Research On Torque Ripples Reduction Of Direct Torque Controlled BLDC Motor

Direct Torque Control was firstly proposed for induction motor,then it was developed for permanent magnet synchronous motor whose waveform of the back EMF is sinusoidal wave.With much more study on Direct Torque Control,the theory of Direct Torque Control was constructed and the Direct Torque Control systems of some other kinds of motor were proposed.One of the advantages of Direct Torque Control system is that it can response very quickly,but one of the disadvantages of it is that more torque ripples can be produced.So the goal of this paper is to improve the Direct Torque Control system of Brushless DC motor at the point of torque ripple resistance and the motor was working under two phase conducting mode.The outline of the research is as follows:Firstly,the definition of zero voltage vector is different among current research results,so three different definitions of zero voltage vectors were concluded,their influence on the currents were analyzed and their effectiveness on reducing torque ripples were compared.It was found that the third kind of zero voltage vector defined was better to resist the torque ripples.Secondly,it was found that the type of using the zero voltage vectors may lead to torque ripples,so a method to reduce the torque ripples was proposed.This method uses two zero voltage vectors in one sector respectively,which not only can improve the reliability of the system,but also be suitable for the case of clockwise rotation and counter clockwise rotation.The results of the simulation and experiments proved the effectiveness of the improvements.Thirdly,the duty ratio modulation technology was introduced into the Direct Torque Control system to reduce the torque ripples,the effectiveness of four kinds of duty ratio producing methods were compared.The four kinds of duty ratio producing methods are the PI control method,the final value method,the mean value method,and the effective value method.The results of the simulations and experiments showed that the duty ratio modulation technology can reduce torque ripples effectively and the effect of the mean value method was not better than the other three methods.