Magnetic gears (MGs) are potential alternatives to conventional mechanical gear boxes. With high performance permanent magnets (PMs) and new topologies, today MGs are capable to transmit a torque that is competitive with mechanical gear boxes in a totally contact-free way. MGs can be categorized by their flux orientations, namely, radial flux magnetic gear (RFMG), axial flux magnetic gear (AFMG) and transverse flux magnetic gear (TFMG). Their torque densities found in the literature are summarized in Fig. 1. The ratio between axial length and outer diameter of the active parts is defined in order to describe the geometrical characteristics of MGs. Fig. 1 shows that torque densities of RFMGs range from 50 Nm/L to 160 Nm/L. A very high torque density was achieved by a flux focusing AFMG [1]. To make a clear comparison in Section III, an AFMG model is simulated by 3D finite element method (FEM) using the same dimensions as those in [1] and the resultant torque density is about 260 Nm/L (design 1 in Fig.1).