Research On Torque Control Strategy Of Brushless DC Motor With Non-ideal Back EMF

The mechanical commutation device,in Permanent Brushless Direct Current Motor(PBLDCM),is replaced by power electronic devices.The PBLDCM has the feature of simple structure,stable working and convenient maintenance,which can be comparable to the AC machines.In addition,The PBLDCM has been successfully utilized in the motion control for its high torque,excellent efficiency and good performance of speed regulation.With the optimization of machine body and improvement of control strategies,the PBLDCM will make more sense in the aerospace,military affairs and other fields.The torque ripple limits the application of Brushless DC motors.In order to solve the torque ripple during commutation,the three-phase cooperative modulation keeps the average current slope of on-going phase equal with the average current slope of off-going phase in each PWM period,but this strategy leads communtation to fault in high speed and heave load.In this paper a limiting condition of duty cycle in high speed is presented which make assures BLDC moters communcate smoothly.The simulation results show that the commutation torque ripple can be mainly eliminated and the commutation time can be adjusted flexibly.The torque ripple limits the application of Brushless DC motors in high precision and stability system.Some problems come out,such as the increasing torque ripple and the descending output torque descending can be caused by high rotated speed.With the analysis of torque ripple in the process of advanced commutation according to the equivalent model,we proposed a simple advanced commutation PWM modulation scheme.This controlling scheme brings the current point of intersection ahead to the Non-ideal BEMF's and leads the angle dynamically adjusted with the changing motor running state.Finally a control method process was provided and the MATLAB/Simulink simulation module was built.The simulation results verified the effect of the controlling method that can suppress torque ripple and descending output torque.