Sudden torque collapse in energy-harvesting BLDC permanent magnet generators

Brushless DC machines utilizing high energy neodymium magnets are commonly used as generators to produce electricity and provide braking torques to a shaft. We examine generators designed for lower varying operating speeds such as those used in micro-wind and human-driven dynamos. We focus on the phenomenon of a sudden loss of braking torque that occurs in smaller generators operated beyond a critical load and speed. We propose a new figure of merit beyond the traditional motor constant, K_m, as critical to proper design of a BLDC generator for variable speed applications.