The Research Of Control Strategies For Position-Sensorless Brushless DC Motors

The advantage of using a Permanent-Magnet Brushless Direct Current (BLDC) motor is that it has no commutated-spark,high reliability, simple structure and no ullage of excitation and it is easy to maintain and control, so BLDC motors have been widely used in household products, automotive, chemical industrial and medical instruments since 1950s. Compared with traditional sensored BLDC motors, sensorless BLDC motors have the advantages of low cost, small size, long duration, easy maintenance and it can resist EMI effectively. Because of the huge market potential, sensorless technologies will be the main direction of the development of BLDC motors.This thesis describes the operation characteristics and control strategies based on the mathematic model of BLDC motors. Then some problems such as low resolution of magnet position measurement, complicated start strategy, torque ripple and limited speed range are analyzed.It is found that it is hard to achieve phase compensation when there is a phase lag because of the coexistence of the zero crossing point and phase change point by analyzing the principle of the third harmonic measurement of rotor flux. A novel method of detecting of the back EMF third harmonic component is developed. Compared with detecting the third harmonic component of rotor flux, the phase change point leads 90 electrical degrees. So the phase lag can be compensated well.Based on the traditional start strategy, motors are hard for mass product and application, because these strategies are almost based on the parameters of the motor, for example acceleration track. When there is a little change of these parameters such as loading torque and moment of inertia, the strategy should also be modified. To solve this problem, the thesis develops a novel method which is Start Control with Cross-Zero Signal. Only minimum start speed is determined, motor would be started successfully with cross-zero signal. This start method can be used for most sensorless BLDC motors, because it needs not to know the parameters of the motor.It is well known that there is torque ripple at the phase change point of BLDC motors. This thesis introduces a new method of leading the phase angle to improve the current wave and minimize the torque ripple.Based on the control strategies discussed above, this thesis develops a novel control system of sensorless BLDC motor drive. A simulation model for this system has been developed with MATLAB/SIMULINK. The simulation result shows that the new system has wide speed range (200rpm-2500rpm), low torque ripple. It can operate well even under various conditions and start the motors smoothly with different parameters. To sum up, this novel sensorless BLDC motor control system is feasible.