Research On Sensorless Brushless DC Motor Drives

The Brushless DC motor (BLDCM) with permanent magnet is used widely due to the advantages of high simple structure, reliability, high power density, high efficiency, and good performance, etc. It grows with the development of microcontrollers, power electronic devices, control theory and low cost rare earth permanent magnet.In this dissertation, the research is achieved comprehensively about control of the BLDCM without rotor position sensors.Firstly, the structure and operation principle of the BLDCM are introduced. The basic equations and the mathematical model are founded based on fundamental electromagnetic laws.According to intensive analysis of the zero-crossing detection of Back EMF, methods for detecting the zero-crossings of Back EMF are analyzed theoretically. The advantages and disadvantages of detecting methods for the zero-crossings of Back EMF are compared, and their application conditions are analyzed. On the basis of this, a detecting method with band-pass filter and its simplified configuration are proposed. The band-pass filter is employed to get the third harmonic also. The negative pole voltage of DC-link is used as the reference voltage for above two methods.The startup method, a key matter for BLDCM drives without rotor position sensors, is investigated. After the implementation process of a typical open-loop method called"3-stage startup method"is analyzed in detail, the synchronous operation condition from the open-loop to the closed-loop is presented.A commutation approach without phase shift limit is proposed based on the study on commutation approaches for BLDCM drives without rotor position sensors. The performance of the BLDCM can be improved with the commutation approach.The filter is an important part to the circuit for detecting Back EMF zero-crossings. The content of terminal voltages of the BLDCM with PWM operation is analyzed with FFT in detail, and then the specifications for filters are given. A passive band-pass filter and an active low-pass filter are designed based on the filter theory. The investigation shows that the simulation results are coincide with the experimental results. This is very useful for detecting the Back EMF zero-crossing signals of the BLDCM.The reasons causing rotor position error in the zero-crossing detecting Back EMF method in a BLDCM without rotor position sensors are analyzed, and the correction methods are also presented in this dissertation. The analysis of effects resulted from rotor position detecting error is provided. Delaying commutation and leading commutation are discussed. The current of the unexcited phase is calculated under an ideal condition. Thecondition of circulating current occurring is also analyzed. The simulation results are compared with that of the experiment.It is important to do the power loss analysis for power devices to design invertors and improve reliability. The simulation is performed under a simplified inverter model with IGBT in this dissertation. The relations between power loss of IGBT and switch frequency, gate resistance are presented, and the value under different switch frequency and gate resistance for power loss is calculated and analyzed.Lastly, a drive, including the hardware and software, is designed and implemented for a BLDCM without rotor position sensors. The rotor position information is detected with zero-crossing of Back EMF in this drive. The hardware of the drive includes a power supply for drive, a switching power, a Back EMF zero-crossing detecting circuit, an inverter, and protecting circuits. The software of the drive includes a module for the BLDCM startup, a module for the BLDCM operation with zero-crossing of the Back EMF and protecting modules. The experiments are carried out with investigated methods and matters on the drive, and the experimental results are presented.