Single-phase brushless DC motors with an equal air-gap make them non-self-starting because they have coincident zero torque positions of excitation(called dead-point). Therefore, single-phase brushless DC motors have a tapered air-gap to allow self-starting. Accordingly, most of the stator(or rotor) shape is designed asymmetrically to solve this problem. Unfortunately, this method affects the cogging torque. Hence, single-phase brushless DC motor should be designed considering the cogging torque.
This paper presents an analytical method and a reduction method for the cogging torque of single phase brushless dc motor.
First, analytical method for cogging torque presented applies a tapered air-gap with notches for single-phase brushless DC motor. To calculate the magnetic flux density, the stator slot function was applied to express the tapered air-gap, and the spreading permeance function was calculated and applied to analyze the cogging torque. The analysis results of the cogging torque were compared using FEA to verify the proposed analytical method. The period and magnitude of the cogging torque of the analytical method were similar to the results of FEA and experiments.
Second, we presented cogging torque reduction method for sloping notch applied two notches on stator teeth. We have confirmed accuracy of FEA through comparison previous model using asymmetric notch for experiment and 3D FEA results, and then cogging torque comparison of 2 notches model and sloping notch model. Also the sloping notch model has been modified to step-sloping notch model to consider manufactur. Optimal design was performed that for cogging torque minimize considering the sloping degree, angle, position, size of the notches. As an optimal design result, the cogging torque on the optimal model was reduced. Finally, the analysis and optimal design results were confirmed by FEA.