Sensorless Control of Low-Power Brushless DC Motor Based on the Use of Back-EMF

The method of sensorless control of brushless DC motor with permanent magnets at 120 degree switching of windings is proposed. To approximate the trapezoidal function of the back-EMF rotation, an approximation based on the piecewise linear function and the inscribed ellipse is proposed. This continuous function also has a continuous derivative, which is an advantage when modeling and calculating the switching moments of the motor windings. Switching of windings is made at equality of back-EMF of the switched-off and switched-on windings. The equations of electrical equilibrium of the motor windings are converted to a two-phase system. Switching moments are determined using a function that depends on the ratio of the back-EMF calculated for the transformed system. This function does not depend on the speed of rotation and can be used in a wide range of speeds. The start-up of BLDC with the subsequent transition to Sensorless control of switching of windings was investigated. The efficiency of the proposed method is confirmed by a computational experiment based on the simulation of a three-section brushless DC motor with permanent magnets.