I. Introduction
In low-speed high-power applications, direct-dive permanent magnet (PM) machine systems are becoming more and more popular. However, the high torque demand on direct-drive PM machines always leads to large machine size and material consumption. In recent years, flux modulation machines, such as vernier PM (VPM) machine and magnetic geared machine, are gaining much interest due to their inherent high torque density [1]. Compared with magnetic geared machine, VPM machine is constructed with only one airgap and has more compact mechanical structure [2]. Therefore, VPM machines have become promising candidate for low speed, high torque applications such as wind power, electric propulsion and robotic servo system.