Reluctance Torque Research Articles

Reduction of Torque Ripple in Inset Permanent Magnet Synchronous Motor by Magnets Shifting

This paper proposes a method of reducing total torque ripple by magnets shifting. The key of this method is to consider the reluctance torque ripple, because the reluctance torque ripple is the main source of torque ripple in inset permanent magnet synchronous motor. This method is realized by appropriately choosing Repeating Unit, which indicates a group of poles producing torques with consistency in waveforms and phases. Meanwhile, the uniform analytical expressions of torques with magnets shifting are established, including cogging torque, reluctance torque, and total torque. Moreover, the total torque ripple can be reduced greatly, and the average total torque loss is acceptable when one pole-pair is chosen as one Repeating Unit and the shifting angles of Repeating Unit are 3.75 and 1.875 mechanical degrees. These theoretical analyses are verified by the finite-element method.

Influence of PM- and Armature Winding-Stator Positions on Electromagnetic Performance of Novel Partitioned Stator Permanent Magnet Machines

Since the permanent magnets (PMs) and armature windings of partitioned stator (PS) PM machines are located in two separate inner and outer stators, their positions can be exchanged to optimize the space utilization, especially in radial field rotating machines. Therefore, in this paper, the influence of PM and armature winding stator positions on the electromagnetic performance of PS-PM machines is investigated based on the novel PS-PM machines (PS-PMMs) with surface-mounted PM (SPM) stator. Similar to the single-stator surface-mounted PMMs (SS-PMMs), flexible rotor pole number, bipolar phase flux linkage, and symmetrical phase back electromotive force (EMF) are also obtained in PS-PMMs. Based on the same 12/10 stator/rotor pole number combination, PS-PMM-I (PMs located in the inner stator) and PS-PMM-II (PMs located in the outer stator) exhibit 120% and 160% higher phase back EMFs as well as 120% and 139% larger average torques, respectively, than the SS-PMM together almost without scarifying the PM utilization efficiency under the same machine size and the same rated copper loss. Further, the proposed PS-PMM-IIs have both higher phase back EMFs and larger average torques than PS-PMM-Is among all the main stator/rotor pole number combinations. Meanwhile, for both PS-PMM-Is and PS-PMM-IIs with 12-pole stator, the machines with the 11-pole rotor exhibit the optimal torque capabilities. Moreover, the reluctance torque is also negligible in the proposed PS-PMMs due to very low saliency ratio. The analyses are validated by experiment results of the prototype machine.

Reducing Acoustic Noise of Axial‐Gap SRMs by Decreasing Axial Electromagnetic Force Ripple

SUMMARYSwitched reluctance (SR) motors are motors that utilize the reluctance torque originating in the magnetic saliency between the stator and the rotor, which are made up of an electromagnetic steel sheet and windings without a permanent magnet. Because of their robustness and simple structure, SR motors are suited for use as the in‐wheel motor for traction motors of vehicles. In a flat space such as is present in in‐wheel motors, the axial‐gap structure has an advantage compared with the conventional radial‐gap structure for the space factor of a motor. An axial‐gap in‐wheel SR motor was tested in a microbus, and the bus could be successfully driven. The test results regarding the motor performance were satisfactory. However, the motor emits a loud sound. The acoustic noise of the axial‐gap SR motor is mainly caused by the axial electromagnetic force. This paper presents a method of reducing acoustic noise that is based on a model of the electromagnetic force on the axial‐gap SR motor.

Analytical Field Calculation and Analysis of Surface Inset Permanent Magnet Machines With High Saliency Ratio

In this paper, an analytical field calculation model is built for surface inset permanent magnet (SIPM) machines with high saliency ratio. The high saliency ratio of SIPM machine is obtained using unequal $d$ - $q$ axis air-gap length based on the SIPM rotor structure. High saliency ratio can produce large reluctance torque and wide speed range. To get a fast and exact knowledge of flux distribution and operation parameters of the SIPM machine with high saliency ratio, the field solving domain is divided into five subdomains, i.e., stator slots, stator slot openings, air-gap, rotor air slots, and rotor magnets. The vector potential in each subdomain is governed by Maxwell’s equation and can be solved using the variable separation method and boundary conditions between adjacent subdomains. In addition, the cogging torque, back electromotive force (EMF), inductance, output torque, and unbalanced magnetic force (UMF) are calculated based on the field model. All the field calculation results are compared with those of finite element analysis (FEA), and it is shown that analytical field calculation model is fast and accurate. Furthermore, the effects of $q$ -axis air-gap length and pole arc ratio on machine parameters, such as cogging torque, back EMF, saliency ratio, output torque, and UMF, are also investigated using the analytical field calculation model.

Torque Density Improvement of Doubly Salient Electromagnetic Machine With Asymmetric Current Control

Due to the introduction of field winding, the doubly salient electromagnetic (DSEM) machine is featured with good torque production capability. However, this capability is limited by the problems of the phase current reversal and excessive back- electromotive force with traditional control methods. In this paper, a new control method, namely asymmetric current control (ASCC), is proposed to further improve the output torque. Two control angles α and γ are introduced to implement the current-angle control. Simultaneously, the pulse width modulation control is utilized to shape the waveform of phase current. Consequently, the torque is increased drastically because of more effective phase current waveforms, which presents the asymmetry of positive and negative half period. Furthermore, the reluctance torque resulting from the interaction between self-inductance and asymmetric phase current exerts favorable effects on the torque output. By field-circuit-coupled analysis, the ASCC method for a 1.3 kW prototype is analyzed and compared with other traditional control methods. Laboratory testing of the 1.3 kW DSEM machine driven by the new control method confirms the validity of theoretical analysis and simulation improvement of torque. The simulation and experimental results show that the torque and power density can be improved to 1.5 times by the proposed control method.

Investigation of Reluctance Torque in a BLDC Motor using Frozen Permeability Method and Equivalent Air-gap Analysis

This work investigates the reluctance torque in a surface mounted Brushless DC motor (BLDC) regarding the saturation effects due to the increase of electric load. At first, the reluctance torque is evaluated considering the line inductance waveforms for three different electromagnetic load conditions. Based on these results, which show an impressive change on the behavior of reluctance torque due to electromagnetic saturation, a further investigation is carried out in order to clarify the role played by each tooth in the reluctance torque, and of course, in the line inductance waveforms. These investigations are carried out with the aid of Frozen Permeability Method and the Equivalent Air-gap Analysis.

Study of IPMSM Interturn Faults Part I: Development and Analysis of Models With Series and Parallel Winding Connections

In this study, the interturn fault (ITF) models of interior permanent magnet synchronous motors (IPMSMs) employing series and parallel winding connections were developed through the addition of saliency modeling to the surface-mounted permanent magnet motor models presented in a previous work. The saliency model was obtained by using deformed flux models based on both the fault winding flux information and inductance variations caused by cross-flux linkages that depend on the distribution of same-phase windings. By assuming a balanced three-phase current injection, positive and negative sequence voltages, and the fault current are obtained in the positive and negative synchronous reference frames. The output torque models were developed by adding the magnet and reluctance torques, which were derived from the developed models. To verify the proposed IPMSM models with an ITF, a finite element method-based simulation and experimental measurements were performed; the results are presented in this paper.

Application of stator shifting to five‐phase fractional‐slot concentrated winding interior permanent magnet synchronous machine

In many applications, interior permanent magnet synchronous machines (IPMSMs) with fractional slot concentrated windings (FSCWs) are considered promising candidates in terms of higher power density and efficiency. In addition, employing a multiphase stator winding improves the drive train availability and increases reliability. This study investigates the effect of applying stator shifting to five-phase FSCW winding IPMSMs to suppress the effect of the slot harmonics by doubling the number of slots. In this case, the winding coil pitch will be two, which stands as a compromise between single-tooth and distributed winding topologies. This highly improves the air gap flux distribution, significantly reduces both rotor core and magnet eddy current losses, and increases saliency ratio and reluctance torque component. Moreover, an improved performance under fault conditions, in terms of lower torque ripple, and core and magnet losses, adds to the main advantages of this technique. Various slot/pole combinations suitable for five-phase machines are investigated. A full simulation case study based on two-dimensional finite element analysis is applied to the 20-slot/18-pole stator with single-tooth winding under both healthy and open-circuit phase fault cases.

Analysis and Design of a Hybrid Rare-Earth-Free Permanent Magnet Reluctance Machine by Frozen Permeability Method

Hybrid-type switched reluctance machine (SRM) by adding permanent magnets (PMs) in the stator of the SRM is gaining significant attention in order to solve the rare-earth PM problems, such as price instability and environmental problems. However, the addition of PMs in an SRM leads to positive and negative effects on torque production at the same time, because PMs in the stator assist the flux linkage of phase windings but aggravate total reluctance due to a slot for each PM. This paper is to propose a hybrid PM reluctance machine (PMRM) (hereinafter referred to as a hybrid PMRM) having a rare-earth-free PM in the stator, and the machine is analyzed and designed in terms of PM thickness by torque separation during freezing permeability in steel. The level of excitation current is not important in the variation of reluctance torque and mutual torque, but the thickness of PM has a major effect on both reluctance torque and mutual torque. Although bigger thickness in PM causes negative effect on reluctance torque production, total torque in the hybrid PMRM has been improved by a distinct margin, since mutual torque enhancement is much larger than reluctance torque drop. As a result, the proposed PMRM has an increase of 10.1% in efficiency compared with the same design without PM.

Reliability Analysis of Permanent Magnet and Air-gap Flux Density of Spoke-type IPMSM for Driving Cooling Fans

One of the key electrical components in a car, a motor has been actively studied based on its reliability. In recent years, as the proportion of the motor has increased in a car, development of the motor with a high output and a low price has been required. IPMSM can cause a high torque per unit current by using a reluctance torque generated from the difference in the magnetic resistance, which is suitable for the motor which requires a high output. In particular, since the spoke-type IPMSM with a concentrated flux structure can utilize non-rare earth metal-based ferrite magnets which are cheaper than rare earth metal-based permanent magnets such as neodymium, this can lower the motor price. In this paper, we study driving characteristics by analyzing the magnetic flux density and cogging torque that causes noise and vibration in accordance with the rotor shapes (Magnet, Rib) of the spoke-type IPMSM.

Comparative study on synchronous reluctance and PM machines

Purpose – The purpose of this paper is to comparatively study a synchronous reluctance machine (SynRM) and a permanent magnet assisted synchronous reluctance machine (PMASynRM) as alternatives of the interior permanent magnet synchronous machine (IPMSM), and to investigate the performance and conclude both advantages and disadvantages. Design/methodology/approach – A unified mathematical model is established for the IPMSM, SynRM and PMASynRM. Then finite element method (FEM) is used to compare the electromagnetic performance. Permeability-frozen method is utilized to distinguish basic electromagnetic torque and reluctance torque. Findings – The PMASynRM can improve the power factor of the SynRM, overcome the drawback of the IPMSM in the high-speed flux-weakening region and is more proper to operate over a wide speed region. The SynRM is mechanically robust for lacking of the permanent magnets, and the PMASynRM can keep similar rotor stress as the SynRM by optimizing the magnets. Assembly of the SynRM is the simplest, and the economic performance of the SynRM and PMASynRM could be much better than the IPMSM which even uses ferrite magnets. Practical/implications – The SynRM can produce identical torque and efficiency compared with the IPMSM except the poor power factor. The poor power factor could be improved by adopting the PMASynRM, which is proved to be able to act as an alternative of the IPMSM for low-cost high-performance application. Originality/value – This paper provides the theoretical model of the IPMSM, SynRM and PMASynRM in a unified format. The electromagnetic, mechanical and economic performances of the three kinds of synchronous motors are compared comprehensively. Then, both the advantages and disadvantages are summarized.

Dynamic and steady state performance comparison of line-start permanent magnet synchronous motors with interior and surface rotor magnets

Abstract A comprehensive comparison of the dynamic and steady state performance characteristics of permanent magnet synchronous motors (PMSM) with interior and surface rotor magnets for line-start operation is presented. The dynamic model equations of the PMSM, with damper windings, are utilized for dynamic studies. Two typical loading scenarios are examined: step and ramp loading. The interior permanent magnet synchronous motor (IPMSM) showed superior asynchronous performance under no load, attaining faster synchronism compared to the surface permanent magnet synchronous motor (SPMSM). With step load of 10 Nm at 2 s the combined effect of the excitation and the reluctance torque forced the IPMSM to pull into synchronism faster than the SPMSM which lacks saliency. The ability of the motors to withstand gradual load increase, in the synchronous mode, was examined using ramp loading starting from zero at 2 s. SPMSM lost synchronism at 12 s under 11 Nm load while the IPMSM sustained synchronism until 41 seconds under 40 Nm load. This clearly suggests that the IPMSM has superior load-withstand capability. The superiority is further buttressed with the steady state torque analysis where airgap torque in IPMSM is enhanced by the reluctance torque within 90° to 180° torque angle.

Effect of Multilayer Windings With Different Stator Winding Connections on Interior PM Machines for EV Applications

The interior permanent magnet (IPM) machine equipped with a fractional-slot concentrated winding (FSCW) has met an increasing interest in electric vehicle applications due to its higher power density and efficiency. Torque production is due to both PM and reluctance torques. However, one of the main challenges of FSCWs is their inability to produce a high-quality magnetomotive force (MMF) distribution, yielding undesirable rotor core and magnet eddy-current losses. Literature shows that the reduction of low-order space harmonics significantly reduces these loss components. Moreover, it has been previously shown that by employing a higher number of layers, although causing some reduction in the winding factor of the torque-producing MMF component, both machine saliency and reluctance torque components are improved. Recently, a dual three-phase winding connected in a star/delta connection has also shown promise to concurrently enhance machine torque of a surface-mounted PM machine while significantly reducing both rotor core and magnet losses. In this paper, a multilayer winding configuration and a dual three-phase winding connection are combined and applied to the well-known 12-slot/10-pole IPM machine with v-shaped magnets. The proposed winding layout is compared with a conventional double-layer winding, a dual three-phase double-layer winding, and a four-layer winding. The comparison is carried out using 2-D finite-element analysis. The comparison shows that the proposed winding layout, while providing similar output torque to a conventional double-layer three-phase winding, offers a significant reduction in core and magnet losses, correspondingly a higher efficiency, improves the machine saliency ratio, and maximizes the reluctance toque component.

Performance Comparison of Line-Start Permanent Magnet Synchronous Motors with Interior and Surface Rotor Magnets

Objectives: The aim is to undertake a comprehensive comparison of the dynamic and steady state performance of Permanent Magnet Synchronous Motors (PMSM) with interior and surface rotor magnets for line-start operation. Methods/Analysis: The dynamic model equations of the PMSM, with damper windings, are utilized for dynamic studies. Two typical loading scenarios are examined: Step and ramp loading. Emphasis was on the synchronization and load-withstand capabilities of the two motor classes. Also, the steady state torque performance was compared using the torque/torque angle relationship. Findings: The interior permanent magnet synchronous motor (IPMSM) showed superior asynchronous performance under no load, attaining faster synchronism compared to the Surface Permanent Magnet Synchronous Motor (SPMSM). With step load of 10Nm at 2 seconds the combined effect of the excitation and the reluctance torque forced the IPMSM to pull into synchronism faster than the SPMSM which lacks saliency. The ability of the motors to withstand gradual load increase, in the synchronous mode, was examined using ramp loading starting from zero at 2seconds. SPMSM lost synchronism at 12seconds under 11Nm load while the IPMSM sustained synchronism until 41seconds under 40Nm load. This clearly suggests that the IPMSM has superior load-withstand capability. The superiority is further buttressed with the steady state torque analysis where airgap torque in IPMSM is enhanced by the reluctance torque within 90o to 180o torque angle. Applications: The findings of this research will serve as guide in taking logical decisions in the design and operation of permanent magnet synchronous motors to achieve desired objectives in specific applications.

軸方向電磁力リプル低減によるアキシャルギャップ型SRモータの振動・騒音低減に関する検討

SUMMARY Switched reluctance (SR) motors are motors that utilize the reluctance torque originating in the magnetic saliency between the stator and the rotor, which are made up of an electromagnetic steel sheet and windings without a permanent magnet. Because of their robustness and simple structure, SR motors are suited for use as the in-wheel motor for traction motors of vehicles. In a flat space such as is present in in-wheel motors, the axial-gap structure has an advantage compared with the conventional radial-gap structure for the space factor of a motor. An axial-gap in-wheel SR motor was tested in a microbus, and the bus could be successfully driven. The test results regarding the motor performance were satisfactory. However, the motor emits a loud sound. The acoustic noise of the axial-gap SR motor is mainly caused by the axial electromagnetic force. This paper presents a method of reducing acoustic noise that is based on a model of the electromagnetic force on the axial-gap SR motor.

High-Performance Low-Cost Electric Motor for Electric Vehicles Using Ferrite Magnets

Permanent-magnet motors with rare-earth magnets are among the best candidates for high-performance applications such as automotive applications. However, due to their cost and risks relating to the security of supply, alternative solutions such as ferrite magnets have recently become popular. In this paper, the two major design challenges of using ferrite magnets for a high-torque-density and high-speed application, i.e., their low remanent flux density and low coercivity, are addressed. It is shown that a spoke-type design utilizing a distributed winding may overcome the torque density challenge due to a simultaneous flux concentration and a reluctance torque possibility. Furthermore, the demagnetization challenge can be overcome through the careful optimization of the rotor structure, with the inclusion of nonmagnetic voids on the top and bottom of the magnets. To meet the challenges of a high-speed operation, an extensive rotor structural analysis has been undertaken, during which electromagnetics and manufacturing tolerances are taken into account. Electromagnetic studies are validated through the testing of a prototype, which is custom built for static torque and demagnetization evaluation. The disclosed motor design surpasses the state-of-the-art performance and cost, merging the theories into a multidisciplinary product.

電機子巻線を界磁源とする新たなハイブリッド可変界磁モータ

In this paper, a new hybrid-excited motor is proposed. Its unique feature is that it has no field exciting coil, despite being a type of bypass yoke core (BYC) hybrid-excited synchronous motor. Based on finite element analysis (FEA), the following facts are clarified. Since the stator coil magnetomotive force (MMF) and claw poles rotate at the same speed and in the same direction, they are motionless relative to each other. Since the claw poles are located at one of its ends near the BYC surface with a gap, it receives immovable magnetization by the stator coil MMF through the BYC; then the claw poles can supply a magnetic field flux via the BYC. This flux via the BYC increases the total torque, because its torque is added the two other torque components: the permanent magnet torque and claw poles reluctance torque. In addition, the magnetic polarity and the amount of the flux of the claw poles can be varied by controlling the armature current phase angle.

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.

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.

3-D Effects of Rotor Step-Skews in Permanent Magnet-Assisted Synchronous Reluctance Machines

Permanent magnet assisted synchronous reluctance machines (PMA SynRM), stemming from the combination of synchronous reluctance and interior PM machine topologies, have recently gained increasing popularity as a viable candidate for electric vehicle traction applications due to concerns regarding the price volatility of rare-earth magnet materials. Their attractiveness can be attributed to their distinct torque production mechanism, which essentially relies on the reluctance torque component enabling the use of less expensive ferrites or bonded NdFeB magnet solutions. A drawback often associated with this machine topology is high torque ripple emanating from its intrinsically high salient nature [1]. Among several machine design techniques available to mitigate the high torque ripple, rotor step-skew is known to be one of the most effective and frequently incorporated solutions. When a step-skew is performed, it causes variation of magnetic flux density along the axial (skew) direction, and thus influences the machine's electromagnetic behavior [2]. Numerous methods are available in literature to account for the effects of skew using (either) 2D or 3D numerical finite element analysis (FEA), analytical, or semi-analytical approaches, the trade-off in each case being computational time versus accuracy. Due to the prohibitive computational time and numerical memory costs of 3D FEA, the prediction of a skewed machine's electromagnetic performance typically relies on 2D domain-based techniques, such as multi-slice 2D FEA [3] or more computationally efficient semi-analytical techniques [2] exhibiting comparable accuracy. A common limitation associated with these techniques is their inability to capture certain 3D phenomena, namely the end-effects and axial interaction between skewed steps, since they explicitly rely on the 2D domain [4]. The significance of such 3D effects is influenced by the axial length to airgap diameter ratio (viz. end-effect), the number of skewed steps, the skew angle and the structure of the machine topology under consideration. The quantification, and most importantly experimental verification, of the 3D effects associated with skewed PMA SynRMs has received limited attention in literature to date.