Effect of air gap between PM segments on cogging torque and acoustic noise of BLDC motor

Abstract

Cogging torque is one of the major excitations of acoustic noise in Brushless DC(BLDC) motors. Many researchers have investigated the characteristics of cogging torque and its effect on acoustic noise. The driving frequencies of cogging torque in BLDC motors are the least common multiples (LCM) of the pole and slot numbers [1]. Ring type PM is widely used in small motors, and segmented PMs are usually applied to large motors as shown in Fig. 1(a) since it is difficult to manufacture a large ring PM. However, the circumferential air gap between PM segments generates the variation of tangential reluctance, and introduces additional harmonics to the cogging torque of BLDC motors [2]. Acoustic noise of BLDC motors originated from PM segments has not been reported. This research investigates numerically the characteristics of the cogging torque in a BLDC motor caused by the air gap between PM segments, and proposes a design method to reduce the effect of the air gap between PM segments on the cogging torque and acoustic noise of a BLDC motor.