Research On Control Strategy Of High-Speed Permanent Magnet Brushless DC Motors

Permanent magnet brushless DC(PMBLDC)motor is one of the main types of high-speed motors,due to its small size,high power density,high efficiency,and simple rotor structure,it has been widely used in many fields such as turbochargers,micro-gas turbines,power tools,air compressors,and turbomolecular pumps.However,limited by the maximum switching frequency of the power device,the carrier ratio of high-speed PMBLDC motor operated under PWM chopping control method will be small,therefore the phase current is severely distorted,the resulting harmonic magnetic field will generate a large amount of eddy current loss in the rotor,causing temperature rise of the permanent magnet,and in severe cases,it may expose the magnet to high risk of demagnetization,which limits the maximum speed of the motor.Therefore,it is necessary to study new control strategies to solve the problem.The main work of the paper includes the following aspects:Firstly,the finite element model and double closed-loop drive system model of high-speed motor are built based on Ansoft and Matlab/Simulink,respectively.Harmonic analysis is carried out on the stator current of high-speed PMBLDC motor under the traditional PWM chopping mode.The relationship between rotor eddy current loss and current harmonic components as well as control parameters is studied.Stepless speed regulation can be realized by adjusting the commutation angle and conduction period dynamically.In this paper,the effects of variable conduction period control strategies using single-pulse,symmetrical dual-pulse and asymmetrical dual-pulse on current waveforms are studied,and the corresponding eddy current losses are compared by finite element analysis.The research results show that adopting asymmetrical dual-pulse variable conduction period control method can make the positive and negative half-cycle waveforms of the motor stator current symmetrical under different loads,thus eliminating the even harmonic components in the current.Besides,by adjusting the commutation angle and the conduction period of the dual-pulse dynamically,the ripple amplitude of the current can be effectively reduced,therefore the high-order time harmonic components in the current are reduced and the purpose of weakening rotor eddy current loss of the motor is achieved.In addition,based on chip TMS320F28035 belonging to 28 series of TI company,the experimental device of the prototype is built,the drive and control hardware circuits are designed,and the corresponding software is compiled.The current waveforms of the experimental prototype under PWM chopping control method as well as variable conduction period control method are measured.The experimental results are in good agreement with the simulation results.It is verified that in the high-speed PMBLDC motor drive system,by replacing the traditional PWM chopping control method with asymmetrical dual-pulse variable conduction period control method,the current waveform is symmetrical and the current ripple amplitude is reduced,therefore the high-order time harmonic components in the current is reduced,and the rotor eddy current loss is weakened.