Research On Torque Control Strategy Of Permanent Magnet Brushless DC Motor

Permanent magnet brushless DC motor(BLDCM)has been widely used in fields such as aerospace,industrial and agricultural production owing to its simple structure,reliable operation and high power density.The control performance of electromagnetic torque will directly affect the operation quality of the motor system in various application fields,so this paper focuses on the high-performance torque control of the BLDCM in electric operation and braking operation.For the three-phase voltage source inverter(VSI)fed BLDCM system,this dissertation profoundly analyzes the torque control performance of PWM modulation methods when the motor operates in electric state and braking state.In electric operation,the analysis shows that the commutation torque ripple in the high-speed region cannot be suppressed by using the modulation method with two-phase switch mode due to the limitation of the DC-link voltage.In braking operation,when the bipolar modulation method with two-phase switch mode is used,the good torque control can be achieved but the effective braking energy regeneration cannot be taken into account at the same time.In view of the problem that the commutation torque ripple in the high-speed region cannot be suppressed with two-phase switch mode for the VSI-BLDCM system under electric state,this dissertation proposes a direct torque control strategy based on the selection of main vector(MV)and subsidiary vector(SV).During the normal conduction period and the commutation period,the mapping between the all possible combinations of terminal voltages and the torque variation rates is revealed,and two switching tables with MV and SV are established respectively in these two periods by analyzing the influence of terminal voltages with different values on the torque variation rates.Especially during the commutation period,a unified vector selection mechanism is constructed in the high-speed and low-speed regions to avoid switching control strategies.The proposed strategy can effectively suppress the commutation torque ripple in the wide speed range.In view of the problem that the commutation torque ripple over the full speed range cannot be effectively suppressed due to the limitation of the DC-link voltage for the VSI-BLDCM system under electric state,this dissertation proposes a torque control strategy based on diode-assisted buck-boost inverter.Four types of switching vectors are constructed according to the working pattern of the diode-assisted buck-boost inverter and the operation mode of the BLDCM.During the normal conduction period and the commutation period,the constraints that the terminal voltages of the windings should be satisfied are clarified when different vectors work together to maintain the phase current steady.The duration and sequence of switching vectors within each modulation cycle is designed in these two periods based on the objective of suppressing the torque ripple and reducing the voltage stress of the switching devices.The proposed strategy can effectively suppress the commutation torque ripple over the full speed range.In order to achieve good torque control in both electric operation and braking operation,and take the effective braking energy regeneration into account at the same time,a non-inductive hybrid energy storage topology is designed in this dissertation.The operating mechanism of the BLDCM in different working modes is explored based on this topology.Especially in braking mode,the effects of different switching vectors on braking torque control and braking energy regeneration are analyzed in detail,and then the priority of the different switching vectors is determined.On this base,a vector selection mechanism based on average ranking is constructed.During the whole braking process,the proposed strategy can achieve both smooth braking torque control and braking energy regeneration.The proposed torque control strategies in electric operation and braking operation are verified experimentally by building a hardware experimental platform.The experimental results prove the correctness of the theoretical analysis and the feasibility of the proposed strategies.