Research On Low Speed Sensorless Control Strategy For Permanent Magnet Brushless DC Motor

The permanent magnet brushless direct current motor(BLDCM)has been used by more and more users in industrial control because of its simple structure and easy control.In the control system of BLDCM,in order to commutate precisely,the mechanical sensor is installed on the rotor shaft for measuring the position and speed of the rotor.However,the installation of sensors will increase the cost of control system and reduce the system's reliability.Therefore,the sensorless technology of permanent magnet BLDCM has become a hot area in the field of electric system.At present,the sensorless technology at high speed of BLDCM is relatively mature.But the low speed sensorless research in two phase conduction mode is rare.In order to realize the sensorless direct torque control of BLDCM at low speed,a new method based on high frequency current injection is proposed in this paper.In this paper,in order to design the permanent magnet BLDCM quickly,the existing design parameter of three phase motor is used.By changing the stator winding and rotor electromagnetic structure,a 1.5Kw salient pole permanent magnet BLDCM is designed.Then,the performance simulation of designed motor is completed in the finite element analysis software Ansoft/Maxwell.Through the comparison between test and simulation,the correctness of motor design is verified further.In order to inject the high frequency current to the conducted stator winding,a double hysteresis loops DTC control structure of torque and high frequency current is adopted in this paper.The scheme ensures that the torque and high frequency current can be controlled at the same time without additional hardware functional modules in system.From the high frequency mathematical model of BLDCM,when a high frequency current through stator winding,the corresponding frequency of voltage component can be inducted.Because the salient-pole effect of motor,the inducted high frequency voltage component includes the information of rotor position angle.In this paper,through the inducted high frequency voltage,band pass filter,multiplier,low pass filter,adder and phase locked loop structure,the rotor position angle observer can be obtained.The BLDCM has the characteristics of commutation conduction,using the low pass filter will case the observed angle is not accurate in a period of time after the commutation.In order to resolve this problem,the observed position angle is replaced by predicted value when the observed angle is inaccurate.Then the smooth position angle can be obtained by the second phase locked loop structure.According to look up table by observed position angle,the observed rotor flux and back electromotive force(EMF)will be obtained.Then the stator flux can be obtained by the current model of stator flux.Through the observed EMF,stator flux observation and the phase current,the observed torque can be obtained.At last,according to using the observed values,the low speed sensorless direct torque control system of BLDCM can be build.By the simulation based on MATLAB/Simulink and the test based on DSP hardware platform,the dynamic and static performance of the control system proposed in this paper is well.