Investigation of Cogging Torque in Permanent Magnet Homopolar Inductor Machines Based on Air-Gap Field Modulation Principle

Cogging torque can affect the performance of permanent magnet (PM) homopolar inductor machines (HIMs). In order to find the reduction methods of the PM HIM cogging torque, it is necessary to investigate its production mechanism and analytical model. In this paper, the production mechanism of the PM HIM cogging torque is revealed from the perspective of air-gap field modulation principle. It is found that the air-gap permeance of PM HIMs can modulate their air-gap magneto-motive force (MMF) and then a large number of modulated air-gap magnetic fields are generated. These magnetic fields with identical production condition but different rotation speeds or rotation directions can interact with each other, hence the production of the PM HIM cogging torque. By the combination of energy method and air-gap field modulation principle, the cogging torque analytical model of PM HIMs is derived. Based on the obtained analytical model, the methods for reducing the PM HIM cogging torque are further analyzed. Finally, a 48-slot/4-pole (48S4P) PM HIM is designed and exampled. The air-gap flux density of the 48S4P PM HIM is simulated by three-dimensional (3-D) finite element analysis (FEA). Based on the harmonic analysis for the simulated results, the correctness of the proposed cogging torque production mechanism is validated. In addition, a prototype of the 48S4P PM HIM is manufactured. The cogging torque in the prototype is obtained by the analytical model, 3-D FEA and experiments, respectively. The simulation and measurement results verify the correctness of the cogging torque analytical model. (c) 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.