Methodology for Determining the Number of Rotor Bars in Induction Motors Considering Synchronous Torque and Vibration

Abstract

The squirrel cage induction motor (SQIM) has long been used in industry and is gaining renewed interest as a rare-earth-free traction solution for electric vehicles. However, selecting an appropriate number of rotor bars remains challenging due to its influence on performance, lack of standardized guidelines, and experimental difficulties. Literature identifies synchronous (parasitic) torque during start-up and vibration from low-order spatial harmonics of radial force as key concerns. Synchronous torque may cause start-up failure under direct-on-line (DOL) conditions or produce torque ripple even at rated slip, while low-order spatial harmonics of radial force can induce mechanical stress and shaft system damage. Despite various studies, no unified or practical method has emerged. This study presents a mathematical framework linking rotor bar count to synchronous torque and the lowest spatial harmonic of radial force. Based on this, an integrated selection guideline is proposed and validated through theory and finite element analysis across multiple pole/slot combinations, offering insights for reliable SQIM design in both conventional and inverter-fed systems.