Direct-drive Applications Research Articles

Investigation of Permanent-Magnet Vernier Machine With Fractional-Slot Nonoverlapping Windings for Direct-Drive Application

Investigation of Permanent-Magnet Vernier Machine With Fractional-Slot Nonoverlapping Windings for Direct-Drive Application

Analysis of a Complementary Dual-Stator Vernier Machine With Reduced Non-Working Harmonics for Low-Speed Direct-Drive Applications

Analysis of a Complementary Dual-Stator Vernier Machine With Reduced Non-Working Harmonics for Low-Speed Direct-Drive Applications

Investigation on Brushless DC Appropriateness to Direct-Drive Generator Wind Turbine.

The paper studies the posibility to increase the power capability of direct-drive application. The system consists in a trapezoidal waveform PM brushless generator connected to the grid via a back-to-back concerter. Numerical model of the the system from the shaft of the generator until the DC bus circuit is setup. The studied parameters of the system are the electromagnetic torque ripple and the quality of the energy supplied to the grid. DC bus voltage adaption is proposed to avoid torque oscilltions and polyphased systems are proposed to reduce the low frequency harmonics of the power transferred to the grid.

Design and Analysis of a New Dual-Stator Hybrid Magnet Flux Modulation Machine

This paper proposes a new dual-stator hybrid-magnet flux modulation machine (DS-FMHMM) for direct-drive applications, which employs NdFeB magnet excitation and Ferrite magnet excitation on the rotor and outer stator sides, respectively. With this design, the proposed DS-FMHMM can not only fully use the bidirectional flux modulation effect, but also effectively alleviate the magnetic saturation issue. The machine configuration is described, together with the operating principle. Then, the design parameters of DS-FMHMM are globally optimized for obtaining high torque quality, and the influence of magnet dimensions on torque is analyzed. To evaluate the merits of the proposed DS-FMHMM, the electromagnetic performances of machines under different magnet excitation sources are analyzed, and a comprehensive electromagnetic performance comparison of DS-FMHMM and two existing dual-stator flux modulation machines (DSFMMs) is developed.

Comparative Analysis of Consequent-Pole Flux-Switching Machines with Different Permanent Magnet Arrangements for Outer-Rotor In-Wheel Direct-Drive Applications

Recently, the interest in consequent-pole flux-switching permanent magnet (CP-FSPM) machines has been increasing because of the flux-focusing PM arrangements and the removal of the flux-barrier effect. A simple and rigid outer-rotor salient pole rotor structure can be adopted in CP-FSPM machines, making them applicable for in-wheel direct-drive applications. In this study, three CP-FSPM machines with II-shaped (II-PM), V-shaped (V-PM), and straight U-shaped PM (SU-PM) arrays are analyzed and compared. Moreover, a CP-FSPM machine with inclined U-shaped PM (IU-PM) arrays is proposed to improve the flux-focusing effect and stator slot utilization. The working principles of CP-FSPM machines are analyzed by adopting a semi-analytical model. Combining the finite element analysis (FEA) results of air gap flux density and the analytical model of phase back electromotive force (EMF), the contributions of multiple working harmonics to the back EMF are quantitatively analyzed. Additionally, 6/16 and 6/17 CP-FSPM machines with different PM arrangements are globally optimized. Both the no-load and on-load performance of the optimized machines are included in the performance comparison. The results illustrate that the 6/16 and 6/17 machines exhibit their respective merits, and the IU-PM machine shows the best torque production ability in these CP-FSPM machines with the same design criteria.

Comprehensive Armature Reaction Modeling and Flux Weakening Optimization of a Surface Permanent Magnet Vernier Motor

Surface permanent magnet vernier motors (SPMVMs) possess the potential for direct-drive applications due to the feature of high torque density. This paper presents a deep investigation into the armature reaction modeling and a semi-analytical optimization framework considering the constant power speed range (CPSR) in the flux weakening region for the SPMVM. The new armature reaction model can identify the armature air gap flux density of each harmonic component and armature flux linkage accurately, by conducting harmonics analysis, slot leakage flux computation and slotting effect evaluation, permitting to calculate the inductance. Then, the results from the armature reaction model and those obtained from finite element analysis (FEA) at the rated operating point together are applied to a comprehensive semi-analytical motor optimization framework developed for SPMVMs. The optimization framework is able to reflect the motor performance metrics over the entire speed range, with torque density, power factor and CPSR as objectives. Finally, a prototype designed according to the trade-offs of the optimization objectives is fabricated to confirm the validity of the armature reaction model and optimization framework.

Demagnetization Modeling and Analysis for a Six-Phase Surface-Mounted Field-Modulated Permanent-Magnet Machine Based on Equivalent Magnetic Network

Demagnetization Modeling and Analysis for a Six-Phase Surface-Mounted Field-Modulated Permanent-Magnet Machine Based on Equivalent Magnetic Network

A Novel Energy Balance Control Method for a Modular Multilevel Converter in a High-Speed PMSM Drive Application

Nowadays, high-speed permanent magnet synchronous machines (HSPMSMs) are widely used in high-speed direct-drive applications. Moreover, modular multilevel converters (MMCs) have attracted a lot of attention. In this paper, the mathematical model of the MMC for the HSPMSM was built on high- and low-speed regions, respectively, and the control of each model is completely decoupled. Moreover, energy balance control schemes based on the common mode current injection of the MMC with HSPMSM drive applications have been proposed for high- and low-speed cases, respectively. Based on this model, the controller is proposed for high- and low-speed cases, respectively. Simulation and experiment results indicate that the proposed energy balance control method for an MMC in the high-speed PMSM drive application exhibits significant performance, which can ensure the energy balance of the MMC and eliminate the circulating current of the MMC directly.

Topology Development and Performance Analysis of a Dual-Stator Permanent Magnet Arc Motor

In this paper, a dual-stator permanent magnet (PM) arc motor (DS-PMAM) with improved torque density is investigated for some specific direct-drive applications. The proposed DS-PMAM can be considered as the combination of the PM arc motor and the stator-PM motor. Benefiting from two different stator structure, two sets of armature windings are employed to improve the inner space utilization ratio and torque densities. The motor structure and working principle of the DS-PMAM are firstly introduced. The simplified analytical model is derived to provide a solid theoretical guidance for optimization design parameters selection. Then, the design considerations including the slot/rotor pole combination, the armature winding design, and the key structural parameters analysis are carried out to obtain the optimal motor performances of the DS-PMAM. After that, the electromagnetic characteristics of the DS-PMAM are evaluated together with the comparison with the inner-rotor PMAM, the outer-rotor PMAM, the surface-mounted PM PMAM, and the DS flux reversal PMAM. Finally, a prototype is manufactured and fully tested. Relevant experimental results verify the correctness of the FE analysis and the feasibility of this proposed design.

Design Trade-Off Between Torque Density and Power Factor in Surface-Mounted PM Vernier Machines Through Closed-Form Per-Unit Equations

Benefited from flux modulation effects, surface-mounted permanent magnet vernier machines (SPMVMs) present the high-torque low-speed properties, which can become the potential candidates for direct-drive applications. However, their power factor is inferior to conventional PM machines due to the large inductance caused by parasitic no-working harmonics. This paper is devoted to propose some design considerations of SPMVMs to make a good compromise between torque density and power factor. Via constructing closed-from per-unit equations, the cross-coupling influences of slot/pole combinations and normalized geometric variables on machine performances are revealed. It is found that there is a tradeoff between torque density and power factor. The high magnet thickness to air-gap length ratio is beneficial to decrease inductance, while the flux modulation effect under the large equivalent air-gap length would be impaired. Moreover, the pole-pair ratio, i.e., the PM pole-pair to armature winding pole-pair, should be carefully selected, where the SPMVMs with high pole-pair ratio have the relatively high magnetic loading owing to the strong flux modulation effects, whereas they suffer from the low power factor due to serious harmonic leakage inductance. In particular, taking the end-winding length into consideration, the high torque advantages of high pole-pair ratio SPMVMs under the same copper losses can be well performed with high length-radius ratio. Combined with the analysis, the preferable selections of critical parameters to satisfy various technical indexes are determined. Then, the general design approach is proposed, and two design examples are built and compared. Meanwhile, finite element analysis (FEA) and prototype experiments are carried out to verify the feasibility.

Thrust Ripple Reduction of Dual-Side Toroidal-Winding Linear Permanent Magnet Vernier Machine by Unequal End-Teeth Width

The linear permanent magnet vernier machines have attracted more and more attention last several years due to its high thrust density at low speed, which is very suitable for the direct-drive applications. A dual-side toroidal-winding linear permanent magnet vernier machine (DSTWLPMVM) with the unequal end-teeth width is proposed in this paper to reduce the detent force and thrust ripple. The low detent force in DSTWLPMVM is achieved by combination of two single-side detent forces with the opposite phase angle, which is obtained by optimizing the unequal end-teeth width of the primary two-sides. In the proposed machine, the symmetry three-phase flux linkage of the toroidal winding also can be obtained by optimizing the unequal end-teeth width. Therefore, the low thrust ripple of DSTWLPMVM is achieved by optimizing the unequal end-teeth width for obtaining the low detent force and symmetry three-phase flux linkage. The vertical magnetic force in the proposed LPMVM is also considered by finite element analysis (FEA) in this paper. Via optimizing the unequal end-teeth width, the detent force is reduced by 90% while the peak-to-peak thrust ripple is reduced by 69% than that of DSTWLPMVM with the equal end-teeth width, respectively. In this paper, the thermal and PM demagnetization behavior of the proposed machine are also analyzed by FEA.

Design and Analysis of New Dual-Stator Flux Modulated Machines With Dual-PM Excitation

This article proposes new dual-stator flux modulated machines with dual-permanent magnet (PM) excitation (DS-FMDPMMs) for direct-drive applications. The proposed DS-FMDPMMs are characterized by employing consequent-pole (CP) PMs on both the stator and rotor sides. By using dual-layer CP PMs, the DS-FMDPMMs can perform the bidirectional flux modulation effect to enhance the torque capability. In addition, the merits of dual-stator structure, such as high space utilization, high torque density, and power factor are inherited in the proposed machines. The design principle of DS-FMDPMM is provided at first, and the formed three representative DS-FMDPMMs with different stator-PM arrangements are proposed. Then, the design considerations of three DS-FMDPMMs are described in detail, which includes the armature winding design, the selection of stator-slot/rotor-pole combination, and the influence of critical structural parameters on torque. In order to find an optimal direct-drive candidate among the three DS-FMDPMMs, a quantitative electromagnetic performance comparison is developed through finite element analysis (FEA). The results show that among the three machines, DS-FMDPMM-I accommodates the highest torque density, lowest torque ripple, best over-load torque capability, and highest efficiency. Moreover, to further verify its performance advantages, the electromagnetic performances of DS-FMDPMM-I and two typical dual-stator flux modulated PM machines are compared through FEA. Finally, a prototype of the optimal DS-FMDPMM is manufactured and tested to validate the effectiveness of FEA-predicted results.

Design of a Dual-Permanent-Magnet Vernier Machine to Replace Conventional Surface-Mounted Permanent Magnet Motor for Direct-Drive Industrial Turbine Application

Surface-mounted permanent magnet motors (SMPMMs) have been widely used in industrial fields. However, when it comes to direct-drive low-speed large-torque fields, SMPMMs have several problems, such as lower slot filling factor, complicated fabrication, low efficiency, and bulky size. Recently, dual-permanent-magnet Vernier motors (DPMVMs) have been investigated due to their high torque density for low-speed direct-drive applications. However, so far, there is little research that demonstrates the application of the DPMVM for real industrial fields and quantitatively compares the performances of the DPMVM and the SMPMM. This paper studies the application of a DPMVM to the direct-drive industrial turbine as a replacement for conventional SMPMM. Rectangular and trapezoidal PMs are first applied to DPMVM in this paper. The performances of the DPMVM are quantitively compared with two typical SMPMMs. One is the design that has the same stator slots as the DPMVM, while the other has the same rotor pole pairs as the DPMVM. Economic costs are considered to form a thorough comparison between DPMVM and SMPMMs. It is revealed that with rectangular and trapezoidal PMs adopted in the DPMVM, the cost of PMs is significantly reduced with little sacrifice of performance. The efficiency of DPMVM is around 12% higher, and the material cost is reduced by 6.87% compared with the SMPMM.

Optimal Pole Ratio of Spoke-type Permanent Magnet Vernier Machines for Direct-drive Applications

Due to magnetic gearing effects, spoke-type permanent magnet vernier machines (ST-PMVMs) have the merit of high torque density, where an extra torque amplification coefficient, i.e., pole ratio (the pole-pair ratio of PMs to armature windings) is introduced. However, different from surface-mounted PMVM, the variation of torque against pole ratio in ST-PMVMs is non-linear, which is increased at first and then decreased. This article is devoted to identify the optimal pole ratio of ST-PMVMs by equivalent magnetic circuit model. It is found that except the P <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> air-gap magnetomotive force (MMF) harmonic having the same pole-pair of PM, the Pa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> air-gap MMF harmonic having the same pole-pair of armature winding is also induced due to the modulation of doubly salient air-gap structure. The P <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> MMF harmonic produces positive torque, while Pa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> MMF harmonic produces negative torque. With the increase of pole ratio, the proportion of Pa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> MMF harmonic as well as negative torque is increased, which reduces the advantages of high pole ratio coefficient. Further, the influence of dimension parameters on the performance of ST-PMVMs under different pole ratio are investigated. Results show that ST-PMVMs with pole ratio 2.6 have high torque density, low cogging torque and high power factor simultaneously. Finally, a prototype is manufactured to validate the analysis.

Deep-Investigated Analytical Modeling of a Surface Permanent Magnet Vernier Motor

Permanent magnet Vernier motors (PMVMs) possess the advantage of high torque density for high-performance applications. However, the low power factor challenge makes it unacceptable for direct-drive applications. A lack of accurate modeling method based on the motor sizing law, i.e., air-gap flux density, linear current density, and motor geometry parameters, raises difficulties for machine designers to further conduct research on the performance metrics. This article presents a deep investigation into the analytical modeling technique for surface PMVMs (SPMVMs). It can identify an accurate approach to obtain the performance metrics, including electromagnetic torque and power factor. The modeling technique is developed based on the conformal mapping method. By using this, both radial and tangential permeability functions can be calculated to obtain the motor magnetic loading accurately, considering the leakage flux. The slotting effect on both air-gap flux density and armature-winding function is analyzed to achieve a precise formula for torque and power factor computations. The new modeling technique is applied to integral-slot SPMVMs with different slot/pole combinations, gear ratios, slot openings, and magnet thickness to evaluate the impacts of motor parameters on high-power-factor and high-torque-density designs. Finally, an SPMVM with the characteristics of high torque density and power factor is fabricated to verify the analytical model at the power rating of 0.8 kW and the speed of 500 r/min. The experimental results show that the prototype exhibits a high power factor of 0.9 and high torque density 22.5 Nm/L.

New Dual-PM Spoke-Type Flux-Reversal Machines for Direct-Drive Applications

In this article, a new dual-permanent magnet spoke-type flux-reversal machine (dual-PM SFRM) for direct-drive applications is proposed. Tangentially magnetized permanent magnets (PMs) sandwiched with iron poles are set in stator slots. To increase the torque density within the geometric restrictions, PMs are introduced in the rotor as well and magnetized radially to form a consequent-pole rotor. Global optimizations are conducted and comparisons are made on torque, torque ripple, and torque/PM volume with the existing single-PM SFRM. It is found that the proposed dual-PM SFRM can produce 22.6% larger torque with an optimal PM volume and a 17.5% larger torque with the same PM volume compared with the single-PM counterpart. Moreover, major magnetic field space harmonics are derived analytically and verified by finite-element analysis (FEA) and the torque components produced by the working harmonics are calculated and compared in terms of different stator/rotor pole number combinations. Comparisons are made on back electromotive force, cogging torque, torque performance, and efficiency between dual-PM SFRM and conventional flux-reversal machine. The influence of different stator configurations, i.e., the slot opening, the iron bridge, and the unequal tooth, on torque performance in the proposed dual-PM SFRM has been studied and highlighted. A prototype has been built and tested to validate the FEA calculations.

A New Hybrid Magnet Dual Stator Field Modulation Machine With Different Split Ratios of Stators

A new type of hybrid magnet dual stator field modulation machine with different split ratios of stators (DSR-HMDSFMM) is proposed in this article, which has the merit of high torque density. The split teeth of stator sever as modulators to modulate air gap flux density based on the magnetic gearing effect for high torque production. The total split tooth number of inner stator is equal to that of outer stator because the pole pair numbers of armature reaction and permanent magnet (PM) excitation on rotor are equal. Furtherly, the inner and outer stators with different split ratios and same split tooth number are employed to increase torque production because armature windings with high winding factor and high turn number can be used. In addition, the employment of hybrid magnet excitation can increase space utilization and enhance the air gap flux density with low harmonic distortion. Therefore, the proposed DSR-HMDSFMM can achieve high torque production with the same volume and PM usage. In order to find the optimal split ratio match of stators, several DSR-HMDSFMMs with different combinations of split ratios are proposed and calculated. The configuration and operating principle are illustrated, together with electromagnetic performances of the proposed machines analyzed using finite element analysis (FEA). The FEA-predicted results show that the proposed HMDSFMM exhibits high air-gap flux density with low harmonic distortion and high torque density, which is promising for the direct-drive applications.

A Mesh Design Technique for Double Stator Linear PM Vernier Machine Based on Equivalent Magnetic Network Modeling

Linear permanent magnet Vernier machines (LPMVMs) is an attractive solution for stroke and direct-drive applications. Several LPMVM topologies have been presented for different applications that operate based on magnetic flux modulation. LPMVMs have some advantages including high force, and torque density at low speeds. This paper introduces a double-excited linear Vernier machine for wave energy conversion. To model the machine, finite element analysis and nonlinear equivalent network model are used. To improve the accuracy of the model and compact the network, a new meshing method is proposed. For better continuity in the output waveforms of the machine, special positioning is utilized. The end-effect is modeled using two separate networks. Finally, the modeling and simulation results are compared with each other and validated experimentally.

Design and Optimization of a High-Torque-Density Low-Torque-Ripple Vernier Machine Using Ferrite Magnets for Direct-Drive Applications

In recent years, Vernier permanent magnet machines (VPMs) have gained much interest in academia. However, due to the large leakage flux, the power factor of Vernier machines is too low to be applied in real industry. In this article, a novel split-tooth, concentrated-winding Vernier machine using ferrite magnets is proposed for high-torque, low-speed direct-drive applications in home appliance industry. It uses consequent-pole magnets in the rotor, as well as additional assisted magnets in the stator teeth, in order to reduce the leakage flux and boost the torque density. A systematic way of reducing the torque ripple from 40% to 4% is proposed by carefully choosing the right combination of stator/rotor pole-pair numbers. A comprehensive design optimization is conducted to achieve the optimal performance. Finally, the ac loading is carefully reduced to meet the torque, saturation, power factor and space requirements. The test results of a prototyped machine show that the proposed design can achieve 34% more torque and 20% less loss with a reasonable power factor of 0.63, compared with an optimized PM synchronous machine under the same volume. It is found that the significant torque boost is attributed to the multiple working harmonics of the air-gap flux density.

AC Losses in Form-Wound Coils of Surface Mounted Permanent Magnet Vernier Machines

Surface mounted permanent magnet Vernier (SPM-V) machines, because of their high torque density and multi-pole structure, are promising candidates for low-speed direct-drive applications. To achieve high torque density, the SPM-V machines are generally designed with high gear ratios. Therefore, their operating frequencies can be much higher than those of the conventional SPM machines. This potentially increases the alternating current (AC) winding losses, especially those with form-wound coils proposed for high-power applications. This article investigates the AC losses (including skin effect, proximity effect, rotor PM induced, and circulating current losses) in form-wound stator coils of a 3 MW direct-drive SPM-V machine with different slot/pole number combinations. The study reveals that the SPM-V machines have significantly higher AC winding losses than their conventional SPM counterparts for similar operating conditions. To reduce the AC winding losses in SPM-V machines, a novel flux shunt concept is proposed along with other conventional techniques such as increasing the number of turns/coil (with terminal voltage being kept constant) and parallel strands/turn, providing extra clearance at slot opening. With the loss reduction techniques implemented, the SPM-V machines can achieve comparable efficiency but much higher torque density than their conventional counterparts. Moreover, the proposed flux shunt was also found to be very effective in reducing the potential risk of permanent magnet (PM) irreversible demagnetization, a key issue for SPM-V machines at high power ratings.