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

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.