Fractional-slot Permanent Magnet Research Articles

Harmonics Traceability and Suppression of Electromagnetic Noise in Fractional Slot Permanent Magnet Synchronous Motors

Harmonics Traceability and Suppression of Electromagnetic Noise in Fractional Slot Permanent Magnet Synchronous Motors

Research on torque ripple suppression of in-wheel fractional slot permanent magnet synchronous motors based on auxiliary slots

Research on torque ripple suppression of in-wheel fractional slot permanent magnet synchronous motors based on auxiliary slots

Investigation of Asymmetrical Winding Layouts for Fractional-Slot Permanent Magnet Machine

Investigation of Asymmetrical Winding Layouts for Fractional-Slot Permanent Magnet Machine

Multiple Sets of M-Phase Winding Composition Principle of Permanent-Magnet Machine for MMF Harmonic Analysis

This paper presents the composition factor of the fractional-slot permanent-magnet (PM) machine equipped with multiple sets of m-phase windings. Besides, the intrinsic relationship between magnet-motive force (MMF) harmonics and the phase shift angle is revealed. Firstly, the structural characteristics and phase shift regulations of multiple sets of <i>m</i>-phase windings are analyzed. Based on a single set of <i>m</i>-phase windings, the MMF composition factors of the odd and even sets of <i>m</i>-phase windings are derived, respectively. Secondly, four PM machines with different slot-pole combinations and winding configurations were designed. Their MMF harmonics are analyzed by the finite-element method to verify the analytical results. Moreover, the performances and MMF harmonics of a 48-slots 22-poles (48s22p) PM machine with dual three-phase windings are analyzed. Finally, the 48s22p machine is prototyped, and experimental results are validated.

A New Flux-Concentrating Rotor of Permanent Magnet Motor for Electric Vehicle Application

This article presents a new interior permanent magnet (IPM) rotor structure/ assembly, which inherits from the Halbach magnet configuration and spoke-type IPM rotor to achieve a high saliency ratio and high torque/ power density. With this rotor structure, the torque density and power density are improved obviously, while the rotor inner diameter can be maximized to reduce core material and increases internal space in the rotor. Accordingly, the electromagnetic performance of three motors consists of a unified stator but different rotors, namely flux-concentrating rotor, Halbach array permanent magnet (PM) rotor and spoke-type IPM rotor is compared by finite element analysis. Nevertheless, due to the segmented structure of flux-concentrating PM rotor without magnetic bridges, the mechanical challenges are analyzed in detail for electric vehicle application to hold the pole pieces validate the structural feasibility at 7200 r/min. Finally, a 72-slot/ 16-pole fractional-slot PM motor with new flux-concentrating rotor is manufactured and tested to verify its performance and feasibility. It can be concluded that the new rotor is superior to the spoke-type IPM and Halbach array PM rotor in terms of torque density and PM utilization.

Torque Ripple and Radial Force Minimization of Fractional-Slot Permanent Magnet Machines Through Stator Harmonic Elimination

This article proposes a torque ripple minimization strategy based on minimizing interactive space harmonics of fractional-slot permanent magnet (PM) machines using a space-shifted wye–delta stator winding. A radial force analysis has also been carried out, which showed a substantial reduction in the second-order mode for these space-shifted wye–delta wound machines; this will improve the noise and vibration performance. The effectiveness of the proposed method is shown by analyzing the performances of a low-power surface-mounted permanent magnet (SPM) machine for the electric power steering (EPS) applications and a high-power interior permanent magnet (IPM) machines for the hybrid electric vehicle (HEV) applications on the 12-slot/10-pole (12 S/10 P) configuration. The proposed winding doubles the stator slot number of the base model, and as a result, the base 12 S/10 P configuration becomes 24-slot/10-pole (24 S/10 P) combination. Compared with the existing ripple minimization techniques of magnet shaping or rotor skewing, the proposed stator winding-based strategy achieves better torque ripple performance and magnet utilization leading to an improvement in average torque while reducing the amplitude of the low-order radial force. A 10-pole SPM machine has been built and tested for experimental validation of both the concept and the finite element analysis (FEA) simulation results.

극 슬롯 조합에 따른 영구자석 표면 부착형 동기전동기의 특성 분석 연구

In this paper, the characteristics of SPMSM (Surface mounted Permanent Magnet Synchronous Motor) applied to EPS(Electric Power Steering) systems were analyzed according to the combination of pole slots. In general, fractional-slot permanent magnet synchronous motors using concentrated windings have advantages such as being more economical than distributed windings, having shorter end windings, and high copper fill factor. However, traditional permanent magnet synchronous motor having the fractional slot has a relatively low winding factor due to the slot pitch and pole pitch ratio. One way to compensate for this is to use a modular motor with fewer slots for a given number of poles. The modular motor is a motor with the number of slots per pole per phase less than 0.5, and the winding factor is larger than that of the traditional motor. Therefore, in this paper, three types of motors: fractional slot modular motors, traditional motors, and integer slot motors, were analyzed and the power characteristics were compared using the FEM(Finite Element method).

Cogging Torque Dynamic Reduction Based on Harmonic Torque Counteract

This article presents a method to reduce cogging torque in permanent magnet (PM) machines from dynamic control based on harmonic torque counteract (HTC). We find the possibility of dynamic cancellation of cogging torque and harmonic torque by studying their generation mechanism and period characteristics, and further study the current harmonic injection scheme for cogging torque reduction. Further, two case studies of an integer slot PM machine and a fractional slot PM machine based on finite element analysis well validate the proposed method. Among them, the cogging torque of a 6-slot/2-pole PM synchronous machine is weakened from 932.0 to 56.2 mN.m, with a weakening ratio of 94.0%, and the cogging torque of the other with 6-slot/4-pole is weakened from 1.23 N.m to 44.21 mN.m, with a weakening ratio of 96.4%. The theory of this article can be extended to PM machines with other different pole/slot combinations and topologies.

Analysis and Design of Dual Three-Phase Fractional-Slot Permanent Magnet Motor With Low Space Harmonic

There are many space harmonics in the stator magnetomotive force (MMF) of fractional-slot concentrated winding (FSCW). These harmonics will produce high eddy current losses in the permanent magnet (PM), which will reduce motor efficiency and affect motor performance. Aiming at the above problems, the harmonic components of traditional stator MMF are first analyzed in this article, and then, a novel space-shifted unequal-turn winding structure is proposed for dual three-phase fractional-slot permanent magnet synchronous motors (PMSMs). Meanwhile, the sub- and super-harmonics in the stator MMF can be simultaneously eliminated using the proposed winding structure. To verify the effectiveness of the proposed structure, an 18-slot/8-pole motor and a 30-slot/14-pole dual three-phase fractional-slot PMSM with low space harmonic are designed by using the proposed space-shifted unequal-turn winding structure based on the analysis of nine-slot/eight-pole and 15-slot/14-pole motors. Finally, by comparing the finite element analysis (FEA) results, it is verified that the dual three-phase PMSM motor with the proposed structure has smaller eddy current losses in PM and better motor performance.

Fractional-Slot PMSMs With One Coil Parallel Branches Made Phases—Part 2: Circulating Harmonic Current Rejection Potential

The paper is aimed at a class of fractional-slot permanent magnet- synchronous machines (FSPMSMs) equipped with phases made up of one coil parallel branches, with emphasis on their potential to reject the circulating harmonic currents. Following the identification and topological characterization of this class of machines treated in Part 1, the star of slots approach is extended to the investigation of their back-EMF harmonic content, and the possible resulting circulating harmonic currents. It is found that all identified candidates exhibit the potential to reject the circulating harmonic currents. A case study dedicated to a candidate equipped with three phases made up of six parallel branches of one coil each, is treated. Following an analysis of its back-EMF harmonic content, the torque-speed characteristic is analytically-predicted and validated by 2D finite element analysis and by experiments. The study is achieved by a special attention paid to the iron loss and thermal analysis.

Influence of Pole and Slot Combination on Torque Characteristics and Radial Force of Fractional Slot Permanent Magnet Machines

The fractional slot concentrated winding structure effectively solves the difficulty of low‐speed and high‐torque permanent magnet (PM) machines with a large number of poles and slots, but it also brings a wealth of magnetomotive force (MMF) harmonic. The winding function method is used to compare and analyze the stator MMF distribution and harmonic content of the fractional slot PM machines with different pole and slot combinations, and the cogging torque waveform and torque ripple are calculated by the finite element method. The influence of pole and slot combination on radial force and vibration modes of fractional slot PM machines is studied. The main vibration mode can be determined from the lowest order of radial force harmonics. The results show that in a fractional slot PM motor, the radial force is the main cause of noise and vibration, rather than the cogging torque and torque ripple. The dominant radial force is mainly produced by the interaction of rotor field harmonics and armature reaction field harmonics, and the vibration and noise will significantly increase with the decrease of the dominant vibration mode order, which largely depends on the pole and slot combination. This paper provides a certain reference for the selection of pole and slot combination in the electromagnetic design process of low speed high torque fractional slot PM motors. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Multi-Phase Fractional-Slot PM Synchronous Machines with Enhanced Open-Circuit Fault-Tolerance: Viable Candidates for Automotive Applications

This paper deals with the winding arrangement of multi-phase fractional-slot permanent magnet (PM) synchronous machines (FSPMSMs), with emphasis on the enhancement of their open-circuit fault-tolerance capability. FSPMSMs are reputed by their attractive intrinsic fault-tolerance capability, which increases with the number of phases. Of particular interest is the open-circuit fault-tolerance capability, which could be significantly enhanced through the parallel connection of the coils or suitable combinations of the coils of each phase. Nevertheless, such an arrangement of the armature winding is applicable to a limited set of slot-pole combinations. The present work proposes a design approach that extends the slot-pole combinations to candidates that are characterized by a star of slots including three phasors per phase and per winding period. It has the merit of improving the tolerance against open-circuit faults along with an increase in the winding factor of multi-phase machines. Special attention is paid to characterization of the coil asymmetry required for the phase parallel arrangement. A case study, aimed at a finite element analysis (FEA)-based investigation of the open-circuit fault-tolerance of a five-phase FSPMSM, is treated in order to validate the analytical prediction.

Fractional-Slot PMSMs With One Coil Parallel Branches Made Phases—Part I: Investigation Study

This work proposes an approach to design fractional-slot permanent magnet synchronous machines (FSPMSMs) equipped with phases made up of one coil parallel branches. These machines exhibit attractive potentialities, especially their enhanced open-circuit fault tolerance capability. Beyond their intrinsic fault tolerance, an open-circuit affecting the targeted topologies leads to a torque step-down limited to the one developed by the concerned coil rather than the torque produced by the total phase in FSPMSMs equipped with phases made up of series-connected coils. Furthermore, these topologies are suitably-adapted to low-voltage power supply that makes them viable candidates in automotive applications. This potentiality is by far vital in battery electric vehicles with the risk of electrocution in case of accident totally-eradicated thanks to the reduction of the DC bus voltage. Part 1 of the developed work is aimed at a star of slots-based identification and topological characterization of the FSPMSMs enabling the arrangement of the armature phases according to one coil parallel branches. The study distinguishes the topologies with odd and even number of phases, arranged in single- and double-layer slots. A case study is treated using a 2D finite element analysis and validated by experiments.

EMF Voltage Distortion Mitigation in Fractional-Slot Permanent Magnet Machines by Varying Coil Distribution and Turn Ratio

Electromagnetic force (EMF) voltage distortion, caused by armature reaction, not only deteriorates torque ripple in constant torque region but also reduces the flux weakening operation region, especially for fractional slot permanent magnet (PM) machines. This article mitigates voltage distortion by varying coil distribution and turn ratio with employment of triple layer (TL) and quadruple layer (QL) windings with varied coil turns. Coil turn ratios of TL and QL windings are optimized to minimize voltage distortion ratio. Reduction mechanisms are exhaustively analyzed. It is found that, for the TL winding with optimal turn ratio, the armature leakage flux and PM leakage flux in tooth tips will not reverse polarities simultaneously, resulting in much lower voltage distortion. On the other hand, for the QL winding, voltage distortion reduction is realized by lowering the amplitude of armature tooth tips leakage flux. Compared with conventional double or single layer (SL) winding machines, torque ripple can be reduced and flux weakening capability is greatly enhanced. The 12-slot/10-pole prototypes with both SL and optimum TL windings are fabricated and tested to verify the analysis.

Influence of unequal stator tooth width on the performance of outer-rotor permanent magnet machines

AbstractOuter-rotor permanent magnet machines for low-speed and high-torque applications have been used due to their high moment of inertia and torque density. In this paper, the 12-slot/10-pole outer-rotor fractional-slot permanent magnet machine model is established by finite element analysis, the influence of unequal stator tooth width and tooth tip width is investigated for the PM machine, and five different schemes of stator tooth and tip width are designed and analyzed. Detailed comparisons of the performance characteristics of the machines are presented including important issues such as output torque, magnetic field distribution, and tooth flux density. It is shown that the torque density and cogging torque and efficiency can be effectively improved by choosing reasonable tooth and tip width, which lays a foundation for further optimum design and efficiency improvement of the machine.

Mechanical and Magnetic Pivot Roles of Tooth in Vibration of Electrical Machines

The magnetic field including tooth effect can be calculated by the tooth harmonic permeance. However, the mechanical pivot role of tooth is always neglected in vibration analysis and the slotted stator is considered as a simple cylinder. In this article, the mechanical pivot role that the tooth plays in vibration analysis is illustrated, where the transfer function of force is proposed to represent the process of force from the top of tooth to the yoke. First, the magnetic pivot role of tooth in magnetic field analysis is described by the tooth harmonic permeance. Then the force on the top of tooth is transferred to the yoke by the transfer function of force, after that the yoke can be considered as a cylinder. Based on the pivot role of tooth, the phenomenon that the high order force can cause the low mode vibration is explained and clarified straightforwardly. Not only the fractional slot permanent magnet synchronous motor (PMSM), but also the integer slot PMSM, are analyzed to demonstrate the universality of these methods. Finally, the finite element method simulation, and the experiments on one fractional slot PM motor and one integer slot PM motor, are applied to validate the theory.

Investigation of Unbalanced Magnetic Force in Fractional-Slot Permanent Magnet Machines Having an Odd Number of Stator Slots

Fractional-slot permanent magnet (PM) machines with an odd number of stator slots are attractive candidates for industrial applications due to their high torque density and low torque ripple. However, the unbalanced magnetic force is detrimental to the bearing and yields noise and vibration. This paper determines the existence criterion of UMF on both open-circuit and on-load conditions by analysis of air gap field and magnetic force waves. Only when the machine periodicity is one, will the open-circuit UMF be generated due to the mutual interaction between the modulated PM harmonics and the original PM harmonics. In addition, the existence of on-load UMF in machines with a periodicity of one is confirmed even when the slotting effect is ignored. Furthermore, among the five investigated machines with a given odd slot number of 9 but different pole numbers, the UMF in a machine with two poles is the smallest since the magnitude of the 2nd harmonic in the armature reaction is the smallest compared with other contributing harmonics. Meanwhile, the UMF in a machine with pole number 2p = Ns-1 is the largest due to the relatively larger magnitude of the (p + 1) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> armature reaction harmonic and the additive effect between the first order radial and tangential stresses.

Deadbeat Direct Flux Vector Control of Surface Permanent Magnet Motor Drives

The predictive control algorithms for electrical drives are currently subject to considerable interest and development. In particular, the predictive torque control is a competitive solution that may replace in the future the conventional control schemes based on linear controllers. Therefore, this article proposes a predictive torque control for surface permanent magnet motor drives requiring a wide constant power range. The predictive algorithm uses a deadbeat direct flux vector control approach for the simultaneous control of the stator flux amplitude and torque-producing current component. The proposed control scheme is suitable for applications requiring a wide speed range with current and voltage constraints. The proposed deadbeat torque controller has been tested on a fractional slot permanent magnet machine and the experimental results demonstrate the full drive controllability, including deep flux-weakening operation with limitation of the load-angle.

Design and Analysis of a Bearingless Permanent-Magnet Motor for Axial Blood Pump Applications

Because of their high power density and compact size, permanent-magnet (PM) motors have been commonly used to drive rotary blood pumps (RBPs), which are focused on the treatment of end-stage heart failure or as the bridge to a heart transplant. In this paper, a bearingless PM motor has been proposed for axial blood pump applications. The finite-element method (FEM) is used to predict the electromagnetic characteristics of the designed motor with improved performance. Two topologies are investigated, namely the integral-slot and distributed-windings method and the fractional-slot and double-layer concentrated windings method. Both motors are analyzed and optimized. FEM reveals that, compared with the integral-slot motor, the fractional-slot motor offers significantly enhanced performance, including reduced cogging torque, improved back electromotive force (back EMF), and decreased magnetic flux leakage. Finally, hydraulic experiments have been conducted in a mock-circulation loop to validate the feasibility of the designed motor for an axial blood pump. The results show that the fractional-slot bearingless PM motor can drive the RBP to produce physiological blood flow with reasonable efficiency.

Novel Modular Fractional Slot Permanent Magnet Machines With Redundant Teeth

In order to ease the manufacture process of very large permanent magnet (PM) machines, such as direct drive wind power generators, modularity technique is usually adopted. Although conventional fractional slot PM machines with all teeth wound windings show good performance, the coils located at the end part of each segment could be easily damaged because of being exposed to the air. In addition, the local winding faults could easily proliferate to the whole machine due to the physical touch of adjacent coils. Thus, a novel modular fractional slot PM machine with redundant teeth is proposed in this paper, while three phases within each set of winding are still balanced. The construction of the proposed modular PM machine having 42-slots/32-poles (42S/32P) combination is described in detail as an example, which can be treated as the integration of six 6S/5P segments and a redundant 6S/2P machine. The reason for using 6S/2P machine is to get rid of the undesirable unbalanced magnetic force (UMF), which usually exists in rotating asymmetric machines, such as 3S/2P PM machine. Since the end part of each segment in the proposed modular PM machine can be covered by half redundant tooth during the production, more protection can be provided for coils located in these regions. Furthermore, the separation of six segments can improve the fault-tolerant capability of the machine. The electromagnetic performance predicted by finite-element analysis (FEA) also demonstrates that the proposed modular machine will perform better if dual three-phase winding is adopted. Finally, the experiments on the prototyped machine validate the analysis in the paper.