Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> The paper describes a short theoretic study into the flux reversal machine and its potential as a high-torque drive motor when compared to other motors, especially a switched reluctance machine. The literature on this type of motor is scarce and there is still little development of the equivalent circuit and design criteria necessary to formulate a design-office procedure for developing such motors. The paper describes the fundamental concepts for the operation of the machine and highlight design criteria that are important for a good machine. A design is compared to a switched reluctance machine using finite element analysis and the SPEED software. The first machine studied was a 12/8 machine, this was then changed for a 12/16 machine. <s> BIB001 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> This paper proposes design and analysis of a new linear motor called flux reversal linear synchronous motor (FRLSM), which is cost effective for long stroke applications. Passive rotor configuration of a typical flux reversal machine (FRM) can be utilized for a linear machine with less permanent magnet (PM). Three different design choices are investigated and compared both in thrust and normal force. The investigation shows extremely low force ripple and good thrust linearity through skewing and three-phase sinusoidal control. Reduced PM configuration generates about 21% higher thrust even with half amount of PM. <s> BIB002 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> Permanent magnet (PM) brushless machines having magnets and windings in stator (the so-called stator-PM machines) have attracted more and more attention in the past decade due to its definite advantages of robust structure, high power density, high efficiency, etc. In this paper, an overview of the stator-PM machine is presented, with particular emphasis on concepts, operation principles, machine topologies, electromagnetic performance, and control strategies. Both brushless ac and dc operation modes are described. The key features of the machines, including the merits and drawbacks of the machines, are summarized. Moreover, the latest development of the machines is also discussed. <s> BIB003 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> Permanent-magnet linear synchronous motors (PMLSMs) are increasingly preferred in the industry wherever linear motion is required because of their high reliability and excellent dynamic characteristics. In this paper, a modified double-sided design of a linear motor for long-stroke applications is introduced. The proposed motor utilizes a flux-reversal topology and hence requires lesser number of permanent magnets (PMs) for operation than PMLSMs with magnet tracks. Comparison is made with conventional single-sided flux-reversal motor configuration, which shows 33% increase in thrust producing capability in the proposed design without an increase in excitation sources. Additionally, mover weight is reduced significantly, which can improve the dynamic characteristics of the motor. A five-phase 5-slot/11-pole flux reversal motor based on the double-sided topology is introduced for use in safety-critical applications. The proposed five-phase motor has low detent force and improved thrust density. <s> BIB004 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> Generally, the discontinuous armature permanent magnet linear synchronous motor (PM-LSM) is composed by the stator block (accelerator, re-accelerator, and decelerator) and the free running section. However, the stationary discontinuous armature design involves the velocity variation of the mover during drive when the armature’s non-installation part changes over to installation part as a result of the outlet edge of the armature. Therefore, we considered deforming the shape of the outlet edge at the armature and apply skew on the permanent magnet by displacing the two magnet segments of each pole. This paper presents the results of a threedimensional (3-D) numerical analysis with a finite element method (FEM) of the force exerted by the outlet edge. <s> BIB005 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> A new linear permanent-magnet (PM) motor, termed as modular linear flux-reversal PM (M-LFRPM) motor, is proposed. This is particularly attractive for high reliability urban rail transit. Its PM materials and armature windings are set in the mover (train), while the long stator (rail) is composed of irons, hence improving efficiency and reducing cost. The key of the proposed motor incorporates the concept of fault-tolerant teeth to provide the desired decoupling among phases. The structure and features of the proposed motor are presented, and an optimal design is performed. By using finite element method, the advantages and validity of the proposed motor are verified. The analysis results show that the proposed M-LFRPM motor not only retains the advantages of the conventional LFRPM motor, but also improves its fault-tolerant capability. <s> BIB006 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> A novel partitioned stator flux reversal permanent magnet (PS-FRPM) linear machine with segmented iron mover is introduced. The operation principle of the PS-FRPM linear machine is described, and the optimisation of the machine leading design parameters for maximum thrust force using genetic algorithm has been carried out. Moreover, in order to investigate the longitudinal end effect on the machine performance a periodic model has been employed to compare with the real model. It is found that the end effect has a significant impact on the machine performance, particularly in terms of unbalanced magnetic circuit and cogging force. Additionally, the influence of the mover iron pole numbers on the electromagnetic performance of PS-FRPM linear machines has been examined, i.e. 6/4, 6/5, 6/7, 6/8, 6/10 and 6/11 stator/mover pole combinations are designed and compared. It is concluded that the machine with a mover pole pitch which is closer to the stator slot pitch exhibits the best performance among the others. <s> BIB007 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> In the recent years, linear propulsion technology has been increasingly adopted in transportation, electromagnetic launch and aerospace applications. Compared with the conventional rotary-to-lin-early propulsion, the linear propulsion can offers many advantages, such as high efficiency, reduced maintenance cost, vibration and noise [1]-[4]. Among various kinds of linear motors for linear propulsion system, the linear permanent magnet (PM) motors has gained more and more attention due to the merits of high power density and efficiency. However, since the PMs or windings are set on the long stator, the conventional PM motors suffers from high cost for the long stroke applications such as rail transit. The linear flux reversal permanent magnet (FRPM) motor combining PMs and windings on the mover has been regarded as a potential solution, but its fault tolerance is poor [4]-[6]. In [4], the fault tolerance of linear FRPM motor was improved by utilizing a modular mover structure. However, it suffers from the low PM magnetic field utilization and force density due to its homopolar flux linkage. Furthermore, its force ripple and cogging force is very large, which deteriorate the force performance. In order to improve these problems, a new linear FRPM motor with modular mover will be proposed in this paper, which not only retains the advantages of the existing modular linear FRPM motor such as fault tolerance and reduced PM consuming, but also significantly enhances its force density and reduces its force ripple. <s> BIB008 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> AbstractAlthough modular linear flux-reversal permanent magnet motors are widely attractive for high reliability urban rail transit because of their advantages such as high power density, high reliability, low permanent magnet flux leakage problem, and improved fault-tolerant capability, they suffer from high thrust ripples. In this article the authors use a layer model for defining different optimization problems to improve thrust density, efficiency, and thrust ripples, independently and simultaneously. Design variables are chosen based on the sensitivity analysis of different objective functions relative to motor different geometrical parameters and some constraints are taken into account to avoid undesirable results. Then the proposed analytical model and optimization process are verified by time-stepping finite-element method. <s> BIB009 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> Starting from the rotating magnetic field theory developed by Nikola Tesla in 1882, the induction machine has become the most popular electric machine for over a century. Actually, the Tesla's AC Induction Motor is dubbed one of the ten greatest discoveries of all time. This popularity has been extended to the latest Tesla Motors which has set a new world speed record for production electric vehicles (EVs). Meanwhile, the permanent-magnet (PM) brushless machines are superseding the induction machine because of their high efficiency and high power density. In order to fulfill the ever increasing demand on electric propulsion, innovation of electric machines accelerates. For instance, the stator-PM machines combine the merits of both PM brushless and switched-reluctance machines to offer energy-efficient robust operation; the vernier PM in-wheel machines incorporate the vernier effect into the PM brushless machines to provide low-speed high-torque direct-drive operation; and the advanced magnetless machines adopt innovative topologies to compensate the reduction of torque and power densities due to the absence of PM material. Increasingly, the development of electric machines has been expanded to incorporate the heat engine for hybrid propulsion. Rather than using the planetary-geared machine system to perform power split for electronic continuously variable transmission (ECVT), the double-rotor machine system is developed to achieve gearless ECVT, and the magnetic-geared machine system is proposed to achieve ECVT without physical gearing or brushes. In this paper, various electric machines and machine systems for electric and hybrid vehicles will be discussed, with emphasis on the challenges and opportunities of those emerging machine topologies. <s> BIB010 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> In this paper, a transverse-flux flux-reversal linear motor (TF-FRLM) is proposed. With magnets on the primary side, the proposed motor is suitable for long-stroke application, benefiting from easy manufacturing and low-cost secondary cores. First, the basic structure and operating principle of the proposed motor are presented. Then, the expressions of back electromotive force (EMF) and electromagnetic thrust are obtained by an equivalent magnetic circuit model. Third, for saving computation time, a simplified half-phase model with a symmetry boundary is employed in the 3-D finite-element analysis (FEA) to investigate the TF-FRLM, and the field distribution, the back EMF, the detent force, as well as the thrust force are emphasized. Finally, a prototype is constructed, and experiments show good agreement with the FEA results. <s> BIB011 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> A novel E-core transverse flux-flux reversal linear motor is investigated in this paper. The magnets and armature coils are both on the primary side, while the secondary side only consists of laminated steels. The toroidal armature coils are placed on the middle tooth of primary core. The magnets are mounted on the primary teeth with alternating polarities. The problem of complex structure of transverse flux motors is addressed. At first, the basic structure and operating principle are introduced. Then, the equation of thrust is calculated based on the equivalent magnetic circuit method. And 3D FEM is employed to get the maximum thrust. Also the performance of this motor is evaluated. The results show that this kind motor benefits from sinusoidal back EMF and low detent force. <s> BIB012 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> Partitioned stator flux reversal permanent magnet (PS-FRPM) linear machine is a type of permanent magnet linear machine with dual active stators and one passive mover. Pervious work was shown that such a machine possesses the advantage of having big slot area leading to reduced phase resistance and consequently improved electric loading capability. In order to verify the finite element (FE) results, a prototype of PS-FRPM linear machine is built and tested. A comparison between the test and FE results is presented in this paper, together with the investigation on the influence of axial length differences. <s> BIB013 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> Linear flux-reversal permanent magnet machines (LFRPMs) have attracted increasing attentions in recent years due to its excellent performances, such as high efficiency, superior acceleration characteristics and simple maintenance [1]. <s> BIB014 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> A novel primary consequent-pole tubular transverse-flux flux-reversal linear machine (TTFFRLM) is proposed in this paper. The permanent magnets (PMs) of the machine are located on the inner surface of the short teeth of the primary iron cores for reducing the amount of PM in long stroke drive systems, and the primary is easily manufactured. The structure and principle of this machine are analyzed in detail. Based on the unit machine, a no-load equivalent magnetic circuit model is established by using the magnetic circuit method. Then, the equations of the no-load back electromotive force (back-EMF) and the electromagnetic thrust force are deduced. The simulation models of the unit machine are established by equivalent 2D finite element method (FEM) for saving computation time, and the static characteristics, including the flux field, the no-load back-EMF, and the electromagnetic thrust force, are analyzed. Detailed simulation and experimental results of a three-phase 4-poles 12-slots machine are given. The results verify the correctness and effectiveness of topology, model, and analysis method of the proposed TTFFRLM. Compared with the conventional TTFFRLM, the proposed prototype has the advantages of a lower cost and smaller electromagnetic thrust force ripple. <s> BIB015 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> A novel tubular transverse-flux flux-reversal permanent magnet linear motor (TTFFRPMLM) for long stroke applications is designed and analyzed in this paper, which not only has the low cost advantage of the existing flux-reversal motor (FRM) by combining both PMs and armature coils in short primary, but also can address the problem of complicated structure of the existing transverse-flux motor (TFM) by using rotating motor technology. The structure of the motor are introduced. The topology and operating principle of the motor are presented and analyzed. The performance of the motor is optimized by equivalent 2D finite element method (FEM). The simulation results of the 3D FEM prove the correctness and effectiveness of the motor topology and equivalent 2D FEM. The research work of this paper provides the theory basis for the engineering application of this novel motor. <s> BIB016 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> Linear flux-switching permanent magnetic (LFSPM) machines are good choices for long stroke applications. These machines deliver high thrust force density in addition to the machine structure where permanent magnetics (PMs) and windings are all on the short mover. For LFSPM machines, their performance is always affected by big thrust force ripple. In this paper, for two C-core LFSPM machines of high thrust force capability, including a 6/13 C-core LFSPM (6/13LFSPM-C) machine and a sandwiched C-core LFSPM (SLFSPM-C) machine, and a thrust force ripple reduction method is proposed. The proposed method is developed by reducing the slot effect component of the cogging force based on staggered stator tooth, and suppressing the thrust force ripple caused by unbalanced three phase back-electromagnetic forces (EMFs) based on two end PMs. Based on finite element analysis (FEA) results, both C-core LFSPM machines can achieve small thrust force ripples as well as high sinusoidal back-EMFs, and at the same time, maintain high thrust force capability with the proposed method. It was also found that, the improved SLFSPM-C machine exhibited the same thrust force capability as the improved 6/13LFSPM-C machine, but with a much smaller thrust force ripple. <s> BIB017 </s> Linear Machines for Long Stroke Applications—A Review <s> B. FLUX-REVERSAL MACHINES <s> Abstract Sustainable management strategies for securing long–term supply of rare earth elements is priority for Europe due to a complex and interlinked production chain and its dependence on Chinese export. Among rare earth elements, neodymium captured most attention due to its essential role in a wide spectrum of applications including green–energy technologies such as wind turbines and electric vehicles. Being complementary to primary production, end–of–life recycling would diversify neodymium supply, relieve the Chinese dominance on primary production and contrast the balance problem. However, neodymium recycling at end–of–life is not yet in place. In this work, we developed a dynamic material flow model to investigate neodymium stocks and flows in the EU–28 to 2016. The analysis enabled a detailed investigation of secondary sources of neodymium, which set essential boundary conditions for material recovery and recycling. We found that roughly up to 50% of the annual neodymium demand in the EU–28 could be met by domestic secondary supply, if latent recycling potentials were turned into actual capacity. Significant energy savings and GHG emissions cut could be also attained. However, product design, end–of–life collection, and scrap price issues are primary obstacles to neodymium recovery. Thus, unless going beyong those limits, establishing and maintaining a sustainable recycling chain for neodymium in the EU–28 will remain problematic. <s> BIB018
Linear flux-reversal machines, much like all the machines in the flux-switching family, are well known thanks to their high efficiency, thrust density and robustness BIB006 - BIB009 , BIB007 , BIB013 , BIB014 , BIB010 , BIB003 . In the case of LFRPMs, two PMs are mounted on the surface of each primary teeth with alternated magnetisation, as shown in Fig. 12 . Due to their placement, the flux from magnets is in series with the flux generated by the armature winding. Thus, these machines suffer from poor demagnetisation immunity under fault conditions BIB011 ,  . The operating principle of these machines is similar to the LDSPMs. Nevertheless, LFRPMs are able to link bipolar flux in their primary coils. Notice how the sense of the flux is reversed when the secondary tooth in Fig. 13 moves from one magnet to the adjacent one. So, higher thrust density values should be expected from LFRPMs when compared to LDSPMs. In this case, the natural back-EMF waveform of these machines is sinusoidal, thus, they are most suited to be driven in a BLAC operation BIB002 , BIB011 , BIB007 . The dq frame inductances of these machines generally display similar values, thus, the best option is to control them with the direct current, I d , set to 0 BIB007 , BIB011 , BIB007 . In the seek of performance improvements, several different solutions can be found in the literature. Due to the existing concerns around the price of rare-earth materials BIB018 , reduction of PM material is one of the main research fields concerning LFRPMs. In BIB002 the authors propose a consequent-pole configuration to achieve a 50 % reduction of the volume of PMs used in the machine. Moreover, the thrust capability with the consequent-pole configuration is also 22 % higher than the one from the original machine. Combining the consequent-pole topology with a yokeless structure, Gandhi et al. increase the average thrust force of the conventional LFRPM machine in 33 % BIB004 . A greater reduction of PM volume is achieved in BIB014 by adopting the consequent-pole structure and suppressing half of the primary teeth. Shi et al. BIB014 manage to achieve an improvement of 51 % of the average thrust while reducing 75 % of the PM usage volume with a large slot-opening LFRPM. Nevertheless, a more comprehensive research of this machine topology is needed, because it has not been found anywhere else in the literature. Improvement of the fault-tolerant capability is another area that has drawn attention of researchers. A LFRPM with a modular and complementary magnetic circuit is proposed in  . Thanks to the adopted structure, the resulting machine achieves a high fault-tolerant capability as the ratio between the mutual-inductance and the self-inductance is reduced to 4.6 %. Besides, a thrust ripple of 10.2 % and a detent force of 5.4 % are achieved by the proposed machine. A similar phase decoupling is also obtained in BIB006 thanks to the insertion of fault-tolerant teeth. However, the flux linkage of the machine in BIB006 is unipolar BIB008 , so Xu et al. employ another modular an complementary structure to reduce the aforementioned ratio to a 4.5 %, while improving the thrust density of the fault-tolerant LSFPM machine from 113.7 kN/m 3 to 158 kN/m 3 BIB008 . The improvement of thrust density is also researched in the literature. Shuraiji, Zhu et al. BIB007 , BIB007 - BIB013 employ a partitioned-primary structure to improve the thrust density of conventional LFRPMs. For machines with same dimensions and current density, a remarkable improvement of 118.9 % is reported in BIB007 . The authors also mention that due to the partitioned-primary structure, the PM immunity is also improved, as the flux from the primary coils is not directly applied to the magnets. Transverse-flux LFRPMs are also proposed in the literature in order to improve the thrust density of these machines BIB012 , BIB011 , BIB015 - BIB016 . A comprehensive analysis of a transverse-flux LFRPM is published in BIB011 . The presented machine can produce a thrust density of 153.5 kN/m BIB017 for the proposed LFRPM. It is a known issue that transverseflux machines require 3D FEM simulations to analyse their performance. In this aspect, Dong et al. BIB015 propose a 2D FEM equivalent model for the analysis of a transverse-flux LFRPM. The measurements obtained with a prototype show a good agreement between predicted and measured values. Although it is not mentioned in the literature for machines with a linear form, rotary flux reversal machines generally suffer from low power factor BIB005 , BIB001 . This is a characteristic that should be improved in order to enhance the feasibility of LFRPM drives.