Cogging torque and torque ripple reduction of a novel exterior-rotor geared motor

The reduction of cogging torque and torque ripple in permanent-magnet motors to suppress the vibration and acoustic noise is a major concern for motor designers. This study presents a novel exterior-rotor geared motor which integrates a brushless permanent-magnet (BLPM) motor with an epicyclic-type gear reducer to form a compact structural assembly without extra transmitting elements. One of the special features of the geared motor lies in the gear-teeth of the epicyclic-type gear reducer merged with the stator of the BLPM motor. The gear-teeth serve as the interfacial medium to connect the BLPM motor with the epicyclic-type gear reducer, which provides functions not only for transmission to achieve a desired speed ratio, but also effectively reduce the cogging torque and torque ripple of the geared motor. Five shape models of pole shoes with different values of the shoe depth and the shoe ramp are presented to effectively reduce the cogging torque and the torque ripple. With the aid of the finite-element analysis, shape model III of the geared motor performs better than the existing BLPM motor on the cogging torque with 87 % decreasing and the torque ripple with 23 % decreasing. Such a unique characteristic of the geared motor is of benefit to the widely applications on accurate motion and position control systems.