Position Sensorless Brushless Dc Motor Position Detection Starting Problems

Based on the investigation of demerits of the "improved free-wheeling diode method", an improved rotor-position detecting method of sensorless brushless dc motors (BLDCM) is proposed, which is referred to as improved free-wheeling diode method with complementary PWM mode. Theoretical analyses and comprehensive investigations by simulation and experiment are conducted. Then in order to solve the common problem of difficult startup of sensorless BLDCM, the starting algorithm referred to as linear acceleration method under limited peak current is presented. The start process is studied by simulation in detail. The theoretical results are verified experimentally. In addition, a prototype control system of sensorless BLDCM with proposed rotor-position detecting method and starting algorithm is constructed based on TMS320LF2407A. All software and hardware are designed and made by the author.The paper makes deeply theory analysis for the improved free-wheeling diode method, finding that although the improved free-wheeling diode method's accuracy and reliability is better than the traditional terminal voltage method, it has some demerits: when the current flowing from the Power MOSFET varies differently, the rotor-position error becomes larger; when the motor has light load resulting in the discontinuum of the current, the correct rotor-position can not be detected. Based on the investigation of demerits of the improved free-wheeling diode method, the paper proposes an improved rotor-position detecting method of sensorless BLDCM, which is referred to as improved free-wheeling diode method with complementary PWM mode.The improved method solves effectively the detection error and also can detect the precise rotor-position when the motor has light load.Simuliation and experiment study are made which confirm the theoretical analysis of the presented technique.In order to make further research about the motor start process and starting algorithm, the model of the driver and control system of sensorless BLDCM in matlab is builded in the paper. Based on the investigation of previous starting approaches, the starting algorithm referred to as linear acceleration method under limited peak current is presented.As compared to previous approaches,the presented technique solves the common problem of difficult startup result of the motor's torque and moment of inertia...