Analysis of the Influence of Permanent Magnet Parameters on Cogging Torque in Line Start Permanent Magnet Synchronous Motors

Abstract

Cogging torque in Line - Start Permanent Magnet Synchronous Motors (LSPMSMs) is influenced by various design parameters, among which the type and grade of permanent magnet (PM) play a significant role. This paper investigates the influence of permanent magnet (PM) parameters specifically magnet grade on the cogging torque, back - EMF waveform, and starting characteristics of a 2.2 kW Line - Start Permanent Magnet Synchronous Motor (LSPMSM). Three types of NdFeB magnets (N30, N35, and N40) were examined using finite element simulations conducted in Ansys/Maxwell. Simulation results show that increasing the magnet grade enhances the synchronization capability of the LSPMSM, reducing the starting time and speed deviation. The best performance was achieved with the N40 magnet, which provided a starting time of 0.92 s and a speed deviation of only 2 rpm under rated load. However, higher magnet strength also increased cogging torque amplitude, with N40 producing a peak cogging torque of 170 Nm over 12 times the rated torque. FFT analysis revealed that higher magnet grades also introduce stronger high - order harmonic components, leading to increased total harmonic distortion (THD) in both cogging torque and back - EMF. The study concludes that while high - grade PMs improve synchronization performance, they also intensify harmonic effects and mechanical vibration due to increased cogging.