High Air-gap Flux Density Research Articles

Analysis and Suppression of Spoke-Type Permanent Magnet Machines Cogging Torque with Different Conditions for Electric Vehicles

Spoke-type permanent magnet (STPM) machines have high power density and low cost due to flux concentrated effect and high air-gap flux density, but they can cause high cogging torque and torque ripple. To reduce the cogging torque, the analytical model considering a rotor slot is established and compared with the finite element mothed (FEM). Then, the cogging torque production mechanism is revealed and analyzed under different conditions, which provides direction to optimize the cogging torque STPM machines. The harmonic content of cogging torque under different conditions is obtained based on the freezing permeability (FP) method. It is found that the fundamental waves mainly generate the cogging torque under a no-load condition, and it is mainly generated by the second harmonics under an on-load condition. In addition, the optimization method is introduced and researched, including rotor slot width, uneven rotor core, and so on. Finally, a 50 kW STPM machine prototype is manufactured and tested to verify the accuracy and efficiency of the analysis method.

Cogging Torque Reduction of Axial-Modular Flux Switching Permanent Magnet Machine by Module Combination Technique

Flux switching permanent magnet machine exhibits high cogging torque due to the doubly salient topology and high air gap flux density. This paper proposes a new axial-modular flux switching permanent magnet (AM-FSPM) machine, which has an improved torque capability and the potential of cogging torque suppression by module combination technique. The cogging torque analytical expression of the machine with axial-modular topology is derived based on a magnetomotive force-permeance model. Then, the AM-FSPM machines are divided into two types by the combinations of stator-slot and rotor-pole, named fundamental harmonic offset naturally and fundamental harmonic offset manually. In addition, four approaches are investigated to reduce cogging torque by changing modular magnetomotive force and changing modular permeance. It can be concluded that the modular rotor tooth combination method has the most attractive torque performances among the four methods. Finally, the effectiveness of the proposed approaches is verified by the 3D finite element analysis and experimental results.

Design and Performance Comparison of Novel Flux-Concentrating IPM Machines for Power Generation System Application of Extended-Range Electric Vehicle

A new spoke-type interior permanent magnet (IPM) machine with enhanced flux-concentrating capability and high saliency ratio rotor is proposed for the extended-range electric vehicles traction applications. The evolution of the proposed flux-concentrating rotor structure from the spoke-type IPM rotor is illustrated to achieve a higher saliency ratio and higher air-gap flux density. Two IPM motors with the same magnets volume are developed and compared with the proposed IPM machine. The rotor configurations are optimized by the multiobjective genetic algorithm to maximize the torque/power and minimize the cogging torque, as well as increasing the saliency ratio. Furthermore, the major electromagnetic performance of the three machines are analyzed and compared by the finite-element analysis (FEA), including the air-gap flux density, cogging torque, saliency ratio, torque-current curves, and field-weakening capability. The results show that the IPM machine with high saliency ratio rotor can achieve higher torque/power, maximum torque per ampere capability, and flux-weakening ability. Finally, a-rated 60-kW PM motor with the proposed rotor is manufactured and experimented to verify the results of the FEA.

Torque Ripple Suppression of Flux-Switching Permanent Magnet Machine Based on General Air-Gap Field Modulation Theory

Compared with the conventional permanent magnet (PM) machines, the flux-switching permanent magnet (FSPM) machine exhibits higher torque ripple due to its special doubly salient structure and high air-gap flux density, leading to the deterioration of smooth operation. In this article, the output torque ripple suppression methods of FSPM machines from both rotor and stator sides are proposed with the aid of general air-gap field modulation theory. According to harmonic modulation mechanism, air-gap PM field can be modified by revising harmonic content of rotor and stator modulation functions and, consequently, three concepts, including cos-shaped edge rotor (CSER), cos-shaped edge stator, and double cos-shaped edge, are proposed and implemented to suppress the output torque ripple due to the cogging torque and electromagnetic torque ripple of FSPM machines. Finally, finite-element analysis and experiments are conducted to verify the proposed approaches. The results reveal that the CSER method exhibits the most effective suppression effect.

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 HTS Stator-Excited Axial-Flux Magnetic Gear With Static Seal

This paper proposes a novel high-temperature superconducting stator-excited axial-flux magnetic gear (HTS-SEAFMG) for the high-torque transmitting system. The proposed magnetic gear combines the merits of both HTS-excitation and stator-excitation. By using the HTS-excitation, a high air-gap flux density can be easily obtained, thus achieving the high torque density. The HTS-excitation windings are placed on the stator with a static seal of cooling Dewar which greatly reduces the manufacturing difficulty and the cost of refrigeration system compared with the dynamic seal. Thanks to the stator-excitation pattern, both the high-speed rotor and the low-speed rotor comprise only iron, thus contributing to the high mechanical reliability. The axially sandwich structure can also simplify its assembly in the high torque transmission applications. The dimensional parameter optimization and the electromagnetic performance evaluation are conducted by using the finite element method, respectively. The results show that the torque density of the proposed HTS-SEAFMG can reach 123 kNm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Theoretical analysis and results are both given to verify the feasibility of the proposed HTS magnetic gear design.

Design Considerations of a Transverse Flux Machine for Direct-Drive Wind Turbine Applications

This paper presents the design considerations and analysis of a double-sided transverse flux machine (TFM) for direct-drive wind turbine applications. The proposed TFM has a modular structure with quasi U-core stators and toroidal ring windings. The rotor is designed with ferrite magnets in a flux-concentrating setup to achieve high air gap flux density. This paper presents the different aspects of the TFM, taking into consideration the critical geometric parameters affecting the back-EMF waveform, phase inductance, torque density, and power factor. Three-dimensional finite element analysis (3D-FEA) was used to analyze and optimize the critical TFM design parameters to meet the performance specification. A proof-of-concept prototype was developed based on the proposed design considerations. The prototype was experimentally tested at different operating points and the results showed good correlation with the 3D-FEA results.

Cogging torque minimisation in FSPM machines by right‐angle‐based tooth chamfering technique

Cogging torque in flux-switching permanent magnet (FSPM) machines dominates the whole torque ripples due to its special doubly salient structure and high air-gap PM flux density. In this study, the chamfering method of right angles is utilised in both stator and rotor sides of FSPM machines to suppress the cogging torque. First, an analytical expression of cogging torque, taking the influence of the introduced right angles into consideration, is derived. On the basis of the derived model, the optimal dimensions of right angles are obtained and verified by both finite element analysis and experiments. Besides, the impacts of the proposed technique on phase electro-motive force and electromagnetic torque are also evaluated. The predicted results indicate that the proposed technique can significantly reduce cogging torque in FSPM machines with negligible reduction of average torque.

Analytical Approach for Cogging Torque Reduction in Flux-Switching Permanent Magnet Machines Based on Magnetomotive Force-Permeance Model

Flux-switching permanent magnet machine typically suffers from relatively high cogging torque due to its special doubly salient structure and high air-gap flux density, causing undesired torque ripples, as well as acoustic noise and vibration, especially at low speeds. In this paper, an analytical expression of cogging torque is derived based on a magnetomotive force-permeance model, through which the optimal design parameters and dimensions, such as combinations of stator slots and rotor poles, skewing angle, stator tooth width, rotor tooth width, and magnet thickness, can be determined. Based on the derived analytical model, two novel approaches for cogging torque reduction are proposed, i.e., asymmetric magnetomotive force and asymmetric permeance, respectively, and consequently, the optimal designs are also acquired analytically. However, it is also found that the optimal choice of the proposed techniques depends on the combinations of stator slots and rotor poles of the machines. Finally, the effectiveness of the proposed approaches is verified by both finite element analytical predictions and experimental results.

An Improved Configuration for Cogging Torque Reduction in Flux-Reversal Permanent Magnet Machines

A flux-reversal permanent magnet (FRPM) machine typically suffers from unfavorably large cogging torque due to its special doubly salient structure and high air-gap flux density, resulting in undesired torque and speed ripples, as well as acoustic noise and vibration, especially at low speeds. Therefore, an improved configuration of FRPM machines by introducing a small space-gap between the two adjacent magnets belonging to the same stator tooth is proposed in this paper. The analytical expression of cogging torque taking this small space gap into consideration is derived, and furthermore, the optimal dimensions of the space gap are obtained analytically for cogging torque minimization. In addition, the influences of the space-gap-based configuration on key electromagnetic performances, including cogging torque, back electromotive force, and electromagnetic torque, are investigated by both 2-D finite-element analysis and experimental results. It turns out to be that by employing this configuration, not only the cogging torque, and hence, torque ripples can be suppressed, but also the electromagnetic torque can be improved.

A model predictive current control of flux-switching permanent magnet machines for torque ripple minimization

Due to high airgap flux density generated by magnets and the special double salient structure, the cogging torque of the flux-switching permanent magnet (FSPM) machine is considerable, which limits the further applications. Based on the model predictive current control (MPCC) and the compensation control theory, a compensating-current MPCC (CC-MPCC) scheme is proposed and implemented to counteract the dominated components in cogging torque of an existing three-phase 12/10 FSPM prototyped machine, and thus to alleviate the influence of the cogging torque and improve the smoothness of electromagnetic torque as well as speed, where a comprehensive cost function is designed to evaluate the switching states. The simulated results indicate that the proposed CC-MPCC scheme can suppress the torque ripple significantly and offer satisfactory dynamic performances by comparisons with the conventional MPCC strategy. Finally, experimental results validate both the theoretical and simulated predictions.

Air-Gap Flux Density Characteristics Comparison and Analysis of Permanent Magnet Vernier Machines With Different Rotor Topologies

In this paper, the air-gap flux density characteristics of three permanent-magnet vernier machines (PMVMs) with different rotor-PM topologies, i.e., surface, Halbach and spoke-array types are analyzed and compared. The investigated machines share the same stator configuration characterized by fractional-slot concentrated windings and multitooth split-pole structure. By using the finite-element analysis, the dominant factors that influence the air-gap flux density characteristics of the proposed PMVMs are investigated and discussed. It demonstrates that the air-gap flux densities of all these machines are predominantly sensitive to conventional parameters, such as air-gap lengths, PM thicknesses, as well as exclusive parameters of the flux modulation pole (FMP) width and gear ratio. The higher pole arc coefficients are desired for PMVMs with surface and Halbach PMs, whereas a small value is preferred for poke-array PMVM. In addition, higher air-gap flux density can be obtained for all three PMSMs with suitable FMP width and optimal gear ratio. Finally, the aforementioned analysis and calculation are verified by one PMVM prototype with spoke-array PM configuration.

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 Variable-Flux Permanent-Magnet Machines Using Alnico Magnets

This paper proposes a novel design for variable-flux machines with Alnico magnets. The proposed design uses tangentially magnetized magnets to achieve high air-gap flux density and to avoid demagnetization by the armature field. Barriers are also inserted in the rotor to limit the armature flux and to allow the machine to utilize both reluctance and magnet torque components. An analytical procedure is first applied to obtain the initial machine design parameters. Then, several modifications are applied to the stator and rotor designs through finite-element analysis (FEA) simulations to improve machine efficiency and torque density. A prototype of the proposed design is built, and the experimental results are in good correlation with the FEA simulations, confirming the validity of the proposed machine design concept.

Six-Phase BLDC Reluctance Machines: FEM-Based Characterization and Four-Quadrant Control

In a yet another effort to produce better permanent magnet (PM)-less rotor-winding-less brushless electric motor drives, this paper reports work related to multiphase $({\rm m} =6)$ high saliency rotor dual-flat-top alternative current control brushless dc (BLDC) reluctance machine drives. The aim is to produce high torque density, low loss/torque in a PM-less rotor-winding-less machine by full usage of machine windings and core and of inverter kVA. A new derivation of the principle of operation, essential rotary and linear machine topologies, and a 2-D finite-element method (FEM) analysis for torque density and torque pulsations on an already built (6 phase, 6 poles, 35 N $\cdot$ m) lab prototype are made available and show promising results. Experimental flux decay test results are presented, which, together with standstill torque measurements, validate the finite-element model. Advanced iron loss computation by finite-element analysis indicates moderate core loss, although high air-gap magnetic flux density and current harmonics occur as a natural behavior of a BLDC machine. Electrical and mechanical parameter identification is followed by the development of a circuit model based on FEM imported data for parameters and by a four-quadrant control strategy proposal. Running experiments (motoring and generating) with speed-reversal and field-weakening modes using a DSpace platform, which drives three three-phase inverters that power a star-connected six-phase BLDC multiphase reluctance machine are presented, thus showing operation with a reduced number of switches in the inverter.

Design aspects of linear motor for a two-stage stirling cryocooler

Two-stage cryocoolers are used to maintain a lower temperature than that can be achieved in one stage. These are also used to have more than one temperature in electronic instruments. A two-stage expander has a large dead volume compared with that in a single stage; therefore two compressors are used to provide the alternating pressure. A dynamically balanced opposed piston compressor not only makes the system quiet in operation but also eliminates vibration. The reciprocating motion of the piston is obtained from a moving magnet type linear motor. The design of the linear motor to power the piston is a critical one for the efficient and reliable operation of the cooler. In this paper, various configurations of linear motor have been identified and electromagnetic analysis has been performed. The design of the motor involves the size estimation followed by the optimisation of different geometrical and electromagnetic parameters. Based on the analysis of existing structures of linear motors, a novel structure has been identified which would meet the characteristics such as high air-gap flux density, small volume, light weight and simple installation.

Performance of Low-Cost Permanent Magnet Material in PM Synchronous Machines

Permanent magnet synchronous machines (PMSM) are considered a viable option in various types of applications. However, particularly in consumer and low-power industrial applications, the price may be a factor that limits the use of PMSMs. In addition to a different technology, the main reason for the high price of PMSMs is the use of expensive neodymium or samarium-cobalt magnets. Their use is necessary only if a high motor torque <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> to linear current density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> ratio ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> ) is required. Ferrite permanent magnets are low cost, abundant, and have negligible eddy current losses in low-frequency applications such as motor drives. They have a much lower energy product ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BH</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) than the most modern magnets. Because of the high prices of rare earth magnets, many parties are seeking for opportunities to use ferrites instead. In the case of rotor surface ferrite magnets, the air gap flux density remains low. The air gap torque producing tangential Maxwell stress is proportional to the product of the air gap flux density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">δ</sub> [ Vs/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ] and the linear current density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> [A/m]. If the flux density is low and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> cannot be increased, the rotor has to be made larger than in machines having a high air gap flux density. In the case of multiple pole machines, the outer rotor approach, with its low rotor yoke height, offers an interesting alternative. The air gap diameter of these machines can be made larger than in conventional inner rotor type motors without increasing the machine outer dimensions. In this paper, an outer rotor PMSM with ferrite magnets is analyzed and tested. The machine characteristics in a fan drive are compared with an induction machine of the same power.

Reduction on Cogging Torque in Flux-Switching Permanent Magnet Machine by Teeth Notching Schemes

The cogging torque in flux-switching permanent magnet machines (FSPMs) is high due to its unique structure and high air-gap flux density. The cogging torque principle in FSPM is different from that in traditional PM machines, which can not be correctly predicted by analytical consideration. The aim of paper is to present the investigation on cogging torque principle in FSPM by analyzing the flux density distribution and a simple cogging torque reduction technique, i.e., teeth notching. Various kinds of notching schemes and their influence on cogging torque are examined along with instantaneous torque and average output torque at different load conditions. Numerical optimization process combined with finite-element analysis, which gives more preciseness to calculations, is performed to minimize cogging torque. The results show that the cogging torque circle depends on the real flux density distribution in the machine rather than the number of stator/rotor poles and the presented method can greatly reduce the torque ripple at only slight cost of average output torque.

Permanent magnet linear synchronous motor with high air-gap flux density for transportation

Permanent magnet linear synchronous motor with high air-gap flux density for transportation

A New Fault-Tolerant Permanent-Magnet Machine for Electric Vehicle Applications

This paper proposes a new fault-tolerant permanent-magnet (PM) brushless machine for four-wheel independently driven electric vehicles (EVs). The key of the proposed machine is to make tooth widths unequal of inner stator, hence, offering more trapezoidal back-EMF waveform. Due to the fact that the radial PMs are embedded in the outer rotor, the proposed machine incorporates the merits of high torque-to-weight ratio, mechanical robustness, and high air-gap flux density. In addition, multiphase and single-layer fractional slot concentrated windings are employed to improve the reliability. The electromagnetic characteristics of the proposed machine are analyzed by using the time-stepping finite-element method (FEM) and field-circuit cosimulation method, such as magnetic field distribution, flux destiny, flux linkage, and torque, verifying the effectiveness of the theoretical analysis.