This paper presents a comprehensive review of high-torque-density permanent magnet (PM) machines for distributed electric propulsion (DEP) systems. First, the torque expressions of PM machines are derived and unified into a general analytical framework to identify the key influencing parameters that guide targeted design optimization. Critical design aspects, including material selection, winding topologies, and magnet configurations, are systematically analyzed. Emerging magnetic materials, such as soft magnetic composites (SMCs), dual-phase alloy materials, and amorphous materials, are all evaluated for their potential to enhance torque density and overall PM machine performance. In addition, advanced winding arrangements and innovative magnet configurations are discussed to achieve a superior output torque. Besides, torque enhancement strategies through multidisciplinary optimization and main material innovation are also summarized. Finally, fault-tolerant design approaches for high-torque-density PM machines in DEP systems, i.e., electric aircraft, vehicles, and ships, are critically reviewed. Overall, this paper aims to provide a unified perspective and highlight future research directions in the development of advanced high-torque-density PM machines.

This article proposes a novel phase-unit axial-modular permanent magnet Vernier machine with multiple U-core stators (PUAM-PMVM-UCS) for distributed electric propulsion (DEP) in electric vertical takeoff and landing (eVTOL) aircraft. The machine topology and operating principle are first introduced, followed by a detailed analysis of key design parameters such as slot-pole combinations and winding configurations. Based on the DEP system requirements, the proposed PUAM-UCS machine is designed and optimized with respect to critical parameters such as the module number and dimensions of UCS. Following, the theoretical models are developed to highlight the distinctions between the PUAM-UCS machines and conventional machines. A comprehensive performance evaluation, e.g., no-load back EMF, output torque, power factor, losses, and efficiency, is carried out by both analytical approaches and finite-element analysis (FEA). Finally, two prototypes of PUAM-UCS and PUAM machine with split-pole stators (PUAM-SPSs) are fabricated and experimentally tested, demonstrating strong agreement with theoretical and FEA predictions. The results can confirm that this proposed machine can achieve improved torque density and better fault tolerance, making it a promising future candidate for the eVTOL DEP system.

This paper proposes and investigates two novel phase-unit modular (PUM) permanent magnet machines with E-core stators (ECS) for electric vertical takeoff and landing (eVTOL) aircraft propulsion. First, the propulsion requirements of eVTOL aircraft are identified, and two representative ECS topologies are introduced, with detailed analyses of operating principles, slot-pole combinations, and winding configurations. Subsequently, the analytical models of the proposed two PUM machines are developed, e.g., armature magnetomotive force (MMF), inductance, back electromotive force (EMF), and torque characteristic. Based on these models, the two machines are designed and optimized, with emphasis on the effects of stator module number, magnet configuration, and geometric parameters. For overall performance benchmarking and validation, a series of conventional machine models are also established for comparison, which can demonstrate the pros and cons of two PUM machines. Eventually, two prototypes of PUM machines are fabricated and experimentally tested, and the results exhibit a good agreement with previous analytical predictions and finite element analysis (FEA). This paper aims to evaluate a series of modular machine topologies as a promising alternative for eVTOL propulsion systems.

Multiunit modular in-wheel motors have a high utilization rate of remaining healthy windings and a high-power density during fault-tolerant (FT) operations. However, when the open-circuit faulty motor operates with FT control, the working windings are asymmetric in the circumferential space, which will cause some problems, such as decreased electromagnetic characteristics and local windings with a higher temperature. This article aims to mitigate the disadvantages caused by asymmetrical working winding operations, and to explore the FT strategies to comprehensively improve the electromagnetic-thermal characteristics under FT operations. First, the main types of open-circuit faults for the modular motor are introduced, and the impact mechanisms of asymmetric winding operations on MMF, torque performance are analyzed, respectively. Thereafter, the FT strategies based on MMF compensation and torque performance improvement are obtained, respectively. Second, the influence of FT strategies on the efficiency, the loss and temperature distributions of the motor under asymmetric winding operation is investigated. And the above FT strategy is upgraded, which not only enhance electromagnetic characteristics but also improve the thermal characteristics of the remaining healthy windings. Finally, the improvement effect of the FT strategies on the electromagnetic-thermal characteristics FT control and asymmetric working winding operations are verified by experiments.

In this paper, a family of phase-unit axial-modular permanent magnet Vernier machines (PUAM-PMVMs) with various stator topologies is proposed and analyzed. Firstly, the machine topologies of these proposed axial-modular designs are introduced, where the basic working principle and feature are analyzed and extracted. Secondly, the theoretical models of the axial-modular PMVMs are derived, e.g., armature MMF, back-EMF, and inductance, etc. Thirdly, the design requirements and specifications are frozen in terms of the basic requirements of propulsion system. Besides, the influencing factors are targeted and analyzed, e.g., slot-pole combination, winding configuration, and motor dimensions. To ensure a fair comparison between the PUAM models and conventional ones, multiple models are also developed and evaluated. Finally, the experimental results from the tested prototypes align well with the finite element analysis.

In the article, the comparative study of core loss and permanent magnet (PM) loss in fractional-slot permanent magnet vernier machines (PMVMs) and permanent magnet synchronous machines (PMSMs) for in-wheel drive are investigated. Firstly, the definitions of “Family” and “Group” for armature magnomotive force harmonics in fractional-slot PM machines are proposed, and the relationship between PMVM and PMSM is illustrated, where the generalized PMSM concept is proposed. Secondly, the core and and PM losses are calculated and compared by the theoretical and finite-element analysis (FEA) methods. The contribution of each harmonic on the core and PM losses is extracted, where the subharmonic and slot-harmonic that not contributed to the torque are mostly responsible for core and PM losses. Thirdly, the influence of stator slot opening, flux barriers, and PWM control on the core loss, PM loss, and torque are all investigated. The losses of PMVM and PMSM under various conditions is calculated and exhibited, and the PM loss is much severer in PMVM. Finally, two prototypes of PMVM and PMSM are manufactured and tested, and the total losses and efficiencies are measured, where the experiments agree well with the FEA and theoretical analysis.

This paper presents a novel split-phase axial-flux permanent magnet motor featuring a printed circuit board (PCB) stator. The motor adopts a multi-rotor configuration with an intermediate stator structure. The stator windings are axially segmented, with each phase integrated on a dedicated PCB layer to achieve electrical, magnetic, and thermal isolation, thereby enhancing fault tolerance. A design methodology for the motor is proposed to analyze and optimize its performance. Finite element simulations and experimental validations confirm the effectiveness of the proposed design.

This paper presents a comprehensive design for permanent magnet vernier motors (PMVMs) applied to in-wheel direct-drive systems. First, the specific requirements and design boundaries are outlined. Some key performance metrics, such as torque density, power factor, and losses, are analytically derived, and the constraining factors are also identified. The study then investigates the impact of these constraints on overall machine performance, highlighting critical tradeoffs between competing design objectives. Based on the previous analysis, optimal slotpole combinations and design specifications are also selected to balance performance tradeoffs. To validate the theoretical and finite element analysis (FEA) results, a 20 kW prototype designed for a 16 -inch tire is fabricated and tested. Experimental results confirm the effectiveness of the proposed design methodology, demonstrating its applicability for high-torque density in-wheel motor applications.

This paper addresses the issue of high-frequency current harmonics in dual-winding permanent magnet synchronous motors (DWPMSMs), which adversely affect motor performance and increase losses due to inverter-induced switching-frequency harmonics. To mitigate this issue, a novel harmonic suppression strategy is proposed, integrating a coupled-inductor inverter topology with interleaved carrier phase-shift techniques. The proposed approach establishes an optimal coordination principle between the carrier phase-shift angle and the voltage modulation index, ensuring minimal harmonic distortion across the entire modulation range. Theoretical analysis indicates that setting the phase-shift angle to p effectively suppresses harmonics near odd multiples of the switching frequency, whereas a phase-shift angle of pi/2 targets even multiples. Finally, experimental results are presented to validate the effectiveness of the proposed method.

Due to the advantages of reduced torque ripple, enhanced fault tolerance, and superior power density, dual three-phase permanent magnet synchronous motors (DTPPMSMs) systems are widely adopted in high-power applications. However, during motors operation, critical parameters exhibit time-varying characteristics due to multi-factor influences such as temperature rise effects and magnetic saturation effects, leading to degradation of control performance. To achieve realtime acquisition of motor parameters, this paper proposes a novel online parameter identification method based on highfrequency harmonic voltage injection. By utilizing the inherent harmonic subspace of space vector decoupling (VSD), voltage signals are injected into the torque-insensitive harmonic plane to prevent coupling with fundamental plane excitation. Through extraction of current responses at the injection frequency, harmonic impedance components are derived via frequencydomain demodulation, enabling precise estimation of stator resistance and leakage inductance. This method effectively isolates identification parameter from non-ideal error sources by frequency-selective processing.

In this article, a novel axial-flux permanent magnet vernier machine with H-core stator modules (PMVM-HCS) was first proposed and investigated. To start with, the machine topology is illustrated and analyzed. In addition, the winding configurations and slot-pole combinations of PMVM-HCS are also investigated. Secondly, according to the aircraft propulsion and dimension requirements, the specifications are frozen and fixed to help the motor design. Then, the influencing factors, e.g., slot-pole combination, winding configuration, and dimensions, etc., are analyzed and extracted. Thirdly, after the optimized model is obtained, to fairly compare the performance of proposed AFM-PMVM-HCS is are simulated and analyzed, e.g., no-load back-EMF, cogging torque, magnetic field, output torque, etc. Eventually, the prototype was manufactured and tested, and the experimental results agree well with the finite-element results, which can also validate the previous analysis. Overall, this article aims to provide a novel modular axial-flux PMVM with superior torque density and fault tolerance for advanced electric aircraft propulsion system.

Silicon carbide (SiC) mosfets with Kelvin-source connection are widely used in power converters based on a bridge configuration circuit, but crosstalk significantly impacts reliability and limits their application potential. For this problem, this article presents an active gate driver (AGD) based on hardware closed-loop control. A simple hardware closed-loop controller is designed to regulate the gate-source voltage of SiC mosfets. For aspect one, the closed-loop structure can reduce the peak voltage of crosstalk online. For aspect two, since the closed-loop structure can ensure the convergence of the gate-source voltage, a higher drive voltage is then permitted to shorten switching time and power loss. Compared with conventional methods, the proposed AGD can suppress crosstalk without adding switching loss. Experimental results verify the superiority of the proposed AGD for the SiC mosfet-based bridge configuration circuit.

In this paper, the thermal characteristic and loss distribution of permanent magnet vernier machines (PMVMs) and permanent magnet synchronous machines (PMSMs) are investigated for in-wheel direct drive. Firstly, the topologies and heat dissipation design of the outer-rotor in-wheel motors are illustrated. Then, the losses of two selected motors under typical conditions are calculated. Besides, the loss distribution feature is targeted too, where the PM loss is stressed in PMVM and core loss is focused in benchmark PMSM. Secondly, the assembly structure of in-wheel motors is described, where the heat path is through the stator support to motor frame directly without air, so the thermal resistance can be largely reduced and the heat can be transferred easily. In order to verify the analysis, a model is established to calculate the thermal conductivity, and the air-gap thermal conductivity is focused. Besides, the finite-element model is also calculated to identify the temperature difference between PMSM and PMVM. Finally, two prototypes of PMVM and benchmark PMSM are manufactured and tested. Then, the experimental results can match well with finite-element analysis (FEA) and theoretical analysis.

This paper presents a novel split-phase axial-flux permanent magnet motor featuring a printed circuit board (PCB) stator. The motor adopts a multi-rotor configuration with an intermediate stator structure. The stator windings are axially segmented, with each phase integrated on a dedicated PCB layer to achieve electrical, magnetic, and thermal isolation, thereby enhancing fault tolerance. A design methodology for the motor is proposed to analyze and optimize its performance. Finite element simulations and experimental validations confirm the effectiveness of the proposed design.

In this article, the thermal analysis of two permanent magnet machines, permanent magnet vernier machine (PMVM) and synchronous machine (PMSM), is carried out for in-wheel drive application. An improved heat dissipation design is proposed and utilized for the in-wheel motors. Firstly, the in-wheel motors are designed and compared, afterwords, two prototypes of PMVM and PMSM are manufactured. Secondly, the losses of two motors are investigated, e.g., copper loss, core loss, and PM loss. The loss distribution feature of two motors has figured out. Thirdly, the single hub-bearing design for two in-wheel motors is illustrated. Besides, the heat transfer coefficient is further calculated, then the lumped model of single-bearing design is clarified, where the good heat dissipation effect of single hub bearing can be demonstrated theoretically. To further validate the feasibility of analysis above, the CFD models of two in-wheel motors are built. The temperature rises under various typical operation conditions for in-wheel drive are simulated and compared. It is noted that the PM temperature rise in PMVM is much severer than that in PMSM, which threaten the operation safety. Finally, the efficiency maps, temperature rise of windings and rotor parts are measured, respectively, which can agree well with the simulated results. This article aims to provide a comprehensive thermal analysis of PM machines with improved heat dissipation design for in-wheel traction application.

This paper developed a high-torque density axial-flux permanent magnet machine (AFM) with modular U-core and H-core stators (UCS, HCS) for advanced electric aircraft propulsion system. To start with, the novel machine topology is introduced and the working principle is illustrated. Then, the basic theoretical models of the proposed AFMs are established, including the armature magnomotive force (MMF), air-gap flux density, no-load back-EMF, electromagnetic torque, etc. Next, the design and requirements of AFMs are conducted, inducing the basic dimensions and performances, where the comparative studies of these models are carried out. Finally, the prototype is manufactured and tested, where the basic performances were also tested and analyzed, and the experimental results match well with finite-element results.

In electrical vehicles (EVs), permanent magnet synchronous motor (PMSM) should have wide constant power speed range (CPSR), wide high efficiency region and high fault tolerant capability. However, traditional PMSMs do not both have these two characteristics. The axial split phase (ASP) PMSM with high inductance and good fault tolerant performance has been proposed and received attention in electrical vehicles (EVs). However, its performances under the whole speed range are still unclear, which is very critical for PMSM in EVs. In this article, the performance of ASP-PMSM under the whole speed range are comprehensively studied. At first, the critical factors that influence the field weakening performance are investigated. Then, the losses of ASP-PMSM at different operating conditions are analyzed and compared with that of fractional slot concentrated winding (FSCW) motor and integral slot distributed winding (ISDW) motor by theoretical analysis and finite element analysis (FEA). It is revealed that the ASP-PMSM both have high inductance and low loss at deep field-weakening region. At last, the loss and efficiency under the whole speed range are investigated carefully by FEA. It is verified that the investigated ASP-PMSM has the characteristics of wide CPSR and high efficiency region.

To mitigate the safety risks and increase the torque capability as well for electric aircraft propulsion system, a novel axial-flux permanent magnet vernier machine (PMVM) with H-core stator modules is proposed and analyzed in this article. The stator of this machine is totally composed of H-core modules with separate toroidal windings, which can isolate all stator modules physically and mostly prevent the propagation of faults. Besides, due to the flux modulation effect, the output torque also increases and both the output performance and safety can be guaranteed. Firstly, the machine topology and working principle are analyzed and provided, and the feasible slot/pole combinations, armature windings, gear ratio are derived and analyzed. Secondly, to fairly compare the performances of this modular axial-flux machine, a benchmark YSAS axial-flux machine is selected and designed as the comparison candidate. The comprehensive performance, such as the no-load back-EMF, inductance, output torque, losses, and efficiency et al., is simulated by finite-element analysis (FEA).

This paper proposes a torque-coupled axial-flux permanent magnet (AFPM) motor with a wide speed operating range. The motor system incorporates an innovative and adaptive mechanical flux weakening method comprising a single-stator dual-rotor type AFPM and a magnetic coupler with a rigid stopper. The two components are concentrically integrated to maximize radial space utilization, enabling the torque generated by both rotors to collaboratively act on the load side through the magnetic coupler. As the load torque varies, the torque of both rotors is also regulated. This enables the automatic adjustment of the angular displacement between the two rotors through the magnetic coupler, thereby changing the combined electromotive force (EMF) to achieve a wide speed range flux weakening operation. In addition, the implementation for calculating the flux weakening degree during constant power operation is presented. Finally, the effectiveness and feasibility of the design are validated through three-dimensional finite element simulations.