You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Linear Machines for Long Stroke Applications—A Review 
Chapter Title:  C. POWER FACTOR 
1. Abstract of Cited Paper (BIB001):  Vernier permanent-magnet (VPM) machines are well known for high torque density but low power factor. This paper deals with the low power factor of VPM machines. The goal is not obtained by reducing the electrical loading or adjusting current advance angle but by proposing a novel vernier topology, i.e., a dual-stator spoke-array (DSSA) VPM topology. In this paper, the characteristics of the DSSA VPM topology, such as active part, auxiliary mechanical structure, and rotor anisotropy, are analyzed in detail. Performances are evaluated based on finite-element analysis, including power factor, torque density, and cogging torque. The results show that the DSSA VPM topology exhibits high power factor, viz., ~0.9, and significantly high torque capability. The verification of the mechanical structure scheme is also done in this paper. Finally, theoretical analyses are validated by the experimental results by a 44-rotor pole 24-slot DSSA VPM prototype. 
2. Abstract of Cited Paper (BIB002):  A novel interior permanent magnet linear vernier machine (IPM-LVM) is proposed and analyzed in this paper. Compared to the existing surface-mounted permanent magnet linear vernier machine (SPM-LVM) and the SPM-LVM with Halbach arrays, the proposed machine can offer higher flux density in both the air-gap and armature iron core, which lead to a higher back electric motive force (back EMF) and higher thrust density under the same mover volume and heat capacity. Besides, it is proven that the proposed IPM-LVM also has a lower demagnetization risk compared to the SPM-LVM. Furthermore, in order to reduce the thrust ripple of the proposed machine, the three-sectional structure is adopted in this paper, which makes the proposed IPM-LVM exhibit high thrust density and low thrust ripple at the same time. 
3. Abstract of Cited Paper (BIB003):  This paper proposes a new flux concentrated version of a consequent pole linear Vernier hybrid permanent magnet machine. The Vernier hybrid machine stator has alternatively magnetised adjacent magnets directly in contact with each other. A salient rotor then switches the direction of flux past the stator coils. Alternate magnets can be removed to make the consequent pole version, where inset rectangular magnets are magnetized in a common direction and separated by soft magnetic poles. In a flux concentrated consequent pole configuration, each magnet pole comprises of two magnets forming a V -shape. This topology hence possesses a combination of consequent poles, flux concentration and flux switching machine topologies. Magnetic flux concentration increases the airgap flux density, which potentially leads to an improvement of the force density and operating power factor. This paper compares the inset and V -shape consequent pole topologies using finite element analysis simulations. Both machines have the same dimensions, magnet volume, rated speed and current density. The simulation results imply that the V-shape machine offers higher electromagnetic performance. To confirm this, two linear prototypes are currently under construction, where the key parts that already produced are presented in this paper. 
4. Abstract of Cited Paper (BIB004):  This paper discusses the development of linear vernier hybrid machines for use in wave energy converters. A number of topology options are explored to reduce magnet usage and improve performance for use as a linear generator. Consequent pole, V-shape, and Halbach arrays are designed for a target 25 kW specification. A discussion of overall cost of energy produced is used to justify using efficiency and power factor to asses fixed magnet mass topology variants. A power factor improvement from 0.38 to 0.72 is presented. Physical integration of the electrical machine into wave energy converters is considered including flooded experimental results of thermal behavior and polymer bearing wear. 
5. Abstract of Cited Paper (BIB005):  In low-speed applications, variable-reluctance permanent magnet machines are often proposed due to their efficient use of magnet material and high torque density. This becomes even more important in large linear applications, where the translator is longer than the stator. Often, however, very low power factors are experienced by this class of machines. This paper proposes a V-shaped flux concentrated version of a consequent pole linear Vernier hybrid permanent magnet machine and compares it to a surface-mounted magnet variant. Using finite-element analysis validated by two laboratory prototypes, it is shown that the flux-concentrated version increases the airgap flux density, which potentially leads to an improvement in the force density and efficiency or can be used to increase the operating power factor. 
6. Abstract of Cited Paper (BIB006):  Cost-effective and high-performance linear motors attract more and more attention for many applications. This paper proposes a novel double-side linear flux-modulation permanent-magnet (DS-LFMPM) motor with low cost, high thrust force density, and high power factor. The novelty of the proposed DS-LFMPM motor is to improve power factor by using double-side topology. Its topology is presented, and its operation principle is investigated. From the harmonic spectrums of the flux density in the air gap before and after the modulation of the mover teeth and the stator teeth, it can be concluded that the proposed motor has good flux-modulation effect. Also, the expression of the orders of the harmonic spectrums is given. The design process of sizes and structure is particularly explained. The electromagnetic performances are predicted by using the finite-element method and the proposed motor is prototyped for verification. The results show that the proposed DS-LFMPM motor can produce high thrust force density and high power factor. 
7. Abstract of Cited Paper (BIB007):  This paper designs a double-side linear vernier permanent-magnet motor, which incorporates the merits of high thrust force capability, high power factor, and high efficiency. Since the parameters of the motor are very sensitive and the high thrust force capability comes along with the high force ripple, a framework of multiobjective optimization is proposed to improve the overall motor performances. The sensitivity analysis is used to select significant variables and the multiobjective differential evolution with ranking-based mutation operator is conducted with the response surface models to generate the Pareto solution set. The electromagnetic performances of the optimal motor are compared with the initial motor by simulating step by step based on finite-element analysis. Finally, a prototype motor is manufactured and the experimental results validate the improved motor performances.

Literature Review:
The low power factor is one of the main issues for LPPMs. So far, there are not many solutions that have been found to be effective in the literature. However, the usage of V-shaped IPMs BIB003 , BIB004 , BIB005 , BIB006 is a good alternative. In this study, this solution has not been found in LFRPMs and LSFPMs but both of them could benefit of this configuration. Another choice is the usage of a double-sided configuration with a series magnetic circuit BIB002 , BIB007 , BIB008 . As mentioned before, an impressive power factor of 0.945 has been reported in BIB008 , but there are no experimental results that validate this value. By means of example, a double-mover C-Core LSFPM is shown in Fig. 18 with a series magnetic circuit. It is a known issue that C-Core LSFPMs suffer from low power factor BIB001 , so the proposed topology can possibly improve the performance of conventional LSFPMs.