Direct-drive Applications Research Articles

A Novel Dual-Stator Vernier Permanent Magnet Machine

Vernier permanent magnet (VPM) machines for direct-drive applications are receiving more and more attention due to its small pulsating torque, high torque density at low rotation speed. This paper proposes a novel VPM machine, which has two stators and a sandwiched rotor. The outer stator is a conventional stator with three-phase windings and semi-closed slots, and the inner stator has an iron core and PMs mounted on the surface of it. The rotor has a consequent-pole structure with Halbach-array PMs. In this paper, the operation principles and the feasible slot-pole combinations of the proposed machine are analyzed. Moreover, to verify the superior performances of the proposed VPM machine, it is compared with a regular VPM machine in terms of back electromotive force, average torque, pulsating torque, power factor, efficiency, and overload capabilities. It shows that the proposed machine has higher torque density, larger efficiency, and lower cogging torque.

Power Loss and Thermal Analysis of a MW High-Speed Permanent Magnet Synchronous Machine

High-speed permanent magnet synchronous machines (PMSMs) have attracted much attention due to their high power density, high efficiency, and compact size for direct-drive applications. However, the consequent power loss density is high, and hence heat dissipation is a major technical challenge. This is particularly the case for high-speed operation. In this paper, a MW level high-speed PMSM is designed and its electromagnetic and mechanical power losses comprehensively investigated using finite element analysis. The transient machine demagnetization performance is studied, and a composite rotor structure is proposed to improve machine antidemagnetization capability. The temperature distribution of the proposed high-speed PMSM is also analyzed using a fluid-thermal coupling method with calculated power loss. Experiments conducted on a prototype of the high-speed PMSM demonstrate the effectiveness of the numerical models developed and validate the results obtained.

Effects of Armature Winding Segmentation With Multiple Converters on the Short Circuit Torque of 10-MW Superconducting Wind Turbine Generators

Superconducting synchronous generators (SCSGs) are drawing more attention in large direct-drive wind turbine applications. Despite low weight and compactness, the short circuit torque of an SCSG may be too high for wind turbine constructions due to a large magnetic air gap of an SCSG. This paper aims at assessing the effects of armature winding segmentation on reducing the short circuit torque of 10-MW SCSGs. A concept of armature winding segmentation with multiple power electronic converters is presented. Four SCSG designs using different topologies are examined. Results show that armature winding segmentation effectively reduce the short circuit torque in all the four SCSG designs when one segment is shorted at the terminal.

Design and Analysis of a Stator HTS Field-Modulated Machine for Direct-Drive Applications

This paper proposes a new stator high-temperature superconducting (HTS) field-modulated machine with the merit of high torque density for direct-drive applications, which incorporates the attractive features of both stator excitation HTS machine and field-modulated machine. The key is to artfully place the HTS windings on the stator to implement static seals as well as utilizing the “magnetic gearing effect” to achieve high torque capability. Based on the geometry of the fabricated double-layer Dewar assembly for cooling the HTS excitation coil, the proposed machine is initially designed. By using the finite element analysis (FEA), the machine performances are analyzed, and its validity is verified

Design Optimization and Comparative Study of Novel Magnetic-Geared Permanent Magnet Machines

This paper presents a novel magnetic-geared permanent magnet (MGPM) machine with dual-layer PM excitations, referred as dual-layer MGPM (DL-MGPM) machine. The key is to employ both the outer rotor and the modulation ring with PM-iron sequences, in which the PMs are separated by iron segments one by one, and can provide PM excitation and flux modulation simultaneously. Both the outer rotor and the modulation ring can achieve flux modulating, namely, bidirectional flux modulating, which can couple the magnetic fields excited by the two sets of PMs and armature currents effectively. The proposed DL-MGPM machine, as well as its existing counterpart which employing all the PMs on the outer rotor and referred as single-layer MGPM (SL-MGPM) machine, are optimal designed using finite element method coupled with genetic algorithm. The electromagnetic performances of the DL-MGPM machine and the SL-MGPM machine are investigated in detail and quantitatively compared. The results show that the proposed DL-MGPM machine can achieve higher torque density than the SL-MGPM machine, and is more suitable for low-speed high-torque direct-drive applications.

Current-independent torque control of permanent-magnet synchronous motors

A current-independent torque equation for the permanent-magnet synchronous motor (PMSM) aiming at direct-drive servo applications is derived from a first principles model. Instead of measuring currents, all required control parameters are derived from optical incremental encoder measurements. The results are verified on a real system in test series showing the effect of static friction and proving the obtained torque model.

A novel axial flux stator and rotor dual permanent magnet machine

A novel structure of axial flux stator and rotor dual PMs Vernier machine is presented as an effective option for high torque-density direct-driven applications. The key is to locate PMs on both sides of rotor and stator. With dual PMs, this proposed machine with a novel structure generates an improved torque per armature current at low speed. It is very suitable for direct drive applications with limited space such as the in-wheel electric vehicle (EV) motors. The theory of the structure with PMs on both sides is analyzed theoretically. The structure and operation principle of machine are introduced and the steady and steady performance of machine is analyzed and verified with time-stepping finite element method (TS-FEM).

A novel stator and rotor dual PM flux modulated machine

A novel structure of Veriner permanent magnet (VPM) machine is presented as an effective option for high torque density direct-driven applications. The key is to locate PMs on both sides of rotor and stator, and design the stator PMs as a cylinder shape, the rotation of which can be controlled by an external driven system. With this design, magnetic field in the airgap can be directly regulated by the rotation of stator PMs. The structure and operation principle are introduced and the performance is analyzed with time-stepping finite element method (FEM). Analysis results show that the novel structure has really high torque density and good field weakening performance.

A Novel Triple-Rotor Axial-Flux Vernier Permanent Magnet Machine

Vernier permanent magnet (VPM) machines are obtaining more and more attentions due to their high torque density and low torque ripple feature. Owing to the “magnetic gearing effect,” VPM machines are capable of achieving high torque at low speed, which makes them suitable for direct-drive applications. However, they suffer from low power factor. In this paper, a triple-rotor axial-flux spoke-array vernier permanent magnet (TR-AFSAVPM) machine is proposed to enhance VPM machine power factor to a reasonable level and maintain their high torque density and adequate constant speed range. Both quasi-3-D finite element analysis (FEA) and 3-D-FEA are employed to investigate the proposed machine performances. Analysis results show that the TR-AFSAVPM machine has high power factor, viz. 0.94 and high torque density, viz. 22.87 kNm/m 3 .

Modeling and Analysis of a Transverse-Flux Flux-Reversal Motor

Transverse flux motors are of great interest in the low-speed high-torque direct-drive applications. In this paper, a novel transverse-flux flux-reversal motor structure is presented, which is more suitable than conventional transverse-flux motors when cost and torque density are both considered. The basic structure and operating principle are introduced. The three-dimensional (3-D) equivalent magnetic circuit model is established to obtain the expression of electromagnetic torque. A 3-D finite element model is built to analyze the influence of structure parameters on the torque density. A prototype is designed and manufactured based on the analysis result of finite element method (FEM), the experiments of no-load back-electromotive force and load torque are carried out. The experimental results agree with the results of FEM, which verifies the accuracy of FEM analysis.

Application of Phase Shifting Technique for Reducing the Magnetomotive Force Space Harmonics of Tooth Concentrated Windings

Permanent magnet machines with fractional slot concentrated windings are very attractive for low-speed direct-drive applications due to their compactness, cost effectiveness, and high efficiency. However, one of the key challenges of using concentrated winding configurations is the high content of space harmonics in the air-gap magnetomotive force distribution, which can lead to undesirable effects such as high rotor losses, vibrations, and noise. In this article, the phase shifting technique is investigated to deal with the harmonic content of such windings. An example of application is given for a three-phase winding with a number of slots per pole per phase = 2/5. It is shown that by using this simple technique, one can cancel some of the harmful magnetomotive force space harmonics without decreasing the winding factor of the torque producing harmonic. Further, calculations revealed a significant improvement in machine performances when compared to the conventional design.

Design Criteria, Modeling, and Verification of Tubular Transverse Flux Machines for Force-to-Current Ratio Improvement in Direct Drive Applications

Tubular transverse flux machines (TFMs) generally exhibit high force densities and are interested in various direct-drive applications. Due to essentially 3-D distribution, the design and modeling of the TFM is quite difficult and time-consuming. This paper presents the design criteria of a tubular TFM to maximize the force-to-current ratio developed for direct drive applications. Analytical equations are derived to establish the relationship between the force-to-current ratio and the leading design parameters of the machine. In addition, the validity of the derivation is verified by extensive 3-D finite-element computations and experiments on a prototype machine. It is shown that for a given set of volumetric constraints, the force-to-current ratio is more sensitive to the stator segment length than other leading parameters. The results have provided a basis for design optimization, and will aid the design process of the tubular TFM, especially when targeting demanding applications where high force density is crucial.

A Novel Coaxial Magnetic Gear and Its Integration With Permanent-Magnet Brushless Motor

A magnetic geared machine (MGM) is believed to be a promising candidate for high-torque direct-drive application. One of the key issues for developing MGMs is how to resolve the contradiction between the good performance and the complex structure. This paper aims at proposing a novel coaxial magnetic gear (CMG), which will not increase the mechanical complexity after integration with a permanent magnet (PM) brushless machine. The prominent feature of the proposed CMG is the introduction of the stator with modulating teeth, which function as the same as the modulating pole-pieces in the conventional CMG. The integrated MGM can then be achieved by inserting the armature windings into the stator slots. The configuration, harmonic analysis, and torque capability of the proposed CMG are compared with the conventional CMG. The operating principle and electromagnetic performance of the proposed MGM are investigated by dividing it into one vernier PM machine, one PM brushless machine and one proposed CMG. The results show that the developed integrated MGM exhibits good torque capability and power factor.

Design, analysis, and implementation of extra low air-gap single-phase axial-flux induction motors for low-cost applications

Summary In this paper, design and prototyping of two-speed capacitor-run single-phase axial-flux induction motor (AFIM) for direct-drive applications is presented. The proposed structure has single stator-single rotor configuration with high magnetic pull between the stator and rotor, which may compel rotor to lock. Although the most efficient method to overcome this problem is using a second rotor or stator in the mirror with the first one, it is not applicable in low-cost applications. So, in this paper, two new and fast construction techniques are proposed to fabricate an AFIM with extra low air-gap length and minimum cost. Beside design and optimization process, a thermal equivalent circuit considering radial and lateral heat transfers is proposed. Then different losses and thermal resistances are exactly calculated to estimate the temperature rise of different motor parts and the maximum specific magnetic and electric loadings. Then, motor performance is evaluated using three-dimensional time-stepping finite-element analysis. Finally, a comprehensive fabrication algorithm for AFIMs is presented, and a prototype of the studied motor is fabricated and tested. The experimental results validate the design, optimization, thermal model, and finite element simulations. Copyright © 2016 John Wiley & Sons, Ltd.

Design and Analysis of Modular Flux-Concentrating HTS Permanent-Magnet Vernier Machine

In this paper, a novel modular flux-concentrating high-temperature superconductor (HTS) permanent-magnet (PM) vernier machine (FCHTS-PMVM) is proposed and designed for the direct-drive application, which can simultaneously offer low-speed large-torque capability and simplify the manufacture process by using the segmented stator modular design. HTS bulks and spoke-array PMs are emplaced in the spaces between adjacent flux-modulation poles and employed in rotor, respectively, which greatly reduce the flux leakage and improve the torque density. The electromagnetic characteristics of the machine, such as winding factor, magnetic saturation, no-load air-gap flux density, flux linkage, back-EMF, inductance, losses, on-load torque, and cogging torque, are comprehensively investigated by using finite-element analysis. It is demonstrated that the electromagnetic performance of the proposed modular FCHTS-PMVM with a slot/pole combination of 24/19/5 can be significantly improved through proper flux-gap width design.

Comparison and analysis of dual stator permanent magnet vernier machines with different pole/slot combinations for low speed direct drive applications

Permanent magnet vernier machine (PMVM) based on flux modulation principle has attracted increasing attention in low-speed direct-drive applications due to its high torque capability. A dual-stator PMVM topology with sandwiched spokearray PM rotor is presented to produce higher air-gap flux density and larger torque in this paper, and the space utilization and the energy conversion efficiency are improved significantly. Two representative dual-stator PMVMs (DS-PMVMs) are designed with this topology, the pole/slot combinations and gear ratios of which are 18 slots/30 poles and 18 slots/28 poles, 5 and 3.5, respectively. In order to reveal the advantages of the proposed topology and assess the torque characteristics of the proposed machines with different pole/slot combinations, the electromagnetic characteristics of these two machines are quantitatively compared and analyzed by using finite element method (FEM). The results demonstrate that the 18 slots/28 poles DS-PMVM can offer higher output torque, lower torque ripples, lower cogging torque and higher power factor owing to its optimal pole/slot combination. In addition, the unbalanced magnetic pulls (UMPs) of the two machines are also investigated, and the UMP of the 18 slots/30 poles DS-PMVM is smaller than that of its counterpart due to its symmetrical winding.

Design of external rotor switched flux memory machine with hybrid magnets

Purpose– The purpose of this paper is to propose the design concepts of external rotor switched flux hybrid magnet memory machine (SFHMMM) to further increase the torque capability while keeping the merits of internal rotor SFHMMM, such as adjustable back-EMF, and good flux weakening performance, etc.Design/methodology/approach– The torque enhancing principle of external rotor SFHMMM, and the design considerations such as feasible stator and rotor pole numbers (Ns/Nr) are discussed by equations. Then, the performances such as back-EMF, dq-axis inductances, torque and flux weakening performances are calculated and compared with the aid of finite element analysis software.Findings– The external rotor SFHMMMs have obviously larger torque capabilities compared with the internal rotor ones under the same copper loss and machine size. The main reason is that the external SFHMMs could fully utilize the inner space of stator, which offers higher slot area, larger split ratio and consequently the higher average torque. For the external rotor machines with larger rotor pole numberNr, the back-EMF adjust ratio as well as the maxim torque are better. However, leakage flux and losses also increase with Nr due to limited machine size and higher operation frequency. Considering torque capability and flux weakening performance (efficiency map), the external SFHMMM withNr=2Ns+1, e.g. 6/13Ns/Nrstator/rotor pole machine, is the optimal choice.Originality/value– This paper introduces the design concept and design considerations of external SFHMMM for the first time. The proposed machine could be a competent candidate for direct-drive electric vehicle applications.

Analytical sub-domain model for predicting open-circuit field of permanent magnet vernier machine accounting for tooth tips

Purpose – Due to high electromagnetic torque at low speed, vernier machines are suitable for direct-drive applications such as electric vehicles and wind power generators. The purpose of this paper is to present an exact sub-domain model for analytically predicting the open-circuit magnetic field of permanent magnet vernier machine (PMVM) including tooth tips. The entire field domain is divided into five regions, viz. magnets, air gap, slot openings, slots, and flux-modulation pole slots (FMPs). The model accounts for the influence of interaction between PMs, FMPs and slots, and radial/parallel magnetization. Design/methodology/approach – Magnetic field distributions for slot and air-gap, flux linkage, back-EMF and cogging torque waveforms are obtained from the analytical method and validated by finite element analysis (FEA). Findings – It is found that the developed sub-domain model including tooth tips is very accurate and is applicable to PMVM having any combination of slots/FMPs/PMs. Originality/value – The main contributions include: accurate sub-domain model for PMVM is proposed for open-circuit including tooth-tip which cannot be accounted for in literature; the model accounts the interaction between flux modulation pole (FMP) and slot; developed sub-domain model is accurate and applicable to any slot/FMP/PM combinations; and it has investigated the influence of FMP/slot opening width/height on cogging torque.

Development of a 20-Pole–24-Slot SPMSM With Consequent Pole Rotor for In-Wheel Direct Drive

In-wheel motors bring a fundamental change in electric vehicle technology by removing conventional mechanical components and freeing up space inside the vehicle body. This approach ultimately helps to realize all-wheel independent control for improved vehicle dynamics and increased vehicle design freedom. However, when space allowed for an in-wheel motor is taken into consideration, high torque density, high efficiency, and wide-speed-range capability are typically required. This paper specifically investigates a 20-pole–24-slot surface permanent magnet synchronous motor with consequent pole rotor for in-wheel direct drive since this topology is well suited for high-pole motors and thus high-torque direct-drive applications. Extensive finite-element analysis is carried out to characterize the proposed motor, and the practical feasibility of the proposed motor is discussed. Finally, the validity of the analysis was experimentally verified.

Design Tradeoff Between Cogging Torque and Torque Ripple in Fractional Slot Surface-Mounted Permanent Magnet Machines

In designing the fractional slot surface-mounted permanent magnet (SPM) machines for servo and direct-drive applications, the cogging torque and on-load torque ripple are both major design considerations. Nevertheless, since the torque ripple depends on load conditions, the optimal tooth-tip design for the smallest cogging torque may not be the same for the smallest torque ripple as well. Usually cogging torque will reduce with slot opening width (b 0 ) [1] as shown in Fig.1(a). However, the variations of torque ripple are more complicated. With the help of frozen permeability method [2], the on-load torque can be seen as composed by PM torque (T PM ), reluctance torque (T r ) and on-load cogging torque (T cog ), which all contribute to torque ripple, Fig.1(b). The variation of these components against different b 0 under full-load are shown in Figs.1(c) and (d). It can be seen that the average torque for T r will not be zero even in SPM machines due to cross-coupling saturation. With the reducing of b 0 , the leakage flux will increase, which reduces the average torque and makes the fluctuations of T cog even increase under full-load condition. However, under half-load condition, the variation of torque components against b 0 is totally different, Figs.1(e) and (f). It reveals that the cross-coupling effect under half-load becomes unobvious while T cog reduces when b 0 is small.