Analysis and Calculation of Commutation Error for Sensorless Brushless DC Motor

Abstract

The back EMF zero-cross detection method is widely used in sensorless position control. Since the back EMF cannot be measured, the terminal voltage is often used to approximate the back EMF. However, the terminal voltage is distorted caused by diode freewheeling current, filter delay and other factors, a phase difference will be generated between the terminal voltage and the back EMF. This phase difference makes position detection signal deviate from the best commutation time, even causes commutation failure in high speed or overload. This paper first analyzes the reasons of phase difference caused by diode freewheeling current and filter delay. Then, the relationship between phase difference angle, current and speed is presented through the simulation fitting method. Finally, simulation and experiment results prove the effectiveness of the calculation method. Moreover, from our study, the BLDCM can still achieve stable operation under sensorless control with different load torque and speed.