Research On Optimal Current Control Technique For Brushless DC Motors

Permanent magnet motors have been widely used in the fields of aerospace,rail transportation,numerical control equipment and domestic applications due to their high torque density,high efficiency and reliable structure,etc.Compared with the permanent magnet synchronous motors(PMSMs)with sine wave back EMF,the brushless dc motors(BLDCMs)with trapezoidal wave back EMF have higher torque density.However,the BLDCMs are usually driven by square wave current.Due to the existence of the commutation inductance,it is difficult to obtain ideal square wave current,resulting in large torque ripples during the commutation process.On the other hand,it is difficult to achieve the ideal trapezoidal wave back EMF for BLDCM due to the motor design and manufacturing process,which will also cause the torque ripple and limit its application in high-performance occasions.Therefore,the research on the torque ripple suppression technology for BLDCM is of great significance for promoting the application of such high torque density motors.This thesis takes the BLDCM and its drive control system as the research object,aiming to increase the torque density and reduce the torque ripple for BLDCM.Therefore,this thesis mainly focused on the optimal current waveform,novel vector control strategy,current regulation,and multi-phase permanent magnet motors control to carry out the research of the optimal current control technology for BLDCM.Firstly,referring to the mathematical relationship between dq axis inductance matrix andαβ axis inductance matrix,the electromechanical energy conversion back-EMF,involving in electromechanical energy conversion,is further simplified for interior type BLDCM.On this basis,the analytical formula of optimal current is derived,combined with the instantaneous power theory.With this method,the influence of the rotor salient pole charactristics and the non-ideal back EMF on the torque performance can be considered,simultaneously.With the finite element simulation and the static torque experimental test of the prototype,the drive characteristics of square wave current,sine wave current and optimal current are compared with each other under the same effective value.Simulation and experimental results both verify that the optimal current is better than the other two current drive methods in the torque ripple suppression.Secondly,the optimal current in the traditional dq coordinate based on Park transformation is not direct current components due to the optimal current being generally not a sine wave.However,the proportional integral(PI)regulator with limited bandwidth cannot accurately track the ripple components.Thus,it is difficult to inject the optimal current into motors.In order to solve the above problems,this thesis proposes a variable amplitude generalized vector control strategy to transform the optimal current into two constant dc components with orthogonality,so that the mathematical model of the BLDCM can be equivalent to the dc motor.Thus,high performance vector control for BLDCM is realized.The control block diagram of the proposed vector control is similar to the traditional vector control.The current loop with the traditional PI regulator can track the optimal current command without static error,which enhances torque control performance for BLDCM.Thirdly,in order to suppress the influence of the inverter dead-time and the cross-coupling between the direct axis and the quadrature axis on the current regulation performance,and further improve the torque control performance,an improved current regulator based on adaptive linear neuron active disturbance rejection controller.This method is based on the principle of adaptive harmonic cancellation,using an adaptive linear neuron(ADALINE)harmonic observer to estimate the harmonic disturbances caused by the dead-time,and using the linear extended state observer(LESO)to estimate the slow-varying disturbances caused by cross-coupling.The above two disturbances can be compensated online at the input end of the system to realize disturbance-free system control,thus effectively compensating dead-time effect and realizing current loop decoupling control.According to the specific frequency of the harmonic disturbance,the ADALINE harmonic observer uses the least mean square(LMS)algorithm to continuously adjust the weight coefficients of ADALINE harmonic estimator to ensure the rapid convergence of the harmonic disturbances estimation.Finally,the proposed optimal current control strategy in this thesis is extended to the multiphase permanent magnet motors with harmonic current injection,which effectively suppresses the torque ripple caused by the back EMF harmonic components being higher than the number of phases.Multi-phase permanent magnet motors can be equivalent to odd number of virtual motors through coordinate transformation.The traditional vector control strategy based on the generalized Park transformation for multi-phase permanent magnet motors only injects harmonic currents components being lower than the number of phases,which cannot suppress the torque ripple caused by the back EMF harmonic components being higher than the number of phases.This thesis studies the uniform analytical formula of the optimal current for each virtual motor in the generalized Clark coordinate system,which considers the influence of the back-EMF harmonic components being higher than the number of phases on torque control.On this basis,the variable-amplitude generalized vector control strategy proposed in this thesis is applied to multiple virtual motors to realize the optimal current injection into the multi-phase motors and further improve the torque control performance.A five-phase permanent magnet motor is used as an example for simulation analysis,and the simulation results indicates that the proposed optimal current control strategy reduces the torque ripple rate from 3.5% to 1%.