Torque Control Of Brushless DC Motor

Brushless DC motor, which not only maintains the traditional DC motor superior speed performance,but also solves a series of problems inherent in traditional mechanical DC motor commutation and brush caused, has wide application in all areas of modern society.The maximum current commutation torque ripple can be achieved 50% of the torque output, while Brushless DC motors works in the two-phase 120° conduction mode. The main reasons for the commutation torque ripple is the presence of inductor that leads to current rising and falling rate not match, current fluctuations result in an output torque ripple.Commutation torque ripple has become the hot point of brushless DC motor, commonly used method of suppressing the torque ripple ara overlapping commutation method, hysteretic current France, PWM chopping method and current predictive control.In this paper, a low-cost brushless DC motor torque ripple suppression without position sensors was studied. Firstly, principle and characteristics of the brushless DC motor is analyzed, followed by a mathematical model and simulation model of a brushless DC motor as a theoretical basis for suppression strategies, then analyze other suppression strategies of BLDC, Finally, the terminal voltage detection commutation torque ripple suppression is proposed. The key to this method is to measure the duration of the commutation, then in this duration another PWM duty ratio can work, at the end of the commutation PWM duty ration should be reset to the former one. In this method, terminal voltage minus half bus voltage,through Zero-crossing comparator, the result can show the duration of commutation.According to the terminal voltage of commutation and non-commutation, PWM duty ratio during commutation can be calculated, and thus the duty ratio applied to the commutation period could reduce the torque ripple. Simulation results show that the application of this method can effectively suppress torque ripple during commutation compare with the application that does not use this method. This method can effectively suppress current waveform spikes, verified the effectiveness of terminal voltage detection method for commutation torque ripple suppression. Based on FPGA chip to complete the system function module design, achieved terminal voltage detection module, commutation signal generation module, commutation PWM duty ratio calculation module. Completed a brushless DC motor speed detection system design, experiments and hardware debugging, the experimental results are given. This paper, not only for improves operational performance of brushless DC machine, but also presents problems in the torque ripple suppression and points out the direction.