Research On Torque Performance Improvement Method Of Brushless DC Motor Based On DTC

Brushless DC motor(BLDCM)has been widely used in applications due to its advantages such as.high power density,simple-control and quick dynamic response.However,the major problem of BLDCM is the large torque ripple,which will damage the power system control characteristics,even destroying the machine bearings,lower the machine life arid arouse the vibration,resonance and the noise.When it comes to BLDCM control system,the specific non-sinusoidal back electro motive force(EMF)and 120° conduction mode make BLDCM direct torque control(DTC)become a hot research issue.The torque ripple reduction methods are mainly discussed in two-phase conduction mode of BLDCM DTC system.Based on the study of direct torque control of two-phase conduction BLDCM,this dissertation conducts a depth research about three modified strategies,namely zero voltage vector selection,pulse width modulation(PWM)strategy and commutation torque ripple suppression strategies,to reduce the torque ripple.1.Analyzing the definition of zero voltage vector in two-phase conduction mode and the perfornance of zero voltage vector selection scheme on current and torque,then propose a zero voltage vector selection scheme theoretically independent of intermittent current.Experiments show that the new zero-voltage vector selection scheme does not cause the diode freewheeling of turn-off phase and improves the torque performance.2.In the traditional direct torque control scheme,one voltage vector is sent in one control cycle,which leads to a large torque ripple.This thesis presents a pulse width modulation method in a single control cycle,designs the corresponding duty cycle calculation method,and completes the design of a new torque regulator.3.Aiming at the problem of large commutation torque ripple,a pulse width modulation scheme suitable for commutation period is proposed based on circuit state analysis during commutation period.Aiming at the limitation of pulse width modulation(PWM)in two-phase conduction mode at high speed,i.e.the duty cycle reaches the upper limit of 1,which cannot continue to compensate for the torque drop,a strategy of pulse width modulation with the tum-oflf phase participating in the control is proposed.Considering the change of current during commutation,a calculation scheme of commutation time is proposed.In the simulation,the correctness of the theory is verified,and then the experimental verification is carried out on the experimental platform.The experimental results show that the proposed control strategy has a good suppression effect on the torque ripple/commutation torque ripple.