Design Considerations, Analysis and Comparison for High-performance Surface Permanent Magnet Vernier Machines

Permanent magnet vernier machines (PMVMs) exhibit the feature of high torque density, which makes them a compelling candidate for gearless direct-drive applications, such as electric vehicle (EV) and servo drive. However, the motor suffers from low power factor issue. And the long end winding, caused by overlaps and long spanning distance, significantly reduces the efficiency and torque density for motors with small size and short stack length. This paper investigates the performance metrics of PMVMs with surface Halbach magnet array for different slot/pole combinations (gear ratios) and winding layouts (short-pitch, fractional-slot). It is found that the fractional-slot winding creates more possibility for vernier machines. First, the working principle of vernier machines is analyzed and the performance metrics are modeled with analytical approach. Then, the motor configuration is classified according to winding layout and some promising structures are selected. Later, a comprehensive comparison is conducted for selected candidates in all electromagnetic performance by using finite element analysis (FEA). The results show that the vernier motors with fractionalslot winding layout and non-integer gear ratios can achieve much lower torque ripple, and higher power factor than integral-slot winding, while sacrifising a little active torque density. Designs with smaller gear ratios can achieve higher efficiency due to short end winding. These findings can provide design guidelines for high-performance PMVMs.