Fractional-slot Concentrated Windings Research Articles

Analysis of MW-Level Offshore Wind Turbine Generators with Dual Star–Delta Fractional-Slot Windings

This paper investigates the use of fractional-slot concentrated windings (FSCWs) in large-scale (MW level) offshore wind generators. It focuses specifically on a power rating of 3 MW and uses an existing direct-drive synchronous PM machine (DD-SPM) with 480s/160p and dual three-phase integer-slot winding (ISW) as a baseline. A multiple of the common 12s/10p FSCW machine is used that matches the electrical frequency of the ISW machine, yielding a 192s/160p dual three-phase machine. The hybrid star–delta connection has grown increasingly popular owing to its unique harmonic cancellation properties, which can help reduce rotor and PM eddy current losses in FSCW machines. In this paper, two dual three-phase star–delta-wound machines are scaled to 3 MW and included in the investigation. Specifically, a 384s/160p dual three-phase and dual star–delta winding machine, which is a multiplication of the 24s/10p machine, and a 192s/176p dual three-phase and dual star–delta winding machine, which is a multiplication of the 24s/22p machine, are used. These machines are investigated using finite element analysis (FEA) and compared on the basis of their air-gap flux density harmonics, open-circuit electro-motive force (EMF), torque performance, and losses and power. It is found that the proposed 384s/160p dual star–delta winding machine has the best electromagnetic performance of all, with a stator power that is 1.2% greater than that of the baseline ISW machine. However, this machine has a coil pitch of 2 and so loses the manufacturing and fault-tolerant advantage of having concentrated windings. If concentrated windings are desired, then the proposed 192s/176p dual star–delta winding machine is the best choice, with the stator power only 2.6% less than that of the baseline ISW machine, but unfortunately still has significant rotor and PM eddy current losses.

Investigation of Torque and Reduction of Torque Ripples through Assisted-Poles in Low-Speed, High-Torque Density Spoke-Type PMSMs

In this article, rotor designs utilizing assisted-poles are investigated for a high-torque density spoke-type permanent magnet synchronous machine (PMSM) with fractional slot concentrated winding (FSCW) to explore the rich air-gap magnetic field harmonics and torque generation mechanism. Due to their higher average torque output, spoke-type PMSMs with FSCW are increasingly used in high-torque density applications. However, slot harmonics generate torque ripples that are difficult to eliminate in FSCW spoke-type PMSMs. Removing slot harmonics from the stator or winding results in a large drop in torque since their winding factors are identical to those of the main harmonic. Therefore, rotor designs having assisted-poles (symmetrical and asymmetrical) are investigated in this work to mitigate slot harmonics and minimize torque ripples. Firstly, the air-gap flux density is analyzed for the machines having assisted-poles, and a model of interaction between the stator and rotor-MMF harmonics is created and validated through Finite element analysis (FEA) to analyze the torque production mechanism. In addition, an analytical relationship between the assisted-poles’ dimensions and the generated torque harmonics is proposed. Furthermore, a generalized torque ripple reduction concept for the FSCW spoke-type PMSM having asymmetrically designed assisted-poles is presented. The proposed design and optimization method are validated through analytical calculations and FEA simulations, and a brief comparative analysis is presented for the analyzed machine prototypes. It has been established that the machine designed by applying the proposed asymmetrical assisted-poles can achieve a reduction in torque ripples while also significantly lowering cogging torque in comparison to the conventional spoke-type PMSMs and other spoke-type PMSMs with rotor having symmetrical assisted-poles.

Analysis and Comparison of Even-Order Back EMF Harmonics of Fractional-Slot Concentrated-Winding Consequent Pole Permanent Magnet Vernier Machines

Fractional-slot concentrated-winding (FSCW) consequent pole (CP) permanent magnet vernier (PMV) machines have received increasing attention due to their less magnet consumption but larger torque capability. However, the PM magnetomotive force (MMF) distribution is asymmetric in the CP structure. Thus, even-order air-gap flux density and back-electromotive force (back-EMF) harmonics are generated, which increases torque ripples. In this paper, to eliminate the even-order back-EMF harmonics and the resulting torque ripples, a comprehensive investigation on even-order flux density and back-EMF harmonics of five-phase FSCW CP PMV machines was presented, and then the general design rules without even-order back-EMF harmonics were summarized. Firstly, considering single- and double-layer windings, the effects of slot/pole number combination and flux modulator distribution on even-order flux density and back-EMF harmonics were analyzed. Secondly, the optimal slot/pole number combinations were selected, and then the rules for eliminating even-order back-EMF harmonics of CP PMV machines were summarized. As an extension, it was revealed that the harmonic elimination mechanism of CP PMV machines is applicable to CP-Halbach PMV machines. According to the summarized general design rules, the five-phase CP and CP-Halbach PMV machines with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> =20/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P_r</i> =21/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z_f</i> =10 were proposed. Their detailed performance has been comparatively analyzed and then the optimal one was selected for prototyping. Both finite element analysis (FEA) and experiments were carried out to validate theoretical analyses.

Design Optimisation Approach of an Outer Rotor Multiphase PM Actuator for Multirotor Aerial Vehicle Applications

The electric urban air mobility sector has gained significant attraction in public debates, particularly with the proliferation of announcements demonstrating new aerial vehicles and the infrastructure that goes with them. In this context, the development of new methodologies for the design and sizing of actuation systems, ensuring high performances of these aerial vehicles, remains an important task in this process. This will allow for better integration within this transport sector. In this paper, a robust design optimisation approach of multiphase fault-tolerant (FT) outer rotor (OR) permanent magnets (PM) for multirotor aerial vehicle applications is proposed. In order to show the effectiveness and the robustness of the proposed design methodology, the number of stator winding phases, with a fractional slot concentrated winding (FSCW) configuration, as well as the PM configuration are considered as variables. Thus, four cases for the number of phases are considered, namely 3, 5, 6 and 7 phases, where for each number of phases case, the PM takes 3 configurations, namely surface PM, interior V-shape PM and interior spoke PM. First, a pre-sizing step is carried out, consisting of selecting the optimal combinations slot/pole, designing the multiphase FSCW layout, and estimating the electric motor (EM) geometry using analytical computations to obtain a preliminary validation of the design specifications. Second, constrained multiobjective optimisation is considered in order to optimise the EM performances, such as motor efficiency and weight, under constraints where the FEMM/Matlab based Finite Element Analysis (FEA) tool is used to perform this optimisation. Finally, results analysis and performance comparisons of different EM configurations are carried out in order to assess the design parameters, such as phases number, PM position, and harmonic currents in the EM design and consequently to select the best configuration for the considered application.

A Natural Position Observer With Vertical Detection Coil for FSCW Machines

This paper presents a sensorless control strategy using the novel detection coil for permanent magnet synchronous motor (PMSM) with fractional slot concentrated winding (FSCW). In order to eliminate the armature component in the coil voltage, the detection coil distribution is determined based on the analysis of synchronous inductance and leakage inductance for FSCW machines. Hence, the rotor position and angular velocity are estimated from the terminal voltage of detection coil, without any knowledge of motor parameters. Both finite element analysis (FEA) results and experiments are carried out to prove the effectiveness of the proposed method at steady and transient states.

D-Q Axis Inductance Analytical Calculation for Fractional-Slot Distributed Winding IPM Motor Based on Big-Small Pole Space Method

The d-q axis inductance is the crucial parameter for the interior permanent magnet (IPM) motor. In order to facilitate the design, analysis, and optimization of IPM motors, it is of great significance to develop an accurate and simplified inductance analytical calculation method. Among the existing calculation methods, the magnetic equivalent circuit (MEC) has advantages in the inductance calculation of the IPM motor due to the simple modeling, the fine processing of the saturation region and the visualization of the magnetic field distribution. However, the traditional MEC method is only applicable to the IPM motor with integer-slot distributed winding (ISDW) or fractional-slot concentrated winding (FSCW), while lacking of in-depth research on fractional-slot distributed winding (FSDW). The magnetic circuit topology of FSDW IPM motors is not as single as that of FSCW/ISDW IPM motors. It leads to problems in the traditional MEC method when dividing the d/q axis flux path in FSDW motors. To solve this problem and expand the applications of MEC method, a big-small pole space (BSPS) method is proposed after fully considering the common characteristics of MMF distribution and reluctance distribution in FSDW motors, which fills the gap of MEC method applied to FSDW motors. The BSPS method divides the d-axis flux path according to the asymmetry of the d-axis magnetomotive force (MMF). Based on the divided flux paths, an improved nonlinear MEC model that considers the saturation of stator and rotor is established to calculate d- and q-axis inductances. The results of finite element analysis (FEA) verify the validity of the proposed MEC model with different geometry parameters. Experimental results of an 84-pole/360-slot FSDW I-type IPM wind generator further confirm the validity of the proposed MEC model

Exact THD Calculations Applied to Minimize Space Harmonic Content of Winding and Rotor Magnetic Fields

As a metric for measuring the combined space harmonic content of electric machine winding magnetomotive force (MMF), the winding total harmonic distortion (THD) is defined with an infinite series. Without truncation, the exact THD can be evaluated via a “family grouping” facilitated by a “family sum” over the fewest possible winding factors. Uniformly applicable to distributed windings and fractional-slot concentrated windings (FSCWs), this procedure also explains the inherent difference in harmonic content between both winding categories. Furthermore, the minimal THD (mTHD) that can be achieved by optimizing a winding is revealed with a closed-form expression. While intended for windings, the mTHD concept also benefits the design of low harmonic content rotor magnetic fields. Examples of both winding and rotor mTHD designs are provided.

Saturation-Induced Variable-Flux Characteristics in a 12-Slot 10-Pole Concentrated Winding Permanent-Magnet Motor

This paper proposes a stator core design to achieve variable-flux characteristics enhanced by magnetic saturation in fractional-slot concentrated winding (FSCW) permanent-magnet (PM) motors without additional excitation systems, complicated structures, or active controls. Magnetic saturation induced at suitable locations can enhance variable-flux characteristics because the PM flux linkage is variable with the magnetic saturation in cores. In distributed winding configurations, magnetic saturation is induced symmetrically for each rotor pole by the fundamental that is dominant in the spatial distribution of the air-gap flux density. The magnetic saturation at each rotor pole can enhance variable-flux characteristics. In FSCW configurations, magnetic saturation is not induced symmetrically for each rotor pole because of several dominant space harmonics. The magnetic saturation at several rotor poles cannot enhance variable-flux characteristics. In the proposed design, the stator core has thin tooth tips for inducing magnetic saturation. The magnetic saturation in the stator core achieves the variable-flux characteristics that are effective for expanding the operating region or decreasing electromotive force and increasing torque. The efficacy of the proposed design is determined by estimating motor performance through a finite element method analysis and demonstrating the actual motor performance of a prototype.

Concentric-Coil Distributed Winding Design for Large Direct-Drive Fully Superconducting Machines

Fully superconducting generators (FSCGs) use superconducting wires for both the field and armature windings to reach a high torque density for wind turbine applications. The AC armature winding of an FSCG design mostly employed racetrack tooth-coils to form fractional-slot concentrated windings. This winding type has high leakage flux and excessive harmonics, especially when used in superconducting machines. Other attempts like applying full-pitch racetrack coils can only be with one slot per pole per phase which also has significant slotting harmonics. This paper proposes an innovative concept of double-layer concentric coils to enable distributed windings with a larger integral number of slots per pole per phase. The concept successfully breaks the limitation that racetrack coils cannot be built with such integral slots. This paper then assesses the performance of the proposed winding concept within a context 20 MW FSCG design. The assessment finds out that the proposed concept has balanced three phases but reduces inductance compared to single-layer windings. The force on each concentric coil can significantly be different, thereby complicating the mechanical design of these superconducting coils.

Investigation on Stator Shifting Technique for Permanent Magnet Vernier Machines With Two-Slot Pitch Winding

This paper investigates the stator shifting technique (SST) for permanent magnet vernier (PMV) machines equipped with two-slot pitch winding (TSPW). By employing SST, TSPWs can be generated from the combination of two sets of fractional-slot concentrated windings (FSCWs), with various shift angles. The effects of various shift angles on armature magnetomotive force (MMF) harmonic contents are explored. It shows that proper shift angles can be selected to preserve the working component while eliminating loss-producing harmonics. Besides, the implementation of various shift angles leads to uneven stator teeth structure, which affects field modulation. Therefore, the output torque capability, phase inductance, power factor, core losses, and fault-tolerance are affected by different shift angles. This paper aims to exploit the full potential of TSPW-PMV machines by SST, considering the synthetic effects on harmonic reduction and field modulation. Through the exemplification of 24- slot, 19-rotor pole pair, and 5-armature pole pair PMV machines, it indicates SST is a more general and flexible design method that can provide new design opportunities and improve various performance metrics by introducing shift angle as an additional design variable. Finally, a prototype of the double-layer TSPW-PMV machine with optimal shift angle is fabricated for concept validations.

Electromagnetic design and performance analysis of large-scale offshore wind generators with modular fractional-slot concentrated winding

Future offshore wind engineering raises demands on higher reliability, better power generating performance, as well as lower manufacturing and transportation cost. This paper presents a direct-driven permanent magnet generator (DDPMG) with modular fractional-slot concentrated winding (MFSCW), to meet the large-scale, low-cost and high-reliability requirements of offshore wind power applications. The corresponding pole–slot combination method and electromagnetic characteristics analysis for this novel generator are proposed and performed in detail, including the winding coefficient analysis, magnetomotive force (MMF) harmonic analysis, cogging torque analysis, finite element simulations and generator parameter optimization. After finite element simulations, a 3.3 MW prototype MFSCW-DDPMG is developed to validate the proposed scheme. The results provide a complete and feasible scheme of pole–slot​ combination and electromagnetic design for future MFSCW-DDPMG engineering applications.

Analysis and Reduction of Pole-Frequency Vibration of Surface Mounted Permanent Magnet Synchronous Machines With Fractional Slot Concentrated Winding Considering the Radial and Tangential Forces

The pole-frequency vibration is significant in permanent magnet synchronous machines (PMSMs) with various pole and slot number combinations, and its weakening is difficult. For conventional analysis in literature, only radial force is considered as the excitation source of pole-frequency vibration. In this paper, the causes of the pole-frequency vibration are considered as the collaborative action of radial and tangential forces, and an optimization method of the piecewise stagger unequal poles is proposed for the reduction of the pole-frequency vibration of the surface mounted permanent magnet synchronous machines (SPM) with fractional slot concentrated winding (FSCW). First, taking a 10-poles 12-slots PMSM as an example, the deep cause of the vibration caused by radial and tangential forces are analyzed, respectively. Then, the optimized structure of piecewise stagger unequal poles is shown, and radial force, tangential force, and vibration spectrum between the common and proposed motors are compared with the finite element model (FEM). The results show that the piecewise stagger unequal poles structure can effectively weaken the pole-frequency vibration while ensuring the torque density. Finally, the vibration experiment of the two prototypes is carried out, and the effectiveness of the optimized structure is verified.

Design Optimization of Four-Layer Fraction Slot Concentrated Winding Spoke-Type Interior Permanent Magnetic Machine for Range Extender

&lt;div&gt;In this article, the design optimization of a four-layer fractional slot concentrated winding (FSCW) interior permanent magnet (IPM) machine for range extender is proposed for high energy efficiency and excellent torque/back-electromotive force (EMF) performance. The design starts with the comparison of four-layer FSCW patterns in terms of efficiency distribution based on a predesign spoke-type rotor model. Magnet segments and rotor auxiliary notches (ANs) are applied and optimized to reduce eddy current losses and torque ripples in the permanent magnets (PMs). Then, an efficient two-step optimization of multiple performances for a machine is presented. The rotor parameters are designed by an analytical model with a Pareto optimizer for torque capacity and ripple. An interpolation-based design method for adaptive stator slot parameters and winding configurations is presented to quickly obtain the optimal stator slot winding designs corresponding to the rotor design to achieve optimal efficiency. The multi-bridge design is applied to rotor laminations to suppress flux leakage, making the rotor core easy to manufacture. Finally, an 18s-16p four-layer FSCW prototype was built and tested to verify the design optimization results, with a maximum efficiency of 96% and rated shaft ripple as low as 3%.&lt;/div&gt;

Design and Finite-Element-Based Optimization for a 12-Slot/10-Pole IPM Motor with Integrated Onboard Battery Charger for Electric Vehicle Applications

Permanent magnet (PM) machines with fractional slot concentrated windings (FSCW) constitute a notably remarkable proposition for electric vehicles. Additionally, an integrated onboard battery charger (IOBC) provides another superiority as it exploits the components of the powertrain to charge the battery without any additional components. Interior permanent magnet (IPM) motor arises as a credible choice due to its high torque density, resulting from the high saliency ratio. The optimal design of an IPM motor has been extensively presented from different perspectives, but the optimal design of a motor employed for IOBC application for both propulsion and charging modes has not been studied extensively. In this paper, the design and optimization of a 12-slot/10-pole IPM motor with IOBC are studied under both propulsion and charging modes. A finite-element-based optimization with the aid of a genetic algorithm technique is proposed to obtain the optimal machine by maximizing the average torque and minimizing the torque ripple, core losses, and magnet size.

Design to Reduce Modulated Vibration in Fractional-Slot Concentrated-Windings PM Machines Considering Slot–Pole Combination

The superior torque performance and unsatisfactory vibration are the inherent contradiction issue in fractional-slot concentrated-windings (FSCW) permanent magnet (PM) machines. This article aims to reduce the modulated vibration in FSCW PM machines while preserving their great torque performance. First, the contradictory relationship between the torque performance and the vibration of FSCW PM machines is revealed from the perspective of the slot–pole combination. Then, the radial force modulation of the FSCW PM machine is investigated from the aspects of spatial order and magnitude modulation. Subsequently, a new vibration reduction design methodology is presented, which combines the optimal slot opening coefficient and modified slot–pole combination. Finally, the prototype of a 12-slot/14-pole FSCW PM machine is manufactured, and several simulated and experimental verifications are carried out to validate the theoretical analysis.

Investigation of Six-Phase Surface Permanent Magnet Machine With Typical Slot/Pole Combinations for Integrated Onboard Chargers Through Methodical Design Optimization

This article presents an analytical magnetic equivalent circuit (MEC) modeling approach for a six-phase surface-mounted permanent magnet (SPM) machine equipped with fractional slot concentrated winding (FSCW) for integrated onboard chargers. For the sake of comparison, the selected asymmetrical six-phase slot/pole combinations with the same design specifications and constraints are first designed based on the parametric MEC model and then optimized using a multiobjective genetic algorithm (MOGA). The commercial BMW i3 design specifications are adopted in this article. The main focus of this study is to achieve optimal design of the SPM machine considering both the propulsion and charging performances. Thus, a comparative study of the optimization cost functions, including the peak-to-peak torque ripple and core losses under both motoring and charging modes and electromagnetic forces (EMFs) under charging, is conducted. In addition, the demagnetization capability in the charging mode and the overall cost of the employed machines are optimized. Since the average propulsion torque is crucial in electric vehicle (EV) applications, it is maintained through the design optimization process. Furthermore, finite element (FE) simulations have been carried out to verify the results obtained from the analytical MEC model. Eventually, the effectiveness of the proposed design optimization process is corroborated by experimental tests on a 2-kW prototype system.

A Novel Dual-Redundancy Permanent Magnet Synchronous Motor With Small Teeth

A novel dual-redundancy permanent magnet synchronous motor (DRPMSM) with small teeth is proposed, which combines the advantages of fault-tolerant permanent magnet synchronous motor (PMSM) and multi-phase PMSM. When fractional slot concentrated winding (FSCW) is used in PMSM, no electromagnetic coupling can be realized by selecting a reasonable slot-pole (SP) combination and using single-layer (SL) winding. However, the goal of no electromagnetic coupling can also be achieved by adding small teeth in the common slot center between two adjacent phase windings under appropriate SP combination and double layer (DL) winding. Compared with the former method, the latter has smaller eddy current loss and a lower saturation level of the magnetic field caused by lower-order harmonics. In addition, the influence of small teeth on electromagnetic properties is analyzed. The correctness of the theoretical derivation is verified by simulation and experiment.

Design of a High Torque Density Robot Joint and Analysis of Force Control Method Applied for a Light Exoskeleton

In many robotic applications, the joint is required to have a small volume, low weight and high torque output. In this paper, based on the finite element analysis (FEA), a 36-slot 40-pole outer rotor surface-mounted permanent magnet (OR-SPM) motor with concentrated winding is designed for the exoskeleton robot. The fractional slot concentrated winding (FSCW) is employed to reduce end winding height, leading better portability. Since the motor is relatively flat, the 3D end effect is critical to the electromagnetic performances. Special attention is paid to 3D end effect during the multi-objective optimization of the OR–SPM motor. In order to increase the ending torque output, the planetary reducer is located between OR–SPM motor and load, and then system level optimization covering motor and reducer is carried out to achieve best torque output. In addition, the force impendence control method with parameter self-adaptive capability is proposed to improve user experience of the exoskeleton robot, where the key parameters in the algorithm vary according to different actions of the exoskeleton. In addition, the inertia of load is calculated using the parameter identification based on least squares method. Finally, the prototype of the joint is fabricated and tested to validate the above FEA results and control method. The user experience of the exoskeleton robot is also covered.

Multi-objective optimization of high torque density spoke-type PM machine for special vehicle drive system

PurposeThis paper proposes a spoke-type fractional-slot concentrated windings (FSCW) permanent magnet (PM) machine for special vehicle driving systems to obtain higher torque and power density, high efficiency and wide field-weakening range. To enhance the efficiencies of multi-objective optimization processes, the respond surface (RS) method and black-hole (BH) algorithm are used.Design/methodology/approachThe spoke-type FSCW PM machine is optimized to meet the requirements of the special vehicle driving system. The combination of the RS and BH algorithm is used to obtain high torque, low torque ripple and high efficiency.FindingsThe optimal spoke-type PM machine is obtained, and it has higher torque density, lower torque ripple, cogging torque and wider magnetic field weakening range. Finally, a 15-kW prototype machine is fabricated and tested to verify the results of the optimization method and finite-element analysis.Originality/valueThis paper designs a high torque density and efficiency spoke-type FSCW PM machine, which is superior for special vehicle driving systems. Meanwhile, the RS model combined with BH algorithm is applied to the field of electrical machine multi-objective optimal design.

Comparison of AC-Superconducting Multiphase Symmetric-Winding Topologies for Wind Power Generators With PM Rotors

In this article, an ac superconducting multiphase symmetric-winding machine is designed for a wind power generator to improve its performance and reduce losses, where four handpicked topological designs were explored and compared. In particular, it is found that using high-phase order of unique phasors further improves the performance. The iron losses are reduced, and the rippling behavior is lowered due to the smoother airgap magnetic flux density. Furthermore, a higher least common multiple is achieved due to a better slot-pole combination for fractional-slot concentrated windings without having space subharmonics. Nonetheless, it is shown that creating a smooth airgap magnetic flux density does not improve the ac hysteretic superconducting losses; thus, further research is needed using additional approaches. Moreover, it is found that the Meisner effect is present in the machine and is inversely proportional to the ac hysteretic superconducting losses. Finally, the work shows that a 13-phase ac-superconducting machine can achieve a theoretical limit approaching <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{101.70 Nm/kg}$</tex-math></inline-formula> for the torque-to-weight ratio, outperforming classic winding layouts.