This paper reviews the historic and current development of magnetic gear technologies. There has been considerable amount of research and development activities on magnetic gear technologies in recent years. However, it is still not clear if this technology is ready for potential industry applications. The purpose of this paper is to summarize the historic and current development work of magnetic gear technologies in an attempt to give readers some insight into the advantages and disadvantages, challenges, opportunities and technology readiness.

Magnetic gears and integrated magnetically geared machines have recently received much attention due to their exceptionally high torque densities. As in the design and analysis of conventional electrical machines, the finite element method (FEM) has been widely used for this class of electrical machine. For the sake of computational efficiency, the standard practice in FEM modeling is to reduce the problem complexity by applying only two-dimensional (2D) modeling and exploiting periodicity, if any. However, it has been shown by many researchers that there exists a significant discrepancy between the results from two-dimensional and three-dimensional (3D) FEM for these machines. Whilst it is generally believed that this discrepancy is due to severe end-effects, there has been no published work specifically looking into the end-effect mechanisms at work in these machines. This paper attempts to identify the origins of these end-effects and explain why they can be especially significant in magnetic gears and magnetically geared machines. The effect of some critical design parameters on the stall torque is investigated by using both 2D and 3D FEM. Special attention is given to the difference between 2D and 3D results, which helps to establish whether or to what extent 2D FEM can be utilized for the design analysis of these special machines. The impact that end-effects have on the stator load factor, a useful concept in the design of magnetically geared machines, is also investigated.

In this paper, an investigation into an alternative topology for reluctance synchronous machine rotor flux barriers is presented. The investigated topology employs a high number of flux barrier variables with an alternative asymmetric rotor structure. The focus in this paper is on maximizing average torque and minimizing torque ripple, using finite element-based design optimization, in order to study the possibility of achieving acceptably low torque ripple. A subsequent investigation into the effect of rotor skew on the proposed optimized design to reduce torque ripple even further is also conducted, as well as the manufacturing and testing of the proposed flux barrier prototype.

In recent years, the permanent magnet Vernier machine (PMVM) has attracted much attention in the research community. Many studies have shown that PMVMs possess superior torque density and are suitable for high torque and low speed applications. Some of them compared PMVMs with the direct-drive permanent magnet synchronous machines (PMSM), but these studies focused mainly on sub-kW power level. There is little work in literature that conducted the comprehensive evaluation of PMVM technology for more specific applications. This paper presents the design optimization of a 15kW surface-mounted PMVM and its comparison with an equivalent conventional PMSM for wind generator applications. It attempts to objectively weigh the relative merits of the two wind generator systems and outline their respective advantages and disadvantages. For validation purposes, an optimally designed 15kW PMVM was constructed and experimentally evaluated, which is one of the largest PMVM prototypes ever reported in literature. It shows that the PMVM can be a competitive alternative to the PMSM for this application.

This paper presents the design and evaluation of a magnetically geared permanent magnet (PM) machine with an inner stator. A brief overview of relevant operating principles is given first. A simplified design and simulation methodology, which can ensure that the magnetic gear and the stator are well matched, is then devised. The method is applied to the design optimization of a small machine resulting in a design with a maximum torque density of 115 kN · m/m ³ per active volume. To validate the design, a working prototype has been built and experimentally evaluated. It shows that this computationally efficient design methodology is well suited for the optimization of magnetically geared PM machines. Finally, a method of analyzing the operating points of the machine is described. Relevant conclusions are drawn and recommendations for future work are given.