The Research Of Brushless DC Motor Double-loop Speed Control System For Torque Ripple Minimization

Brushless DC Motor(BLDCM) is a new kind of special dc motor using electronic commutation in place of mechanical commutation. BLDCM has the advantage of high efficiency and high power density, good speed regulation performance and simple structure. It is these advantages make BLDCM widely used in various fields, which is considered to be one of the most promising motor. The shortcomings of BLDCM limits its development. In the traditional double closed-loop speed control system, the Brushless DC Motor with un-commutated phase current feedback has large torque ripple because of non-ideal trapezoidal wave back EMF and non-ideal square wave phase current. Torque ripple suppression has attracted much attention all over the world. This article mainly analyze the torque ripple of BLDC motor control strategy.Firstly, this paper introduces the development of brushless DC motor and application field, analyzes in details on structure and operation principle, present its mathematical model.Secondly, this paper analyze the reasons of torque pulses produced by BLDCM. this paper introduces resultant current variable, which is proportional to instantaneous torque, and derives the relational expression between the resultant current and the un-commutated phase current, then the torque ripple reduction can be achieved by controlling the un-commutated phase current corresponding to the resultant current. In addition, to reduce the torque ripple in the commutation region, a new modulation method based on PWM-on is adopted. With this modulation method, the rate of change of current in the upcoming phase can be controlled to equal to that of the outgoing phase during the commutation region by modulating three phase cooperatively, in which the duty ratio in each phase is calculated and distributed simultaneously.Thirdly, simulations by Matlab are presented in this paper, including open loop control simulation, current closed-loop control simulation, Resultant Current control simulation, Resultant Current control simulation with new modulation method during the commutation region simulation. Through these comparisons are presented to verify the feasibility and effectiveness of the proposed strategy.Finally, in order to further verify the correctness of control method, experimental platform is built with DSP28335 as the control core, experimental result coincide with simulation result.