Analysis Of Direct Torque Control Structure Of Two-phase Conduction Brushless DC Motor

Brushless DC Motor (BLDCM) has been widely used in many fields thanks to its high efficiency, simple structure, large power density, and many other advantages.Compared with the traditional vector control method, direct torque control (DTC)method aims at controlling the torque directly, which avoids the complex coordinate transformation and makes the control structure much simpler, dynamic and static performance is much better. It's a hot topic in the field of high performance motor control.Therefore,the study on DTC applied in BLDCM control system has obvious theoretical and practical significance. In this paper,several key issues of DTC system were mainly discussed, and which is based on two-phase conduction brushless DC motor.First of all, according to the characteristics of two-phase conduction mode, the characteristics of voltage vector in BLDCM DTC system and its definition method were analyzed in detail. Zero voltage vector (ZVV) was mainly studied and zero vector suitable for two phase conduction mode BLDCM DTC system was obtained. The results of simulation and experimental shows that the newly defined zero vector can make the two-phase windings short-circuit, which meets the requirements of the DTC system.Secondly, through further analysis of the new zero vector in the BLDCM DTC system, this thesis has pointed out that the freewheeling diode was conducted due to the amplitude variation of back EMF of the off-phase under the effect of upper ZVV or lower ZVV, which makes the off-phase appear continuous current and the motor torque ripple increased consequently. It has been validated by simulation and experimental results. According to the characteristics of zero vector, a new control method of the twelve sectors subdivision was proposed,which gave the basis of sector subdivision and corresponding voltage vector selection table. The simulation and experimental results showed that this control method can effectively prevent the off-phase appear freewheeling current phenomenon, using this control method can significantly reduce the torque ripple and improve the static performance of the system.Finally, an experiment platform of BLDCM was designed and made, which based on TI DSP TMS320F28335. The experiment and simulation validation of BLDCM on this platform show that it is effective and reliable.