Research On Sensorless Control Method Of Brushless DC Motor Based On Line Back EMF Estimation

Permanent Magnet Brushless Direct Current(PM BLDC)motors have many advantages over the brushed DC motors such as high efficiency,power density,reliability,long operating life,wide speed range and simple control method,which have been widely used in a variety of applications especially in electrical tools.The PM BLDC motors are inherently electronically controlled and require rotor position information for proper commutation of currents in its stator windings by using the position sensors.However,the use of position sensors not only increases the motor's size and cost,but also limits the application environment of the motor where the reliability is of most importance.Therefore,this paper has carried on a thorough research to the sensorless control methods for BLDC motors.Firstly,the motor starting strategy based on the short-time-pulse detection is studied.The operation principle of the BLDC motor is analyzed in detail from three aspects including the motor structure,the mathematical model and the commutation control principle.The relationship between the equivalent inductance of stator windings and the initial position interval of the rotor is discussed,and then the principle and the control methods of the initial rotor position estimation as well as the position closed–loop starting are presented.The simulation model of three-phase BLDC motor control system is established using MATLAB/SIMULINK tools to verify the motor starting performance of the above method.Secondly,the commutation principle of the sensorless control based on the line back electromotive force(back-EMF)is analyzed.The calculation formula of the line backEMF is simplified properly,which only includes the line voltage,the phase current and the stator winding resistance of the motor.At last,the commutation rules for different positions of the rotor are presented.Thirdly,the commutation error in the proposed control method is discussed in detail and the corresponding compensation schemes are put forward respectively.The phased delay of the low-pass filter is considered,and a new compensation strategy based on the motor speed for “60°-?”&“120°-?” is come up with in this paper.The simulation results verify the effectiveness of this compensation strategy.The influence of the PWM drive on the estimation of the line back-EMF is taken into account.The freewheeling circuit of the inactive phase under the control of the PWM-ON mode is given and the variation of this freewheeling current as well as the estimation error is quantitatively analyzed.The simulation results show that the PWM-ON-PWM mode can reduce the commutation error effectively and achieve a more precise commutation.Finally,the hardware structure of the control system is briefly introduced.The program for motor starting and sensorless drive is designed.The experimental platform for BLDC motor sensorless control is constructed to verify the feasibility and correctness of the proposed control strategy.