Demagnetization Withstand Capability Research Articles

Driving Cycle Design Optimization of Less-Rare-Earth PM Motor Using Dimension Reduction Method

In this paper, a driving cycle design optimization approach using the dimension reduction method is proposed for a less-rare-earth permanent magnet (LRE-PM) motor. In order to improve the efficiency of driving cycle optimization, the k-means clustering method, sensitivity analysis, and principal component analysis are utilized to reduce the dimensions of operating points, design parameters, and optimization objectives respectively. Furthermore, based on multi-objective genetic algorithm, improved motor performances of the whole driving cycle can be realized, including relatively high output torque, low torque ripple, low motor loss, and high demagnetization withstand capability. In addition, motor performances over the driving cycle before and after optimization are compared in detail. Finally, a prototype motor is built and tested. Both simulation and experimental results indicate that motor performances can be improved efficiently and comprehensively, which provides a potential research path for high-efficiency driving cycle optimization.

Demagnetization Analysis of Modular SPM Machine Based on Coupled Electromagnetic-Thermal Modelling

This paper investigates magnet demagnetization characteristics of the modular permanent magnet machine. The influence of flux gaps on magnet flux density, losses distribution, torque and demagnetization are analyzed for different operating conditions. The magnet demagnetizations caused by three sources, such as the PM field, the armature field, and the magnet temperature rise, are individually investigated using the frozen permeability method. Furthermore, coupled electromagnetic (EM)-thermal modelling is also adopted in this paper to fully reveal the advantages of the modular machine in improving machine EM performances. This is essential due to the temperature-dependent properties of the machines, such as the magnet remanence, coercivity, and copper resistivity. For comparison propose, the EM performances with a particular focus on the demagnetization withstand capability for both the modular and non-modular machines are investigated based on the EM-thermal coupling. It is found that, compared to the non-modular machine, the modular machine can achieve higher torque, higher efficiency, and better demagnetization withstand capability.

A Novel Delta-Type Hybrid-Magnetic-Circuit Variable Flux Memory Machine for Electrified Vehicle Applications

This article focuses on a newly developed 30-kW hybrid-magnetic-circuit variable flux hybrid magnet memory machine (HMC-VFMM) with a delta-type magnet arrangement for automotive traction applications. The proposed HMC-VFMM can improve the torque capability with effective utilization of flux concentration due to delta-type hybrid magnet arrangement while decreasing the required magnetization current level due to the dual-layer (DL) permanent magnet (PM) design. The presence of the parallel magnetic circuit can permit a satisfactory magnetization state (MS) manipulation range. Meanwhile, the DL series-type PM configuration can maintain beneficial unintentional demagnetization withstand capability. The machine topology and operating principle are described. Furthermore, the magnetic circuit model of the proposed machine structure is developed to identify and confirm the feasibility of the electromagnetic performance enhancement. Besides, the electromagnetic characteristics of the proposed HMC-VFMM are comprehensively analyzed and compared with those of a benchmark delta-type PM machine based on the Nissan Leaf drive motor. Finally, a 30-kW prototype is manufactured and tested to verify the feasibility of the proposed HMC-VFMM design in the potential context of electric vehicle applications.

Investigation of a Novel Consequent-Pole Flux-Intensifying Memory Machine

This paper mainly focuses on the investigation and analysis of a novel consequent-pole flux-intensifying memory machine (CP-FIMM). The proposed CP-FIMM exhibits the advantages of a satisfactory flux-regulation range, reduction of the required magnetizing current magnitude, as well as similar torque with much less PM utilization compared to its conventional counterpart. By designing the q-axis flux barriers, the flux-intensifying structure can be realized to enhance the demagnetization withstand capability of the CP-FIMM. The machine topology and operating principle are described. Moreover, the equivalent magnetic circuit model is developed to highlight the performance improvement of the proposed CP-FIMM. Finally, the electromagnetic performance of the proposed CP-FIMM is compared with that of a benchmark conventional FIMM by 2-D and 3-D finite element analysis.

A Novel Asymmetric-PM Hybrid-Magnetic-Circuit Variable Flux Memory Machine for Traction Applications

In this paper, a novel asymmetric-permanent- magnet hybrid-magnetic-circuit variable flux memory machine (AH-VFMM) is proposed. The low coercive force (LCF) permanent magnets (PMs) are located on one biased side of the hybrid magnetic circuit branch, forming an AH configuration. Compared to its conventional symmetric-PM hybrid-magnetic-circuit (SH) counterpart, the proposed design benefits from the reduced usage of high coercive force (HCF) rare-earth magnet, alleviating the magnetic saturation to improve the flux regulation (FR) range, while the excellent load demagnetization withstand (LDW) capability can be still maintained. The topology evolution and operating principle of the proposed machine are introduced and described, respectively. Then, based on the combination of the magnetic equivalent circuit (MEC) method and Schwarz-Christoffel (SC) transformation, a hybrid analytical model is established to quantitatively reveal the mechanism of FR range extension, and facilitate the initial design of the machine. The electromagnetic characteristics of the proposed machine under different magnetization states (MSs) are investigated and compared with those of the conventional SH counterpart. Finally, an AH-VFMM prototype is manufactured, and the experimental measurements are carried out to verify the validity of the proposed design.

Design and Analysis of Interior Permanent Magnet Motor for Electric Vehicle Application Considering Irreversible Demagnetization

The design of permanent magnet motors for the battery-powered electric vehicle has different aspects. In this study, the conceptual design process of an electric machine for Sedan Low car is presented. Based on the vehicle requirements, an interior permanent magnet motor with a maximum power of 120 kW with different slot/pole combinations is designed. To obtain the best slot/pole combination, The New European Driving Cycle in terms of crucial performance characteristics is compared. Given the importance of irreversible demagnetization phenomenon to an electric vehicle application, an accurate approach to determine the irreversible demagnetization rate is first introduced. Design tips are then proposed to reduce the probability of the irreversible demagnetization. Finally, fault-tolerant design in case of irreversible demagnetization withstand capability is carried out using a two-level Taguchi optimization approach. The results obtained from this study indicated that the proposed approach is effective in the fault-tolerant design of the permanent magnet machine for electric vehicle applications.

Investigation of Hybrid-Magnet-Circuit Variable Flux Memory Machines With Different Hybrid Magnet Configurations

This article investigates and compares the electromagnetic performance of two novel hybrid-magnetic-circuit variable flux memory machines (HMC-VFMMs) with different hybrid permanent magnet (PM) configurations. The investigated HMC-VFMMs are both geometrically characterized by a combination of series and parallel magnetic circuit topologies, but with different positions of series and parallel PM branches. The developed HMC design can combine the merits of wide flux adjustment range in parallel structure and excellent unintentional demagnetization withstand capability in series structure. The topologies, features, and working principles of the studied HMC-VFMMs with different hybrid magnet configurations are described, respectively. The magnetic circuit model is utilized to analytically reveal the performance metrics of the two machines. The electromagnetic characteristics of the radially magnetized (RM) and circumferentially magnetized LCF types of HMC-VFMMs are evaluated and compared by the finite-element method. Furthermore, the performance comparison of the proposed RM-type HMC-VFMM with a benchmark interior PM machine is conducted. The experiments on RM-type HMC-VFMM prototype with RM-structure are carried out to validate the FE analyses.

Analysis of Split-Tooth Stator Slot PM Machine

In this article, split-tooth stator slot permanent magnet (PM) machines, which have high fault-tolerant capability, are proposed. Compared with the existing 12-slot 10-pole (12S10P) stator slot PM machine, the torque can be improved by 66% by properly choosing the split tooth number and other geometric parameters. It is found that two split teeth stator slot PM machine (ST2) shows the best torque characteristics among the stator slot PM machines with different split tooth number. To further illustrate the merits and demerits of ST2, a conventional 6S4P surface mounted PM machine (SPMM) and a 6S4P variable flux reluctance machine serve as the benchmarks to compare with the 6S11P ST2, at both open circuit condition and load condition. The 6S11P ST2 shows 68% higher torque over the 6S4P VFRM. Although the torque of the 6S11P ST2 is around half of the 6S4P SPMM, the former has 70% higher torque per PM volume than the latter. Furthermore, demagnetization analyses under three-phase short-circuit and pure d-axis currents show that the 6S11P ST2 has much better demagnetization withstand capability over the 6S4P SPMM. A Prototype is manufactured and tested to validate the theoretical analyses.

Novel Hybrid-Pole Rotors for Consequent-Pole PM Machines Without Unipolar Leakage Flux

This paper presents two novel consequent-pole permanent magnet (PM) machines featuring N-iron-N-iron-N-S-iron-S-iron-S and iron-S-iron-S-iron-iron-N-iron-N-iron sequences. In order to provide assistant flux and suppress the subharmonics of the air-gap flux density, the tangential PMs are embedded into the proposed rotors of the consequent-pole PM (CPM) machines to form the novel hybrid-pole PM (HPM) machines. The principle of eliminating unipolar leakage flux and the performance improvement by hybrid pole are analyzed based on the simplified PM magnetic circuit. Then, the proposed rotors of CPM and HPM machines, as well as their conventional counterpart with an N-iron sequence, are optimized by finite-element analyses. Furthermore, the electromagnetic performances of the proposed machines, including the open-circuit air-gap flux density, average torque, torque ripple, loss, efficiency, unbalanced magnetic force, demagnetization withstand capability, and unipolar leakage flux, are compared with that of the conventional surface-mounted PM (SPM) and CPM machines. It is demonstrated that both the conventional CPM and proposed HPM machines have >20% higher PM material utilization ratio than the SPM machine. The proposed HPM machine can obtain almost equivalent torque and efficiency compared to the conventional CPM machine. Moreover, unipolar leakage flux does not exist in the end shaft, and thus the magnetization of the mechanical components can be effectively eliminated. Finally, a 9-slot/10-pole HPM machine is prototyped and measured to validate the analyses.

A Novel Hybrid-Magnetic-Circuit Variable Flux Memory Machine

This article proposes a novel hybrid-magnetic-circuit variable flux memory machine (HMC-VFMM) by combining series and parallel hybrid magnet structures. Thus, the synergies of wide flux regulation range in parallel type and excellent on-load demagnetization withstand capability in series type can be simultaneously obtained with the proposed HMC design. Meanwhile, two sets of the permanent magnets (PMs) with high coercive force and low coercive force (LCF), i.e., NdFeB and AlNiCo PMs, are employed to achieve high torque density and energy-efficient magnetization state adjustment. The topologies and tradeoffs of traditional parallel and series VFMMs are addressed first. In addition, the structure evolution, features and operating principle of the proposed HMC-VFMM are described, respectively. A simplified equivalent magnetic circuit is modeled to reveal the performance improvement of the machine. Then, the design improvements with q-axis barriers are presented to elevate the LCF PM working point for preventing the on-load demagnetizing effect, while maintaining the torque capability. The electromagnetic characteristics of the HMC design are investigated and compared with the parallel/series counterparts. Finally, the experiments have been carried out to validate the finite-element analyses.

Torque Improvement and Cost Reduction of Permanent Magnet Machines With a Dovetailed Consequent-Pole Rotor

This paper presents a consequent-pole permanent-magnet (CPM) machine to reduce the volume of permanent magnets (PM) and the cost of retaining sleeve, and meanwhile the average torque and efficiency can be improved due to the reduced airgap reluctance. The proposed configuration uses dovetail consequent-pole to protect quasi-trapezoidal PMs with lower total rotor loss than a conventional surface-mounted PM (SPM) machine due to the half number of PMs and the removed sleeve loss. In addition, the stress and deformation of the rotor are investigated. The electromagnetic performance including back EMF, average torque, torque ripple, loss, efficiency, unbalanced magnetic force, demagnetization withstand capability of the proposed CPM machine is investigated by finite element analyses. It is demonstrated that the output torque of the CPM machine can be enhanced by 3%, while the PM volume is reduced approximately 20% compared to that of the conventional SPM machine. Simultaneously, the CPM machine can have lower total harmonic distortions of phase back EMF, lower torque ripple, higher efficiency, and stronger demagnetization withstand than the SPM machine. Finally, the 12-slot/10-pole CPM machine is prototyped to validate the analyses.

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.

Demagnetization Withstand Capability Enhancement of Surface Mounted PM Machines Using Stator Modularity

The flux gaps in alternate stator teeth of the modular permanent-magnet machines can have a significant impact on the total magnet flux density, and hence the potential magnet reversible/irreversible demagnetization under flux weakening operations or short-circuit conditions. Such a problem has not been studied in literature and will be investigated in this paper. The influence of flux gaps on the d -axis inductance and the potential peak short-circuit current is analyzed for different slot/pole number combinations. It is found that the flux gaps will affect both d -axis inductance and open-circuit flux linkage, and hence reduce short-circuit current of machines with pole number (2 p ) smaller than slot number ( Ns ), while they will increase the short-circuit current of machines with 2 p > Ns . However, the opposite phenomena can be observed for demagnetization withstand capability. For machines having 2 p , the flux gaps tend to lower withstand capability, while for machines having 2 p > Ns , this capability can be improved. Other parameters such as magnet thickness and temperature have also been accounted for in the demagnetization analysis. Tests have been carried out to validate the predictions of inductances and short-circuit current, as well as performance such as phase back electromotive force, cogging torque, and static torque for machines with one defective magnet, which represents the case of partially demagnetized magnets.

Design and Analysis of a Spoke-Type Hybrid Permanent Magnet Motor for Electric Vehicles

This paper proposes a new spoke-type hybrid permanent magnet brushless (H-PMBL) motor for potential applications in electric vehicles, where the high-energy rare-earth PM and the cost-effective ferrite-PM are combined. In order to realize low-cost design and improve the operating points of the rare-earth PM and the ferrite-PM, the proposed motor is designed based on the calculation of PM cost and analysis on the operating points. Moreover, to fairly estimate electromagnetic performances of the H-PMBL motor, a conventional spoke-type non-rare-earth ferrite-PMBL motor with the same overall dimensions is also analyzed and compared using the finite element method. The simulation results indicate that the H-PMBL motor cannot only retain low PM cost, but also exhibit better flux-weakening ability and enhanced demagnetization withstand capability.

Partitioned Stator Machines With NdFeB and Ferrite Magnets

Partitioned stator (PS) machines employ two separated stators to accommodate armature windings and permanent magnets (PMs), respectively, alleviating the space conflict in stator-PM machines, and hence boosting the torque density. However, the PM usage volume in PS machines is relatively large, and thus the material cost is high, which is undesirable for the cost-sensitive applications. In this paper, a PS machine employing ferrite PM is presented, which takes advantage of larger available space for PMs while balances the performance and cost. The electromagnetic characteristics of an optimized ferrite PS machine and an NdFeB PS machine are evaluated together with a classic Prius2010 interior PM (IPM) machine as a baseline. Meanwhile, the economic issues are considered. The results reveal that the NdFeB PS machine has higher torque density than the Prius2010 IPM machine, but is more expensive, while the ferrite PS machine can significantly reduce the cost and still exhibit good performance. The demagnetization behavior of the ferrite PS machine is evaluated and a new method to improve the demagnetization withstand capability is proposed. Moreover, the mechanical analysis is conducted for the cupped rotor of PS machines. Finally, a pair of small-scaled prototypes are manufactured to validate the predictions.

Comparative Study of Fault-Tolerant Switched-Flux Permanent-Magnet Machines

The fault-tolerant capabilities are compared in this paper for the conventional double-layer switched-flux permanent-magnet machine and its single-layer counterparts, i.e., C-core, E-core, and modular. The comparison includes the interturn short-circuit and irreversible demagnetization faults. A combination of Simulink and finite element models is used in the study. Based on the predictions, it is found that the modular topology produces the lowest short-circuit current and also has the best demagnetization withstand capability while the conventional one produces the highest short-circuit current and has the worst demagnetization withstand capability. The frozen permeability method is employed to separate the flux produced by armature current and magnets, and the results showed that, besides the influence of short-circuit current, the available magnet volume and magnetic circuit configuration play an important role in the demagnetization process. It is also found that removing half of the magnets, such as using C-core, E-core, and modular topologies, generally improves the demagnetization withstand capability and also increases the torque per magnet volume. Measured results are also presented to validate the short-circuit current predictions and magnet demagnetization.

Demagnetization investigation of a partitioned rotor flux switching machine with hybrid permanent magnet

In this paper, the partitioned rotor flux switching permanent magnet machine with ferrite permanent magnet is proposed. By the adoption of the partitioned rotor configuration, the stator flux leakage is eliminated and the permanent magnet utilization is improved. The ferrite permanent magnet machine often suffers from irreversible demagnetization due to the inherent relatively low coercivity of ferrite permanent magnet. To mitigate the machine irreversible demagnetization risk, an improved partitioned rotor flux switching permanent magnet machine with hybrid permanent magnet topology is also proposed. Two little pieces of rare-earth permanent magnet are installed at the corners of ferrite permanent magnet, thus forming the hybrid permanent magnet topology. And the demagnetization mechanisms of both machines are clarified by the magnetic equivalent circuit method, which prove that the rare-earth permanent magnet offer magnetic protection function for the ferrite permanent magnet. Furthermore, by the 2-D finite element analysis, the demagnetization characteristics and the electromagnetic performances of the two machines are quantitively assessed, revealing that the demagnetization risk is reduced significantly. Both theoretical analysis and simulation results verify that the improved machine can not only maintain low-cost design, but also possess enhanced demagnetization withstand capability and competitive electromagnetic performances as expected.

Three-Dimensional Modelling of Demagnetization and Utilization of Poorer Magnet Materials for EV/HEV Applications

High performance electric motor designs with ferrite magnets have recently gained interest due to the high and volatile price of rare earth magnets. However, due to the relatively poor coercivity of ferrite magnets, these designs are highly susceptible to demagnetization, as a result of which accurate modelling and better understanding of this phenomenon is particularly important. In this paper, the impact of the motor stack length and level of magnetic saturation on the demagnetization risk are studied based on 3-dimensional finite element simulations and a proposed lumped circuit model. It is shown that reducing the stack length can significantly enhance the demagnetization resistance with the effect being more pronounced for designs with a higher level of magnetic saturation. To benchmark the practicality of the concept, a previously presented high-performance ferrite-based design is modified by using a 30% weaker grade of ferrite magnet whilst shortening the stack length. It is shown that the demagnetization withstand capability of the design was significantly enhanced and exceeded the short circuit requirement with a good safety margin. The theoretical findings have been supported by prototype testing.

Analytical On-Load Subdomain Field Model of Permanent-Magnet Vernier Machines

Permanent-magnet vernier machine (PMVM) is a relatively new type of PM machines. An analytical subdomain model accounting for tooth-tips and flux modulation poles is developed to accurately predict on-load field distributions in PMVMs. Based on two-dimensional (2-D) polar coordinate and magnetic vector potential, this method solves the Maxwell’s equations in slot, air-gap, flux modulation pole slot (FMPS), and PM regions. Consequently, the electromagnetic performance such as cogging torque, back-electromotive force (EMF), electromagnetic torque, power factor, and magnet loss are calculated. In addition, the model can also be used for the evaluation of demagnetization withstand capability. The finite-element analysis (FEA) and experimental results validate the accuracy of the developed analytical model.

Hybrid-Excited Switched-Flux Hybrid Magnet Memory Machines

The memory machine concept is recently extended to switched-flux (SF) machines, forming a series of SF hybrid magnet memory machines (SF-HMMMs). They exhibit the merits of good demagnetization withstand capability and effective flux adjustability. Nevertheless, the torque density is inevitably compromised due to the geometric conflict within the stationary part. Meanwhile, the inactivated dc coil, excluding online magnetization transients, results in system redundancy. Hence, in this paper, a new hybrid-excited (HE) concept is developed and implemented in a partitioned-stator SF-HPMM (PS-SF-HMMM). Thereby, the distinct synergies of a dual-magnet memory machine and an HE machine are achieved. With slight compensated field excitation, the torque can be improved at low-speed operation. Meanwhile, the high-speed constant-power region can be further extended without sacrificing high efficiency. The stator/rotor pole numbers are optimized first. The operating mechanism and optimal stepwise HE implementation over a whole operating envelop are then addressed. In addition, a comparison between PS-SF-HMMM with hybrid-excitation and its pure HE counterpart is established. Finally, both the finite-element simulation and experiments are carried out to verify the utility of the proposed HE concept.