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

Due to the advantage of simple driven, high efficiency, and high power density，brushless DC motor is used more and more widely.The traditional BLDCM control systemuses hall effect sensors to get rotor position signal. However,installing the hall effect sensorinside a BLDCM is not only a difficult work, which will increase the manufacturingcosts,but also that the sensor will easily fall down when the motor is running.So the researchon the sensorless control of BLDCM has important theoretical significance and practicalvalue.The method based on back electromotive force (EMF) of BLDCM is the most matureway for rotor position estimation because of the advantages of simplicity and reliability. Thetraditional zero-crossing of back EMF detection methods requiring a voltage divider networkconsist of resistors and capacitors to form the floating phase’s back EMFsignal,however,this method will not only bring signal attenuation problem,but also willbring the phase angle shift problem,so in low speed case,it can not get the zero-crossingsignal accurately.In this thesis,a new zero-crossing detect method and its electric circuitdesign is proposed,which have the advantage of great anti-interference performance and highsensitivity,and is proved to be work well when the motor is running in the speed as low as100r/min,it really broader the back EMF method’s speed range.However,when the motor isstarting up,its initial velocity is0,or nearly0,and the back-EMF voltage amplitude is0,andwe cannot get the rotor position under this case,we must use another method to start up themotor without position sensor, after a systematic and theoretical analysis,we found that underthree-phase-six-state control strategy,the two stator windings which have current flowingthrough will produce induced voltage in the floating winding,and the induced voltage havethe same zero-crossing with the back EMF signal. So we proposed a new start-up method,when the motor is starting up,we connect resistors in series with the stator windings, whichwill reduce the time constant of the stator windings,this method will increase the amplitudeof the induced voltage in the floating winding,and then we can detect the zero-crossing of theinduced voltage to get the rotor position information,once we get the rotor positionaccurately,we can start up the motor smoothly.After the motor is started up, the resistance inseries will be short-circuit using electronic switch,and then we detect the zero-crossing ofback EMF to get the rotor position information.In the end,the hardware and software design of the sensorless control of BLDCM basedon PIC24HJ128GP306is introduced in details, numerous experiments are given in order to illustrate the efficiency of this new start-up method,and it is proved to work well wheneverthe motor is running under no load,light load or heavy load,and this method is proposed tosolve the start-up problem of the brushless DC motor without position sensor.