For inverter-driven permanent magnet synchronous machines (PMSMs), the transient overvoltage phenomenon at the machine line-end has attracted widespread attention because of its potential to degrade the winding insulation, thereby causing premature failure. For this issue, various line-end ground-wall (GW) insulation monitoring methods have been proposed. However, recent research indicates that, in addition to the line-end, significant overvoltage also occurs inside the windings. In some cases, the internal overvoltage even exceeds the line-end overvoltage. Therefore, it is necessary to develop an online internal GW insulation monitoring method. This article first establishes a high-frequency winding model, which can be used to study the effects of GW insulation aging at different positions. This study demonstrates that when the internal GW insulation degradation occurs, the common-mode (CM) impedance antiresonance frequency changes. It also shows that the natural frequency (NF) of the CM circuit is equal to the CM impedance antiresonance frequency. Therefore, this article proposes an insulation monitoring method based on the system's NF. This method can quantitatively detect the internal GW insulation aging. To accurately extract the NF of the system, a transfer function-based frequency extraction method is proposed. The performance of the proposed method is not affected by switching frequency and modulation index, demonstrating its strong robustness. Both distributed and concentrated winding PMSMs are employed for experimental validation.

In this paper, a 6-slot/2-pole ultra-high-speed permanent magnet (UHSPM) motor with non-overlapping 2 coil-pitch windings is designed for industrial and domestic applications. The proposed UHSPM motor is optimized for maximum torque density considering stator thermal limitation. Its electromagnetic performance and mechanical strength are evaluated by the finite element method, together with the investigation on the influence of winding connection types and rotor eccentricities. As will be shown, the non-overlapping 2 coil-pitch windings offer advantages of short end-winding axial length, high torque density, and high rotor mechanical strength. The series winding connection is superior to the parallel connection since the unbalanced inductances and resistances of two parallel-connected coils in one phase result in circulating currents and unbalanced magnetic force. Several predicted results are validated by experiments based on a prototype motor.

Multiple synchronous reference frame (MSRF) control is widely used for current harmonic suppression in surface-mounted permanent magnet synchronous motors (SPMSMs) drives. However, conventional MSRF-PI schemes suffer from unclear tuning mechanisms for proportional-integral (PI) and low-pass filters, as well as severe transient oscillations caused by static zero harmonic references. This paper proposes a dynamically decoupled MSRF strategy with a highly simplified structure. By analyzing the core harmonic suppression mechanism in the fundamental dq-frame, the complex PI and filter in MSRF are entirely replaced with pure integral controllers . This innovation drastically reduces the tuning effort from multiple coupled variables (Kp, Ki, and filter cutoff frequencies) to a single integral gain. Furthermore, to eliminate transient coupling, a predicted fundamental current—derived from an ideal reference model—is injected as a dynamic reference for the harmonic loops. In addition, the impact of digital delays is explicitly evaluated to derive a theoretical selection criterion for this single integral gain, balancing harmonic suppression convergence speed and transient response. Experimental results confirm that the proposed scheme effectively eliminates transient surges and simplifies implementation while maintaining superior steady-state harmonic rejection for industrial applications.

The flux-weakening (FW) method is employed to extend the operation speed range and maximize the torque/power capability. Compared to the feedforward FW control method, the feedback FW control method is simple and robust against parameter mismatches. In this paper, based on the field-oriented control strategy and small signal analysis, the stabilities of two forms of feedback FW control methods are investigated: d-axis current-based voltage feedback FW control (DCVFC), utilizing the dq-axis coordinate system, and current angle-based voltage feedback FW control (CAVFC), employing the polar coordinate system. Firstly, the stability characteristics of two FW control methods are analyzed in interior permanent magnet synchronous machines (IPMSMs) and surface-mounted permanent magnet synchronous machines (SPMSMs), respectively. Subsequently, comparative analyses are performed between IPMSMs and SPMSMs under the two FW control methods. Experiments are conducted on both IPMSM and SPMSM to validate the stability performance. The results indicate that there are inherent differences not only between the DCVFC and CAVFC methods when applied to the same machine but also between IPMSM and SPMSM machines when the same FW control method is utilized.

In this paper, permanent magnet synchronous generators, electrically excited synchronous generators, and hybrid excited synchronous generators are compared to evaluate the difference in performance and cost efficiency for 3 kW medium-speed-low-torque and 3 MW low-speed-high-torque wind turbines. These generators with outer rotor or inner rotor are optimized for maximum torque, and the optimal pole numbers are identified. The results of comparison show that the outer rotor generators have higher torque than the inner rotor ones while the difference between them decreases as the power level and radial dimension of generators increase. In the 3 kW case, the torque density and torque per material cost of the optimized electrically excited synchronous generator are 11.6% and 28.2% those of the optimized surface-mounted permanent magnet synchronous generators, and are increased to 51.0% and 211.3% in the 3 MW case, respectively, because electrically excited synchronous generators have better torque capability at heavy load scenarios. For the same reason, the torque advantage of hybrid excited synchronous generator over electrically excited synchronous generator is decreased from 149.1% in 3 kW case to 9.6% in 3 MW case. When considering lifecycle cost, however, electrically excited synchronous generators and hybrid excited synchronous generators have higher operational expenditures due to the maintenance of brush + slipring. Therefore, although the electrically excited synchronous generator is more than twice cost efficient (material cost) compared to the surface-mounted permanent magnet synchronous generator for low-speed-high-torque wind turbines, the surface-mounted permanent magnet synchronous generator is still an ideal choice for both wind turbines due to the highest torque and possibly lower lifecycle cost than the other two generators.

This paper investigates the effect of various cooling strategies, including forced air, water jacket, and shaft cooling, as well as their combinations, on the electromagnetic performance of rotor PM and stator PM machines. These cooling strategies are analyzed by computational fluid dynamics, and the corresponding convection heat transfer coefficients are evaluated, which are further used in the 3D thermal finite element models to determine the maximum reachable output torque and power capabilities of the stator and rotor PM machines, considering thermal limits of PM and winding insulation. Amongst different cooling strategies, it is shown that the hybrid water jacket and shaft cooling can significantly extend the operational region of surface-mounted PM machines, while only water jacket cooling is required in switched-flux PM machines to produce a competitive torque. Both machines are fabricated for experimental validation under different cooling strategies and load conditions, where a good agreement between experimental and numerical results is achieved.

Critical thermal limits of winding insulation and permanent magnet (PM) impose decisive constraints on the maximum output power and power density of high-speed PM machines (HSPMMs). This paper evaluates the electromagnetic performance of 3-slot/2-pole (3s/2p) and 6-slot/2-pole (6s/2p) HSPMMs with different winding configurations, all satisfying the requirement of 300 W at 180 kr/min. Considering the interactive effect of electromagnetic and thermal characteristics, several crucial factors are considered, i.e., different coil-pitch windings (CPWs), stack and machine lengths, sleeve materials, and cooling conditions. It is demonstrated that when all 2-pole machines have the same active length but different machine lengths, the 3s/2p HSPMM exhibits superior output power and power density, followed by the 6s/2p HSPMM with 2-CPW. In contrast, when all 2-pole machines have the same machine length but different active lengths, the 3s/2p HSPMM shows its superiority in both power and power density. The finite element analysis is used for analysis, which is verified experimentally based on four prototype machines.

This article presents a novel hybrid poles asymmetric interior permanent magnet (HPAIPM) motor topology for electric vehicle applications featuring the combined use of rare-earth and ferrite permanent magnets (PMs). It fully uses the magnetic-field-shifting effect in improving torque density by adding a ferrite at the upper part of the V-shaped asymmetric rare earth PM pole and optimizes the harmonic distribution of the air gap magnetic field to achieve a significant reduction in torque ripple. In response to the optimization challenges of complex rotor structure motors, this work applies surrogate-assisted methods to achieve rapid optimization. First, a constraint space Latin hypercube design (LHD) algorithm is proposed to solve the problem of selecting sampling points in a confined space composed of strong constraints on design variables. This not only avoids the generation of invalid points but also optimizes the spatial distribution characteristics of sampling points, reducing the prediction error of the surrogate model by 29.1%. Second, a surrogate modeling method based on transfer learning neural network is given, which freezes some hidden layer coefficients of the neural network to avoid repeated training of surrogate models corresponding to different ferrite materials, reducing the number of training samples by 60%. According to the optimal design scheme, the proposed HPAIPM topology can reduce torque ripple by over 80% and maintain high torque density compared to conventional interior PM and asymmetric interior PM topology. Finally, a small prototype of the proposed HPAIPM topology is designed, manufactured and tested for validation.

In Halbach machines, the potential demagnetization of permanent magnets (PMs) should be considered to avoid irreversible demagnetization (ID). In this paper, the Halbach machines with air- and iron-cored rotors are optimized for maximum average torque and minimum torque ripple, and their electromagnetic performances with consideration of PM demagnetization are compared. It is found that due to low and high permeabilities of air- and iron-cored rotors, the air-cored machine suffers from the larger ID area while the iron-cored machine suffers from more severe ID level in the ID area. When the influence of ID on torque reduction is considered, it is found that due to the field interaction between PMs in air-cored machines, ID influence is more severe at low current and high PM temperature. However, due to significantly strong armature field in iron-cored machines, ID influence is more severe at high current and high PM temperature, which even results in lower torque than that of the air-cored machine. Besides, a notched iron-cored rotor is proposed to improve the ID withstand capability in the iron-cored machine. Prototypes with air- and iron-cored rotors are fabricated and tested to verify the finite element analysis.

Although the field-oriented control (FOC) with flux-weakening (FW) function has been investigated in dual-three-phase (DTP) PMSM drives, the research on model predictive control (MPC) with FW is few. In this paper, the MPC with FW and using hybrid voltage vector (VVs) is proposed. To avoid overmodulation when using FOCs with FW, a margin needs to be kept in the voltage constraint of FW or constraints needed to be applied on the z1z2 voltage reference. In contrast, in the proposed hybrid MPC both the margin and the constraints are freed with sequential optimization, resulting in larger torque-speed operation range and lower current THD. In addition, hybrid VVs are used in the proposed MPC. Two long VVs are used for αβ regulation per control cycle, which increases the torque-speed operation range, while two z1z2 virtual VVs are used for z1z2 regulation, which ensures high z1z2 harmonic suppression capability. The VVs are selected based on voltage reference estimation for light computation burden, while their duty ratios are calculated by the projected quadratic programming (QP) algorithm for the same reason. With the proposed hybrid MPC, wider torque-speed operation range and lower current distortion can be obtained, compared with the traditional MPC using virtual VVs and FOC with FW. Finally, experiments are conducted to verify the feasibility and advantages of the proposed method.

In this paper, electrically excited synchronous machines (EESMs) using copper (Cu) and aluminum (AI) windings are compared for the feasibility of replacing Cu windings with Al windings in electric vehicle (EV) applications since Al windings have lower mass density and cost per weight, but higher resistivity and lower thermal conductivity than Cu windings. The EESMs with four winding configurations are optimized with an electromagnetic-thermal co-optimization method. The optimized EESM with only Cu windings is considered as the baseline in this study. Results show that the EESM with stator-Cu/rotor-Al windings has the least torque reduction (12.1%) compared to the baseline among the three EESMs with Al windings and the highest torque mass density among all EESMs. Meanwhile, although the new European driving cycle efficiency of the stator-Cu/rotor-Al EESM is 1.8% lower than that of the baseline, the torque per cost is 71% higher, and the maximum rotor mechanical stress is 8% lower. Therefore, the EESMs with stator-Cu/rotor-Al windings are prospective substitutions of those with only Cu windings for EV applications considering the trade-off between performance and cost.

This article proposes two novel hybrid magnet rotor topologies, spoke-V and spoke-delta, for interior permanent magnet synchronous machines (IPMSMs) for reducing rare-Earth magnet usage while maintaining high electromagnetic performance in electric vehicles (EVs). Using the Tesla model 3 IPMSM as a reference, the models are parametrized and multiobjective optimization is performed. The proposed spoke-V and spoke-delta configurations achieve NdFeB volume reductions of 19.4% and 25.8%, respectively, while matching the torque performance of the reference machine. This results in a 20.5% and 35.1% improvement in torque per unit volume of rare-Earth material. Detailed electromagnetic analyses are conducted under open-circuit, peak-load, and torque-speed characteristic analysis under full operating conditions, demonstrating the feasibility of the proposed topologies for high-speed EV applications. A small-scale prototype is manufactured and tested to verify the accuracy of the calculations. These findings highlight the potential of asymmetric hybrid magnet rotor designs to deliver less rare-Earth and high-performance IPMSMs, contributing to the development of more sustainable and cost-effective electric drive solutions.

Although resistance-estimation-based method is widely used for winding temperature monitoring in electrical machines by using copper temperature dependence, its primary limitation is the inability to predict maximum temperatures. To address this issue, this paper proposes a method that utilizes the average coil temperature as the input variable to predict the maximum winding temperature caused by the copper loss for both overlapping and non-overlapping winding layouts, based on the conduction heat transfer principle. Meanwhile, a simple approach is developed to distinguish the thermal states of active and end windings for overlapping windings by installing a thermal sensor on the end-winding front surface. This enables the separation of average active- and end-winding temperatures, allowing for rapid maximum temperature estimation using simple equations related to winding geometric and thermal parameters. The proposed method does not require knowledge of machine losses and thermal parameters outside the winding region, which is validated numerically and experimentally based on a prototype electrically excited synchronous machine.

This paper aims at torque enhancement of electrically excited synchronous machines (EESMs) with focus on cross-coupling braking torque mitigation. Firstly, cross-coupling and torque components of a baseline EESM are analyzed, which is designed referring to a commercial interior permanent magnet (IPM) machine in electrical vehicles (EVs). By analyzing the flux paths of the baseline EESM under d-axis, q-axis, and field-excitation currents, a cross-coupling braking torque mitigation method is proposed for torque enhancement by using general shaping on the pole shoes of EESMs. Then, the EESMs with symmetrical and asymmetric pole shoe general shaping are optimized and compared to baseline and IPM. The results show that both A and B exhibit around 4% higher torque, higher efficiency, and higher drive cycle efficiency than baseline, while A has 6.5% lower torque ripple, 2% higher output power, and 14.8% lower mechanical stress on rotor pole shoes than B. The performance difference between EESM and IPM for EV application is narrowed by the proposed method. The scaled EESMs are designed and prototyped to validate the mitigated cross-coupling braking torque and enhanced torque due to the proposed method.

Due to system faults, manufacturing imperfections, assembly tolerances, and operational conditions, the filter capacitors of current-source inverters (CSIs) for permanent magnet synchronous motor drives may be asymmetric in real systems, which results in significant torque ripples. To deal with this issue, this article first studies the influence of asymmetric capacitive filters on torque ripples and then proposes three compensation control schemes. Based on the phasor method, the motor currents under asymmetric capacitive filters are first obtained. Then, the torque ripples can be reduced by eliminating the negative-sequence currents (NSCs), which can be achieved by PI controller (NSC-PI), the calculating compensation matrices (NSC-CM-Calculation), and the measuring compensation matrices (NSC-CM-Measured). The control scheme based on NSC-CM-Measured has the best compensation performance among these three methods. However, it requires the measurement of compensation coefficients and d- and q-Axis currents caused by back electromotive forces (back EMFs). It shows that the control scheme based on NSC-CM-Calculation has good compensation performance and fast dynamic performance and can be extended to other asymmetric conditions easily, such as open-circuit filter capacitor conditions. Finally, the experimental results have verified the effectiveness of the proposed compensation schemes.

This article proposes a novel model-based Luenberger state observer for interturn short-circuit (ITSC) fault diagnostics. The residuals between the observed currents and the measured currents in the alpha- and beta-axes serve as fault indicator, which can be used to detect ITSC faults not only at an early stage with contact resistance but also at the fully short-circuited stage. These currents are observed by the Luenberger observer, which is designed under the assumption that the machine is operating in a healthy condition. In addition, the investigation results indicate that with a greater fault ratio, larger load current, and higher speed, detecting the ITSC fault becomes easier. Moreover, three sets of Luenberger observers, assuming the ITSC fault is in phases A, B, and C, have been designed to identify the faulted phase. A series of experiments have been carried out to validate the developed fault detection method.

This article proposes a novel mixed hybrid permanent magnet (HPM) asymmetric V-shape interior permanent magnet synchronous machine (IPMSM), which is compared with a rare-earth permanent magnet (REPM)-based symmetrical V-shape baseline. These machines are optimized for the same torque with a minimum volume of high-cost REPM at the same specification and size as a commercialized IPMSM, with due account for the mechanical von Mises stress at high speed. The results show that the proposed mixed HPM asymmetric V-shape IPMSM requires similar to 40% less REPM than that of the baseline. Meanwhile, the electromagnetic performance comparison shows how the synergies of magnetic field shifting effect and HPM utilization of the proposed mixed HPM asymmetric V-shape IPMSM improve the torque per REPM usage. As a result, assuming that the REPM is ten times more expensive than the ferrite magnet, the proposed HPM machine benefits from similar to 21% less cost of permanent magnets (PMs) than the baseline. In addition, the demagnetization analyses of REPM at high temperature and ferrite magnet at low temperature prove a high withstand capability of both PM types in the proposed HPM machine. A small size prototype is made and tested to verify the finite element analysis.

This paper investigates the mechanism of asymmetric permanent magnet (PM) eddy current loss and temperature distributions of surface-mounted PM synchronous machines (SPMSMs) with different slot/pole number combinations for the first time. A finite element analysis (FEA) based harmonic restoration method is utilized to evaluate and quantify contributions of individual armature reaction spatial harmonics to PM eddy current loss and temperature distributions for 12-slot SPMSMs with different pole numbers. It shows that there is one dominant armature reaction spatial harmonic contributing to the majority of PM loss for each SPMSM with different slot/pole number combinations. The average PM loss distribution is symmetrical to the center line in circumferential direction with armature reaction or PM magnetic field only, while the interaction between the armature reaction and PM field causes the asymmetric PM loss distribution. Besides, the maximum loss density in one PM pole tends to move from the circumferential center to the edge when the rotor pole number increases. Consequently, the resultant PM local hotspot temperature has been calculated through 3-D thermal finite element method, which shows the same trend as loss density distribution. Finally, two prototypes are manufactured, and the corresponding electromagnetic and thermal tests are carried out to verify the validity of the FEA simulation.

In this paper, a novel pulse-width-modulation (PWM) strategy to operate in overmodulation range with low switching-to-fundamental frequency ratio is developed for permanent magnet synchronous machine drives based on a two-level voltage source inverter (VSI). The proposed strategy is able to operate at low switching-to-fundamental frequency ratio and presents an inherent overmodulation capability. The strategy applies a fixed switching sequence in a fundamental period, similar to six-step modulation, but including zero-vectors to regulate the voltage magnitude, and then, conventional control techniques can be employed. The duty cycles of the output voltage pulses and the switching angles are computed as linear functions of the modulation index. Two switching angles are used to reduce the number of commutations and avoid the problems associated to low switching-to-fundamental frequency ratios. A harmonic analysis of the generated voltage is also presented. The performance of the proposed strategy is tested and validated with simulations and experiments on a laboratory prototype.