to a large-scale FE model-based drive-cycle design optimization. Various pole-slot combinations are methodically compared in terms of drive-cycle losses, active material cost, torque ripple, and PM demagnetization level. More than 20 000 designs were analyzed over ten energy-centric torque-speed points to identify an optimal design solution. The results of multiphysics analysis incorporating electromagnetics, computational fluid dynamics, and structural analyses are provided, including a study on three different water jacket concepts. A final design is prototyped and primary experimental results are provided.

Novel magnetizing technique using high temperature superconducting bulk magnets for permanent magnets in interior permanent magnet rotors

A novel magnetizing technique for permanent magnets (PMs) embedded in interior permanent magnet (IPM) motors has been proposed as a safer and more effective process than the conventional processes using copper pulse-coils. In the new process, the high temperature superconducting (HTS) bulk magnet, emitting an intense magnetic field of over 3 T, was employed to activate the rotor and PMs inside. A magnetic pole containing the HTS bulk magnet was scanned on the surface of the IPM rotor, resisting against the extremely strong attractive force between the magnet and rotor. Following experiments in the past, numerical simulation studies have been conducted to evaluate the magnetic field-trapping performances of the PMs. In this study, the numerical analysis of the magnetization behaviours was conducted for the PMs embedded in the rotor of the commercial air-conditioner motor in a hybrid car. The simulation results showed us that the demagnetized Nd-Fe-B plates in the rotor were fully magnetized in the intense static magnetic fields on the magnetic pole containing the HTS bulk magnet. The magnetizing behaviour in this peculiar magnetizing operation was made understandable by estimating the anisotropic easy-direction of the PM plates embedded in the rotor. The novel activation method for PM is characterized as a no-shock and no-heating method, which would promote the degree of freedom in the IPM motor design.

Computationally efficient steady‐state finite element simulation of multiphase PM AC machines

In this study the authors propose a generalised procedure for computationally efficient steady-state finite element (FE) simulation of symmetrical multiphase permanent magnet (PM) AC machines with an arbitrary number of phases. The procedure is based on newly derived general equations for phase waveform reconstruction that fully exploit the phase symmetry of multiphase electrical machines. This result can be used to reconstruct the full electrical period of any phase waveform from the smallest possible number of magnetostatic FE simulations executed within the minimum angular displacement of the rotor. Reconstruction of phase waveforms is done during post-processing and does not contribute to the execution time of FE simulations. The applicability of the proposed method is demonstrated by deriving specific equations for two-phase, three-phase, five-phase and six-phase machines. Results of classical time-stepping and newly proposed computationally efficient FE simulation are presented for four rotor PM flux switching machines with the different number of phases. Obtained results show that the relative reduction of simulation time increases with the number of machine phases.

Decoupling Control Analysis of a Flux Reversal Linear Rotary Permanent Magnet Actuator

In order to meet the requirement of industrial production, a control system of a novel flux reversal linear rotary permanent magnet actuator (FR-LRPMA) is developed in this paper, and the electromagnetic and motion decoupled process is analyzed in detail. Firstly, the control equation of FR-LRPMA is derived in the nine phase stator coordinate system and dual dq-axis mover coordinate system. Then the control model of FR-LRPMA is built in MATLAB/Simulink using improved space vector pulse width modulation (SVPWM), and the linear, rotary and spiral motions of the proposed actuator are achieved. Finally, the prototype of the actuator is manufactured, and the experiment platform of FR-LRPMA is set up to measure the cogging torque and linear detent force, validating the simulation results. It is concluded that the control system of FR-LRPMA can be converted into two independent single freedom motors by dual dq transformation. The improved SVPWM is effective in the control system, and the method can be used to solve the electromagnetic and motion decoupling problems.

Analysis of Magnetic Field and Torque Features of Improved Permanent Magnet Rotor Deflection Type Three-Degree-of-Freedom Motor

This paper proposes a novel layered permanent magnet motor (N-LPM), which is based on a three-degree-of-freedom (3-DOF) permanent magnet motor. Compared with the former, the improved N-LPM air gap magnetic density, torque and structure stability have been significantly improved. The proposed N-LPM has three layers of stator along the axis direction, and each layer of stator has three-phase winding. In order to calculate the magnetic field and torque distribution of the N-LPM, an analytical method (AM) is proposed. For performance verification and accurate calculation, finite-element analysis (FEA) is adopted. The two kinds of motors before and after the improvement are compared, and their magnetic field, torque and stability are analyzed. The optimization rate is defined to evaluate the performance of the motor before and after improvement. The results show that the radial flux density, rotation torque, deflection torque and the volume optimization rate of the permanent magnet of the improved motor are 80%, 25%, 50% and 54.72% respectively, and the comprehensive performance is improved significantly.

Scaling Effect on Electromagnetic Performance of Surface-Mounted Permanent-Magnet Vernier Machine

This article investigates the impact of scaling on the electromagnetic performance of surface-mounted permanent-magnet Vernier (SPM-V) machines with a main focus on open-circuit induced Electro Motive Force (EMF). Three different power ratings, i.e., 3 kW, 500 kW, and 3 MW, have been chosen for this article. For each power rating, the SPM-V machines are analyzed for different slot/pole number combinations to compare their optimal performance with a conventional SPM machine. Step-by-step development of an analytical equation is presented for the prediction of induced EMF taking into account the interpole leakage of rotor permanent-magnets. 2-D finite-element analysis (FEA) has been used to validate the analytical equation across different power ratings. The analytical equation is thereafter utilized to study the influence of different geometric parameters on the performance of the SPM-V machines. It reveals that the back EMF and torque of SPM-V machines, for a given normalized pole pitch (rotor pole pitch to magnetic airgap length), are unaffected by the increase in airgap length due to scaling. However, the power factor of SPM-V, unlike the conventional SPM, reduces significantly with the increase in electrical loading due to the scaling effect. The analytical model for induced EMF and the 2-D FEA predicted results are validated by experiments using conventional SPM and SPM-V machine prototypes.

Electrical Machine Permanent Magnets Health Monitoring and Diagnosis Using an Air-Gap Magnetic Sensor

This study proposes a magnetic sensor design and application for monitoring the health of rotor magnets in permanent magnet (PM) electrical machines through in-situ observation of the air-gap magnetic flux density. The reported device employs the concept of Fibre Bragg Grating (FBG) strain sensing fusion with magnetostrictive material to deliver a machine stator slot wedge integrated sensor that allows straightforward installation and retrofit with no invasive action to core elements of the machine. The sensing theory, design, prototyping, calibration and installation of the proposed magnetic sensing scheme are detailed in the paper. The sensor was installed into an inverter driven surface mount PM synchronous machine (SPMSM) and its performance for in-situ observation of rotor PM magnetization conditions validated in a range of healthy and demagnetised PM conditions tests. The obtained experimental data demonstrate the reported device's capability to enable recognition of rotor PMs' magnetisation level and thus their health monitoring. Finally, a fault index is proposed and experimentally validated that allows the application of in-situ magnetic sensor measurements for relative quantification of PM demagnetization fault severity.

Estimation of Rotor Temperature of Permanent Magnet Synchronous Motor Based on Model Reference Fuzzy Adaptive Control

For high-speed permanent magnet synchronous motor (PMSM), its efficiency is significantly affected by the performance of permanent magnets (PMs), and the phenomenon of demagnetization will occur with the increase of PM temperature. So, the temperature detection of PMs in rotor is very necessary for the safe operation of PMSM, and direct detection is difficult due to the rotation of rotor. Based on the relationship between permanent magnet flux linkage and its temperature, in this paper, a new temperature estimation method using model reference fuzzy adaptive control (MRFAC) is proposed to estimate PM temperature. In this method, the model reference adaptive system (MRAS) is built to estimate the permanent magnet flux linkage, and the fuzzy control method is introduced into MRAS, which is used to improve the accuracy and applicable speed range of parameters estimated by MRAS. Different permanent magnet flux linkages are estimated in MRFAC based on the variation of stator resistance, which corresponds to different working temperatures measured by thermal resistance, and the PM temperature will be obtained according to the estimated permanent magnet flux linkage. At last, the back electromotive force (BEMF) is measured on the experimental motor, and the flux linkage and PM temperature of the experimental motor are deduced according to the BEMF. Compared with the experimental results, the estimated PM temperature is very close to the actual test value, and the error is less than 5%, which verifies that the proposed method is suitable for the estimation of PM temperature.

A Review of the High-speed Permanent Magnet Rotor Stress Analysis used for Automotive Air-handling Machines

Machines incorporating high-speed electrical machines (HSEM) are becoming increasingly common place in applications including air handling, energy storage and medical devices. They are of increasing interest within the automotive field for air handling applications. HSEM’s use surface-mounted permanent magnet (PM) rotors, manufactured from rare earth metals. However, these PM’s have low tensile strength and are susceptible to failure under the centrifugal load produced at high speed rotation. Retaining sleeves which are an interference fit around the magnets, provide compression and hence resistance to tensile stress. The ability to predict the stresses within the rotor assembly is essential for robust design. This review paper examines existing analytical calculations and finite element analysis (FEA) models. The analytical approaches include both plane stress and plane strain models and the limitations of these are discussed. For relatively long rotors, a generalised plane strain approach is suitable, however it is seldom used. In addition, this latter approach has not been extended to assemblies where the magnets are assembled onto a carrier or shaft. Optimisation of rotors has been addressed in a relatively small number of papers. However, further work is required in this area to ensure that the optimised rotors can be manufactured.

Review of the High-speed Permanent Magnet Rotor Stress Analysis used for Automotive Air-handling Machines

Machines incorporating high-speed electrical machines (HSEM) are becoming increasingly common place in applications including air handling, energy storage and medical devices. They are of increasing interest within the automotive field for air handling applications. HSEM’s use surface-mounted permanent magnet (PM) rotors, manufactured from rare earth metals. However, these PM’s have low tensile strength and are susceptible to failure under the centrifugal load produced at high speed rotation. Retaining sleeves which are an interference fit around the magnets, provide compression and hence resistance to tensile stress. The ability to predict the stresses within the rotor assembly is essential for robust design. This review paper examines existing analytical calculations and finite element analysis (FEA) models. The analytical approaches include both plane stress and plane strain models and the limitations of these are discussed. For relatively long rotors, a generalised plane strain approach is suitable, however it is seldom used. In addition, this latter approach has not been extended to assemblies where the magnets are assembled onto a carrier or shaft. Optimisation of rotors has been addressed in a relatively small number of papers. However, further work is required in this area to ensure that the optimised rotors can be manufactured.

Open-Circuit Air-Gap Field Calculation of a New PM Machine Having a Combined SPM and Spoke-Type Magnets

This article presents the open-circuit analytical magnetic field distribution model for a new permanent magnets (PMs) rotor configuration designed to improve the utilization of rare earth magnets for flux density production. The air-gap flux density is determined with the superposition method according to the magnetomotive force (MMF) and the air-gap permeance model. Since the new rotor consists of two different rotor topologies, e.g., lateral and spoke magnets, their MMF functions are determined separately. The rotor flux barrier (FB) effect on the rotor MMF harmonics is included and studied closely. The investigations carried out on different rotor topologies show high positive impact of FBs on the third or the operating wave. Compared to the conventional rotor, the utilization of PMs for air-gap flux density production significantly is improved. To verify the analytic model, finite elements (FEs) are used and several simulation results are presented.

Saddle-Shaped Post-Assembly Magnetization Coil for a 300 kW 2-Pole High-Speed Permanent Magnet Rotor

This paper presents the design and test results of a saddle-shaped post-assembly magnetization coil for a 2-pole high-speed permanent magnet (PM) rotor. The magnetization coil can generate pulsed transverse magnetization fields of magnitude equal to 6 T/10 ms in a bore having a diameter of 85 mm and a length of 280 mm, thus meeting the needs of parallel magnetization of a 300 kW 2-pole high-speed PM rotor. The saddle-coil structure improves the coupling between the coil and the rotor, and its magnetic energy reduces to 61% compared to the existing triple-coil structure. The influence of eddy current on the magnitude and direction of the magnetization field is analyzed and the pulse-width and coil supporting structure are optimized. The energy storage of capacitor bank pulse power supply is reduced to 230 kJ, which is 50% of the triple-coil structure's storage. The coil is fabricated and tested, and it is shown that it can successfully magnetize a 300 kW 2-pole high-speed PM rotor.

Analysis, fabrication and detailed comparative study of surface and interior rotor PMSM prototypes of identical nominal ratings and stators

This paper presents an in-depth analysis, performance evaluation and comparative study of two 5-kW, 8-pole, 750-rpm laboratory prototypes of a permanent magnet synchronous motor (PMSM) of identical nominal ratings with surface and interior permanent magnet (PM) rotor structures having same stator and armature winding (fractional slot distributed winding). The key electrical (such as rated voltage, current, power, speed, number of poles, etc.) and mechanical variables (such as overall volume, air-gap length, rotor diameter, shaft dimensions and magnetic material) of the fabricated prototypes have also been kept same to pin-point the direct influence of the two different rotor configurations (surface and interior PMSM) on the parameters, performance and operation of these PMSMs. For the two machines, a detailed comparison of air-gap flux density distribution, THD in induced voltage, torque ripple, losses, efficiency, torque–speed characteristics, field weakening capability, steady-state parameters at different operating conditions, etc. has been conducted. The salient observations from this comparative study have been duly highlighted. This paper also includes an in-depth comparison of volume and cost of PM used in the two types of PMSMs. The short-time performance figures of the said motors have also been presented. The possibility of demagnetisation of PMs, during a sudden fault, has also been investigated for both PMSMs. Challenges of making of both rotors have been discussed. The theoretically determined parameters and analytically evaluated performance figures have been verified through standard FEM packages and then validated experimentally on the prototypes.

Design and Experimental Investigation of a Hybrid Rotor Permanent Magnet Modular Machine with 3D Flux Paths Accounting for Recyclability of Permanent Magnet Material

Rare-earth metals used for manufacturing Permanent Magnets (PMs) remain classified as critical raw materials by the European Commission. In order to secure the supply of electrical machines due to the increasing demand of Hybrid and Full Electrical Vehicles ((H)EVs), recycling has emerged as a valuable alternative. Hence, this paper presents the concept of a modular PM machine with a hybrid rotor and 3D flux paths, for application in ((H)EVs). The proposed machine topology is intended to facilitate the extraction of PM material towards a recycling process. The selection of a machine for prototyping is carried out by investigating the effect of the variation of the number of rotor teeth and stator modules on various parameters, with models developed in Finite Element (FE). Finally, the models developed of the selected combination were validated with a detailed experimental evaluation of the prototype.

Characteristic analysis of parallel-rotor hybrid generator based on exciter types

In this paper, a characteristic analysis and a performance comparison of a permanent magnet (PM) exciter and a conventional winding exciter for a parallel-rotor-type hybrid generator is presented. The proposed rotor of the hybrid generator comprises two separate structures, a PM and a field winding. The two rotors are attached in parallel to improve the efficiency and self-starting characteristics. Using this configuration, the PM rotor produces base flux to generate enough no-load voltage and the field winding rotor is used to regulate that output voltage. Because of the PM rotor flux, the automatic voltage regulator can self-start the generator by taking the voltage induced by the PM flux, regardless of the exciter. The self-start capability, dynamic response and efficiency of both exciters are compared and investigated. It can be confirmed through experiments that the PM exciter shows better efficiency and dynamic response with reduced mechanical complexity.

A Novel Fractional Slot Non-Overlapping Winding Hybrid Excited Machine With Consequent-Pole PM Rotor

A novel fractional slot non-overlapping winding (FSNW) hybrid excited (HE) machine with consequent-pole permanent magnet (PM) rotor is proposed in this article. The abundant harmonics in the stator magnetomotive force (MMF) of FSNW, which are harmful and tend to cause parasitic effects in conventional PM machines, are utilized to regulate the magnetic field and produce torque through field modulation. The DC excitation current can be injected by open-winding drive circuit and a biased current excited HE machine without field winding can be achieved, which solves the spatial confliction of field and armature windings. The electromagnetic performances of the proposed HE machine are evaluated with finite element method (FEM). It is revealed that the DC excitation can effectively regulate the magnetic field, and consequently, the back-EMF as well as electromagnetic torque. Finally, a prototype is fabricated and tested to validate the theoretical analyses and FE calculations.

Comparative analysis of spoke‐type, consequent‐pole and hybrid‐pole permanent magnet machines

In recent years, consequent-pole permanent magnet (CPM) machines draw much attention due to their high utilisation ratio of permanent magnet (PM) material. However, they have some disadvantages, such as the even-order harmonics of airgap flux density, which may cause asymmetric phase back-EMF and the increase of torque ripple. In this study, a novel hybrid-pole PM (HPM) machine is proposed, in which the rotor combines consequent-pole and spoke-type PM rotors. The working principle of the proposed HPM machine is analysed based on the equivalent magnetic circuit, and validated by the finite-element method. Then, the electromagnetic performance of the HPM machine is compared with those of the existing CPM and spoke-type PM machines. It is shown that the HPM machine obtains similar average torque, PM utilisation ratio, torque ripple and efficiency to the spoke-type PM machine, whilst it has a higher PM utilisation ratio and much lower torque ripple than the conventional CPM machines. Finally, these analyses are validated by the tested results of a 9-slot/6-pole HPM machine.

Decoupling control of a dual‐stator linear and rotary permanent magnet generator for offshore joint wind and wave energy conversion system

Wind and wave energy are the main forms of offshore energy. In this study, a new joint wind and wave energy (JWWE) power conversion system, which simultaneously features wind power generation and wave power generation, is investigated. In the JWWE power generation system, a dual-stator linear and rotary permanent magnet generator (DSLRPMG) is employed to directly convert the wind and wave energy. The topology and operating principles of the power generation system are analysed, and the rectifier topology with two three-phase bridge converter is introduced for the JWWE power generation system. On the basis of the vector control method, the mathematical model of the DSLRPMG is deduced. The virtual flux direct power control method without any AC voltage sensors is applied for the power generation system. A new decoupling control method including flux decoupling and power decoupling is proposed and analysed. Meanwhile, an experimental setup is constructed, and the experiments are done. Both the simulation and experimental results show the validity and correctness of the control strategy.