Phase Back EMF Oriented Control Of Brushless DC Motor For Torque Ripple Minimization

With the technique of modern power electronics combining with control theory and electric motor technology, brushless DC motor(BLDCM) has been invented, and it is a new kind of electromechanical integration product which using electronic commutation in place of mechanical commutation, because of this, the peculiarity of the torque- current for brushless DC motor is similar to the peculiarity for the traditional dc motor, and compared to the traditional dc motor, the reliability of the brushless DC motor system has been improved, the useful life has been extended, and it has been widely used in some high speed and high precision occasions. The traditional control mode of BLDCM is two-phase 120-degree conduction, but the non-ideal trapezoidal back-EMF waveform can cause obvious torque ripple when the 120-degree square wave phase current has been taken in, in addition to this, because of the influence of phase current inductances, the non-ideal commutation of phase current also can cause obvious torque ripple. BLDCM is a special permanent magnet synchronous motor, and the difference between them is the waveform of the back-EMF, the ideal back-EMF waveform of PMSM is sine wave, and the ideal back-EMF waveform of BLDCM is trapezoidal waveform. Therefore, using for reference to the vector control of PMSM, and considering the characteristic of the non-ideal trapezoidal back-EMF waveform, this paper has proposed a new vector control approach of phase back-EMF oriented. In this approach, the relational expression of torque- current can be obtained by easily vectorial resultant and equation deduction, and completed the study of the minimization of the torque ripple.Firstly, this paper introduces the development and application field of brushless DC motor, and analyzes the structure and operation principle, presents its mathematical model, then introduces the application of vector control in PMSM. Based on these studies, this paper proposes a new vectorial resultant approach of back-EMF, and gains the conclusion that electromagnetic torque is direct proportion to the q-axis component of the resultant current vector, then gains the relational expression between q-axis component and the three-phase currents, it’s easier than the transformation of coordinates of traditional vector control. The value of the d-axis component of the resultant current is depended on the copper loss of the motor.Secondly, simulations by Matlab are presented in this paper, including current closed-loop control simulation and the double-loop system simulation with the current as the inner-loop and the speed as the outer-loop.Finally, in order to further verify the correctness of the proposed strategy, experimental platform is built with DSP28335 as the control core, experimental result coincide with simulation result, and these results are presented to verify the feasibility and effectiveness of the proposed strategy.