Analysis of the end-effects in magnetic gears and magnetically geared machines

Abstract

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).

Magnetic gear technologies: A review

“…Fig. 5 shows the effect of the modulator thickness on the maximum torque of an MG [49]. Both 2D and 3D FE results confirm the existence of an optimum thickness corresponding to the stall torque of the MG.…”

“…The studies reported in [61] (2D FE) and [49] (3D FE) confirm that for an MG with fixed outer diameter, there exists an optimum gear ratio where torque capacity reaches a maximum.…”

“…This is due to the fact that 2D FE models ignore the 3D end-effects within a magnetic gear. The mechanism of these 3D endeffects within MGs are studied in [47], [49]. Fig.…”

“…However, as explained in [62], severe deflection of the modulator is likely to happen with long stack lengths. It has been shown in [49] that aspect ratio has the greatest impact on the end-effects of MGs and that a critical aspect ratio exists, above which the 3D end-effects are significantly reduced.…”

Cogging Torque Mitigation in Axial Flux Magnetic Gear System Based on Skew Effects Using an Improved Quasi 3-D Analytical Method

“…7(a) by dashed line. It can be seen that, some of the flux coupling to the ferromagnetic polepieces does not penetrate through it, but returns to the same rotor as leakage flux [21]. On the other hand, the flux density concentration at the edges of ferromagnetic pole-pieces ( Fig.…”