| Brushless DC motor(BLDCM) is a kind of electromechanical integration machinery, and it has been rapidly developing in recent years with the help of the development of power electronic technique and the appearance of new type of permanent magnet material. With it's widely used in many fields, sensorless control especially back-EMF method is more useful than sensor control in many applications.The common methods of sensorless control of BLDCM are introduced, and the principle of back-EMF method is illustrated in detail. Three kinds of methods of the zero-crossing of back-EMF sensing are analyzed,and the zero-crossing of back-EMF sensing circuit is designed. The reasons causing rotor position error in the zero-crossing of back-EMF sensing method are analyzed, and the compensation methods are also proposed. In this system, the BLDC motor of air conditioner compressor is used as a sample and the hardware of the BLDC motor control system on back-EMF method are designed, each part of this system is introduced in detail. In additional, the methods to reduce electromagnetic interference are also analyzed.The common start methods of sensorless BLDC motor control system are introduced, and the three-step start technique, including the rotor pre-setting, accelerating and state-changing step, is discussed carefully. The advanced or lagged commutation performance during the accelerating step is also investigated.A new method of implementation on the best commutation logic by means of the zero-crossing of back-EMF is suggested, which can not only regulate speed successfully, but also make the state-changing step during the starting easier. The experimental results show the method correct.
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