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.

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 the paper is on maximising average torque and minimising torque ripple, using finite element-based design optimisation, in order to study the possibility of achieving acceptably low torque ripple. A subsequent investigation into the effect of rotor skew on the proposed optimised design to reduce torque ripple even further is also conducted, as well as the manufacturing and testing of the proposed flux barrier prototype. I. INTRODUCTIONWith the increasing emphasis on efficiency and cost reduction, the interest in reluctance synchronous machines (RSMs) has grown during the past decade. This interest is driven by not only the robustness, efficiency and simplicity of RSMs, but also by the fact that the cost of rareearth magnets is increasing and their market stability is decreasing.The main focus of most RSM design optimisation, depending on application, is on maximising average torque (T A ), within the limits of an allowable volume, and minimising torque ripple (T R ). The latter is conventionally achieved by rotor skewing and stator cording in order to reduce the air-gap harmonics that produce a high T R . In this paper, the possibility of both maximising T A and minimising T R without implementing rotor skew techniques to achieve acceptable T R values is investigated. A further investigation into the effect of rotor skew on the proposed topology to reduce T R values even further is also presented.A large part of the design of RSMs is focused on the rotor creation, and more specifically the type of flux barrier topology and its creation procedure. With the latter in mind, two creation techniques are summarised by Vagati [1], the first of which implements generalised lumped-parameter modelling of the rotor magnetic circuit and the second, a numeric design optimisation of a rotor flux barrier structure.The second procedure is implemented in this research. This procedure is based on a predetermined basic barrier structure with a fixed number of variable parameters, such as barrier tip angle and barrier width. These parameters are numerically optimised by implementing a finite element method (FE) package that calculates each iteration's relevant machine performance parameters, for example torque and torque ripple. Examples of this design procedure can be found in [2]-[5]. The advantage of this approach is that the optimisation inherently takes complex phenomena, such as torque harmonics and cross saturation, into account.In RSM design, three basic rotor topologies have emerged with combinations and small variations between specific

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 endeffects, 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 endeffects have on the stator load factor, a useful concept in the design of magnetically geared machines, is also investigated.978-1-4799-4389-0/14/$31.00 ©2014 IEEE Rong-Jie Wang (M'00-SM'09) received the MSc(Eng) degree from the University of Cape Town in 1998 and the PhD(Eng) degree from Stellenbosch University in 2003, all of South Africa. He has been affiliated with the Department of Electrical and Electronic Engineering of Stellenbosch University since 2003, where he is currently an Associate Professor. His research interests include special electrical machines, computer-aided design and optimization of electrical machines, computational electromagnetics, thermal modeling of electrical machines and renewable energy systems. He has published more than 40 journal and conference papers and was a coauthor of the monograph Axial Flux Permanent Magnet Brushless Machines (2nd ed., Springer 2008).

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