Research On Zero Vector Of Direct Torque Control System Of Brushless DC Motor With Two - Phase

Brushless DC motor (BLDCM) has found wide applications in various drive systems thanks to its compactness, high efficiency and high power density. PWM modulation technique was traditionally adopted in the control strategy of BLDCM, by which the torque was indirectly controlled through current regulators. Nowadays, Direct torque control (DTC) has found use in the control system of induction motor and PM synchronous motor with sinusoidal power. A high-performance both in dynamic and steady state operation can be achieved by applying the appropriate voltage vectors according to the required torque. However, It is important to realize that there are some different issues in BLDCM control system as the non-sinusoidal back EMF should be taken into consideration in two-phase conduction mode. These issues, especially with voltage vectors in two-phase conduction mode, have been investigated in this thesis.First of all, the structure and control principles of BLDCM are presented and the modeling of BLDCM with star connected stator windings is proposed in this thesis. The conducting phase voltage synthesis vector is deduced from the two-phase conduction mode, which plays a key role with respect to the DTC of BLDCM.Nonzero voltage vector of synthesis vector and all switch-off voltage vector are investigated in this thesis. The theoretical analysis shows that all switch-off voltage vector is not zero voltage vector in DTC system of BLDCM and it is actually the negative resultant voltage vector of the corresponding synthesis voltage vector. The analysis of simulation and experiment also shows that the voltage waveform as well as the current and torque waveforms on all switch-off voltage vector are basically similar with those on the negative resultant voltage vectors under the condition of continuous current.According to the characteristics of the two-phase conduction mode, zero vector of DTC system of BLDCM is investigated. Two kinds of new zero vector are obtained by theoretical analysis. The results of simulation and experiment show that two conducting phase windings are short circuited by the new zero vector. The validity of the analysis of new zero vector is verified by experiment results. The comparison of the simulation and experiment also shows that the DTC system with the new zero vector has the higher performance with reduced torque ripples than that with nonzero voltage or all switch-off voltage vectors.A BLDCM experiment platform with circuit diagram, based on TI DSP TMS320F28335, was presented and has been used for the experiment and simulation validation in this thesis.