Comparison of Analytical Models of Cogging Torque in Surface-Mounted PM Machines

Abstract

This paper compares four analytical models for predicting the cogging torque in surface-mounted permanent-magnet machines, viz., lateral force (LF), complex permeance (CP), and exact subdomain (SD) models, together with an SD model based on a single slot/pole. The models are evaluated for the air-gap flux density, cogging torque, back electromotive force (EMF), electromagnetic torque, saturation effect, and computational complexity, with particular focus on the influence of design parameters, such as the slot-depth-to-slot-opening ratio and the optimal pole-arc-to-pole-pitch and slot-opening-to-slot-pitch ratios, on the cogging torque. It shows that all studied analytical models have similar high accuracy for the fundamental back EMF and average torque. However, their predictions are quite different for the ripples in the back EMF, cogging torque, and electromagnetic torque waveforms. Overall, the SD models have high accuracy, while the exact SD model is the most accurate among the studied models by accounting for mutual influence between slots, albeit with high computational complexity. The computation of the SD models is slower than that of the LF model but can be quicker than that of the CP model. The comparison is validated by the finite-element and experimental results.