Study And Design Of Brushless DC Motor Direect Torque Control System

Brushless DC motor (BLDCM) has apparent advantages in terms of volume, efficiency,controllability, reliability and economy in a variety of industry, aviation and householdappliances application fields. But the torque ripple of BLDCM is bigger. Torque ripple willbring vibration and noise, so it is not equal to systems that require lower noise, higher speedaccuracy and higher positioning accuracy. BLDCM has cogging torque ripple, nonideal backEMF electromagnetic torque ripple and commutation electromagnetic torque ripple. Ingeneral, the first two are smaller, so reducing or eliminating the commutation electromagnetictorque ripple is the key to improve system performance.The traditional pulse width modulation current control for BLDCM will produce largercommutation electromagnetic torque ripple. In case of nonideal back EMF, it will produce theprinciple electromagnetic torque ripple. In order to restrain the nonideal back EMFelectromagnetic torque ripple and commutation electromagnetic torque ripple, the directtorque control (DTC) for BLDCM is proposed. On the basis of traditional mathematics modelfor BLDCM, a model for BLDCM with nonideal back EMF is presented. For ease ofcomputer control, discrete-time sliding mode observer (DSMO) is designed to obtain the backEMF, and the sliding and convergence condition for DSMO are proved. Rules of selectingvoltage space vectors, the track of stator flux linkage and the electromagnetic torque controlmechanism are analysed. Restrained by switch frequency of existing power electronic powerdevices, the relationship between switching frequency and hysteresis loop width is deduced,and it is the basis to calculate hysteresis loop width. Based on these, the DTC for BLDCM isproposed. DSMO is used to obtain the back EMF to estimate electromagnetic torque, andvoltage space vectors are decided by both the torque hysteresis loop output and position ofmagnetic poles. According to the structure of DTC and control parameters, a simulationmodel is built. The condition of DTC to restrain the commutation electromagnetic torqueripple and the reason of failure at high speed are analysed from simulated results. Finally,hardwares and softwares of the control system are presented.The nonideal back EMF electromagnetic torque ripple and commutation electromagnetictorque ripple at low speed could be restrained effectively by DTC. The DSMO algorithm withgood robustness is simple and suitable for computer control. It can be used to provideelectromagnetic torque feedback for the DTC system of BLDCM.