Study On BLDCM Sensorless Control System

Brushless DC Motor(BLDCM) is a kind of electromechanical integration machinery,and it has been rapidly developing in recent years with the development of power electronic technique and the appearance of new type of permanent magnet material.It takes electrical commutator instead of mechanical brush and mechanical commutator.To control the speed of a BLDCM,hall sensors are usually used for speed sensing and position sensing.However,it would be more economical,if position sensors can be removed from the motor.Thus,this thesis proposes the sensorless control techniques for BLDCMs.This paper details working principle and characteristics of sensorless BLDCMs,analyses the hardware circuit design and software programs design of this control system,introduces disturbances of the hardware and disturbances of the software,presents methods to eliminate these disturbances.This paper introduces "the three-step"start technique which including the rotor pre-setting, accelerating and state-changing step is discussed carefully.Phase change and speed calculation make use of traditional "Back Electromotive Force(BEMF) method",and the principle of "BEMF method" is dissertated in detail.In addition,the zero-crossing of BEMF sensing method is concretely studied in the paper,and the circuit about simple zero-crossing of BEMF sensing is designed.The reason of rotor position error in the zero-crossing of BEMF sensing method is analyzed,and the compensation methods are also proposed in the software design.The results of the experiment show that the control system can make the motor start successfully and achieve the motor's correct commutation and smooth running.It proved design of the system is feasible.