Detection and analysis of additional cogging torque harmonic components in permanent-magnet synchronous motors

Several high performance applications like power steering and robotics require motor drives that produce smooth torque with very low components of the cogging torque. Though there is a variety of techniques known to reduce the cogging torque in permanent-magnet synchronous motors, when requirements are very stringent, minimization of cogging torque becomes a challenging task. The paper presents solutions for reducing the cogging torque. Theoretically, it is possible to minimize or eliminate the cogging torque when proper methods are employed. The FFT analyses of the cogging torque in mass-produced motors have shown additional harmonic components which are in correlation with assembly tolerances and/or permanent-magnet imperfections. A finite-element method (FEM) was used to study sensitivity of different motor models in terms of manufacturing tolerances and to determine rules for detecting imperfections in mass-production. Chapter 2 describes fundamentals about the cogging torque by introducing natural cogging torque components (NHK). The number of cogging cycles per slot pitch, denoted as F, is also defined. Generally, motor designs with a higher F value are preferred as the cogging torque results in a lower peak value and higher frequency. Nevertheless, such motors are more sensitive to the phenomenon of additional harmonic cogging torque components (DHK) than the ones with lower values of parameter F, which is an important fact for manufacturers of permanent-magnet motors. Chapter 3 presents in details the developed theory about additional cogging torque components effected by production tolerances and manufacturing imperfections resulting in stator and rotor anomalies, which are typical for mass-production. In chapters 4 and 5 some simulation and experimental results of a 27-slot 6-pole motor are discussed. They illustrate theoretical thesis given in previous chapters proving introduced expressions (1)-(15). Geometrically measured irregularities of mass-produced motors are applied to create a realistic FEM model thus enabling an adequate comparison between the simulated and experimental results. On a base of numerous FEM simulations and FFT analyses along with experimental results, it follows that manufacturing assembly tolerances and material imperfections cause the phenomenon of additional cogging torque harmonic components in accordance with the presented expressions. Considering introduced theory motor designers are able to predict the entire cogging torque harmonic spectrum, thus predetermine required manufacturing tolerances to minimize cogging torque and fulfil the market demands.