Linear Machines for Long Stroke Applications—A Review <s> A. DOUBLY-SALIENT MACHINES <s> A linear doubly salient permanent magnet (LDSPM) motor is particularly suitable for long stator applications due to its simple and low cost stator, which consists of only iron. This paper proposes a new LDSPM motor design with complementary and modular structure. The key of this structure is that the primary mover is composed of two modules whose positions are mutually four and one half of the stator pole pitch apart and there is a flux barrier between them. Hence, the back electromotive force (EMF) waveform and cogging force of the two modules have 180 electrical degree differences. This design results in the total cogging force being significantly reduced and the back-EMF of each phase becoming symmetrical because the even harmonics are canceled. For fair comparison, an existing linear LDSPM motor is designed based on the same electromagnetic parameters and compared by the means of finite element analysis (FEA). The results reveal that the proposed LDSPM motor can offer symmetrical back-EMF waveforms, smaller cogging force, lower force ripple, and higher magnet utilization factor than the existing one. <s> BIB001 </s> Linear Machines for Long Stroke Applications—A Review <s> A. DOUBLY-SALIENT 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> BIB002 </s> Linear Machines for Long Stroke Applications—A Review <s> A. DOUBLY-SALIENT MACHINES <s> This paper overviews various switched flux permanent magnet machines and their design and performance features, with particular emphasis on machine topologies with reduced magnet usage or without using magnet, as well as with variable flux capability. In addition, this paper also describes their relationships with doubly-salient permanent magnet machines and flux reversal permanent magnet machines. <s> BIB003 </s> Linear Machines for Long Stroke Applications—A Review <s> A. DOUBLY-SALIENT 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> BIB004 </s> Linear Machines for Long Stroke Applications—A Review <s> A. DOUBLY-SALIENT 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> BIB005
Doubly-salient machines, are commonly grouped within the flux-switching machine family, together with LSFPMs and LFRPMs. All these share a similar topology and working principle, but there are some slight differences in both their operation and performance aspects. Doubly-salient machines are an already very mature technology BIB004 . They consist of a primary with a concentrated winding and PMs inserted in the primary yoke, and a doubly-salient passive secondary. Figure 9 shows one of the machines presented in BIB001 . Overall, the LDSPMs exhibit a great PM immunity and robustness BIB001 ,  , BIB004 , BIB002 . Together with the high thrust density and efficiency achieved thanks to PM material, LDSPMs offer really interesting capabilities for applications where high reliability is the main requirement. Regarding manufacturing costs, LDSPMs require the lowest amount of PM volume between flux-switching machines for a given average thrust value BIB002 . Besides material cost, LDSPMs are also easily manufactured BIB002 , which is also another important benefit of these machines. In general, these machines generate a trapezoidal waveform of the back-EMF. Thus, they are naturally suited for a Brushless DC (BLDC) control operation BIB001 ,  . Nonetheless, the performance of LDSPMs is strongly affected by the detent force and on load thrust force ripple BIB001 ,  . Moreover, the unipolar nature of the flux in LDSPMs is a known issue BIB003 , BIB005 , BIB001 , BIB002 . Notice how the flux linked by a coil in Fig. 10 keeps a constant direction over a full period. This reduces the force capability of these machines BIB003 , BIB004 . However, LDSPMs are still an interesting solution for long stroke applications, and their usage in applications such as urban rail transit has already been proposed in some research papers BIB001 ,  . Not many publications involving LDSPMs have been found in the literature. In BIB001 discussed later on, to reduce the remaining ripple of the machine. In this case, the proposed LDSPM achieves lower thrust density and shear stress values with 66 kN/m 3 and 5.12 kN/m 2 respectively, which are rather low for PM excited machines. The authors propose the usage of a BLDC operation for the unskewed configuration, and a BLAC operation with all the current injected to the q-axis when a skew is applied to the machines.