Drive Strategy Of Bldcm With Controllable Commutation Freewheeling

Current freewheeling caused by commutation has negative effect on the performance of brushless DC motor(BLDCM).On one hand,freewheeling delays phase currents and decreases system efficiency.On the other hand,torque pulsations emerging during freewheeling are main origins of vibration and acoustic noise.In addition,when commutation angle exceeds 30 electrical degrees sensorless drive will be infeasible because zero-crossings of back electromotive force(EMF)can no longer be detected.At present,control strategies suitable for short-time commutation freewheeling has been intensively studied while investigation on the control of long-time commutation freewheeling is still absent.Along with the tendency pursuing higher power density,BLDCM drive systems with long-time commutation freewheeling become increasingly common due to high speed and large winding inductance.This dissertation focuses on the drive strategy of BLDCM with controllable long-time commutation freewheeling,which is of great significance and practical value.The objects of this dissertation are:(1)influence factors of commutation freewheeling,(2)novel drive circuit topology with controllable commutation angle,(3)novel control torque strategy with controllable commutation angle,(4)Performance of 12-step commutation modes(CM)considering long time commutation freewheeling.Influence factors of the commutation freewheeling are analyzed.Based on the circuit model of BLDCM drive system,analytical expressions of phase currents are derived.Then change laws of commutation angle along with winding impedance angle and voltage ratio are revealed.Analytic investigation on the influence factors of commutation freewheeling is presented from the aspect of system design and inverter control.The results show that: During stage of system design,commutation angle can be controlled by adjusting inductance,back EMF coefficient and dc-link voltage.The influences of PWM modes on commutation angle depend on chopping mode.Minimum commutation angle can be achieved with ON_PWM modulation.Phase advance is helpful for reducing commutation angle.Investigation of drive circuit topology with controllable commutation angle is undertaken.Firstly,it is proven in theory that adjustment of commutation angle is feasible by piecewise controlling the dc-link voltage.Then based on the double dc-links idea,a three-port inverter is proposed by separating the dc-links of forward energy path and backward energy path.A novel bipolar C-dump converter is also proposed to adjust the dc-link voltage of backward energy path.Experiment results show that: The proposed drive circuit has obvious advantages of simple control and high reliability compared with traditional drive circuits due to the elimination of freewheeling-end-point detection unit.By duty cycle control,the output voltage of the bipolar C-dump converter can be adjusted real-timely to regulate the commutation freewheeling procedure.Torque control strategy with settable commutation angle is also investigated.For torque fluctuation suppression considering non-ideal back EMF,commutation freewheeling and phase advance,an expected current function is constructed taking commutation angle as adjustable parameter.Then the phase voltage is derived and the influence of dc-link voltage on available minimum commutation angle is analyzed.The duty cycles for inverter control are calculated before simulations are conducted to verify the proposed control strategy.Results show that: By the construction of expected current function,accuracy control of commutation angle and suppression of torque fluctuation are both feasible at the same time.The range of commutation angle is restricted by dc-link voltage.The proposed torque control strategy can be realized by real-timely controlling the duty cycle of PWM signals and the torque stability is comparable with sine-wave drive systems.A comparative analysis of 12-step CMs is presented.At first,the switching function model of CM is established taking mode-angle as variable.The influences of commutation freewheeling and phase advance on switching function time sequence are analyzed.Then based on the analytic expression of phase voltage,the influence of 12-step CMs on phase voltage harmonics is analyzed.At last,torque is calculated using harmonic analysis method.Comparison between 12-step CMs and the traditional 120° conduction 6-step CM is also presented.Results show that: Phase voltage is not ideal staircase when 12-step CMs are adopted.Its harmonic amplitudes are mainly but not uniquely determined by mode-angle.They are also influenced by voltage ratio,commutation angle and phase advance angle.Under specified condition of CM-angle,12-step CMs can attenuate phase voltage harmonics compared with 6-step CM.When commutation angle is large,12-step commutation modes with optimal CM-angle can significantly attenuate torque ripples.