Magnetization Analysis for Outer Rotor Brushless DC Motors Using Polar Anisotropic Ferrite Ring Magnets

Ferrite-bonded magnets have been extensively used in small brushless DC (BLDC) motors for low torque applications such as fans and pumps. In particular, some motors of them with ferrite-bonded magnets, frequently called as plastic magnets, do not have a rotor core in order to reduce manufacturing costs and inertia of the rotating part. Accordingly, the magnetization distribution of the bonded magnets has a polar anisotropic direction to secure more magnetic flux. As a result, the magnetization orientation of the magnets is highly significant to accurately predict the performances of the BLDC motors. This paper deals with the magnetization analysis of the polar anisotropic ferrite ring magnet applied to an outer rotor type BLDC motor. The motor is employed for an electric water pump, and its rotor including the impeller consists of only the ferrite-bonded magnet material and made by injection molding. Consequently, a magnetizing fixture and an impulse magnetizer are not used for the magnetization of the outer rotor type motor. Instead, samarium-cobalt (SmCo) magnets having a relatively high curie temperature are applied for generating magnetic field. Finally, this paper presents a process to determine the anisotropic orientations of a ferrite ring magnet by finite element method. In addition, the validity of the analysis method is verified by test results.