Research On The Start-up Control Of Sensorless Brushless DC Motor

Developed on the basis of the DC motor, brushless DC motor not only has speed controlperformance of the DC motor, but also has high efficiency, reliable operation, and longservice life. The method based on back electromotive force (EMF) is the most widely used forrotor position estimation, becoming an important trend of the development of BLDCM.As the back-EMF method used for BLDCM has insensitive detection from standstill tostart up, the paper proposes a novel method to start BLDCM. In the initial rotor positionidentification stage, through the three-phase secondary winding conduction positioningmethod, the rotor is simply and effectively positioned from unknown position to the targetposition with a good stability. When motor is accelerated to start, the back-EMF can not bedetected. Through open-loop acceleration method, the motor is accelerated to a speedthreshold which is back EMF can be accurately detected in a short time. In the low-speedoperation stage, back EMF method is not sensitive. By quantitative and qualitative analysis ofthe relationship between back EMF and induced voltage of non conduction phase, theinducted voltage zero crossing detection method is designed. In the circuit, JFETs are cleverlyused to divide voltage, and low resistances are used to increase induced voltage. Experimentsshow that the new zero crossing detection has high accuracy and great anti-interferenceperformance, extends the sensorless detection range in the low speed as well. When motor isoperated at high speed, the circuit is switched to the back EMF zero crossing detection.In the end, a sensorless control system of BLDCM based on PIC microcontroller andMPLAB IDE is built to verify the proposed methods. Through hardware and softwareplatforms, the new start-up method under constant load conditions is proved to start fast andhave good stability at low speed.