New Modulation And Common-mode Voltage Suppression Methods For A Six-phase Induction Motor

Motor drive systems are used in a wide range of applications,such as heating,ventilation and air conditioning,elevators/lifts and liquid pumps.In recent years,it has been increasingly applied to electric vehicles,nam ely the electrification of transportation.A study by the International Energy Agency（IEA）shows that motor-driven systems consume about 43%of the world’s electricity supply,70%of which is in industrial applications,and predicts a steady increase in m otor consumption.As the power electronics technology rapidly developing,the motor drive system is no longer limited to the three-phase power supply system.Compared with three-phase motors,multi-phase motors have the advantages of low-voltage high-power output,good fault tolerance,and small torque ripple amplitude.This paper takes the six-phase induction motor with a phase shift of 30°as the research object,and conducts in-depth theoretical research and experimental verification from the aspects of mathematical modeling,pulse width modulation technology（PWM）,common-mode voltage（CMV）suppression,harmonic flux linkage analysis and optimization.Firstly,this paper introduces the mathematical model of six-phase induction motor in the natural coordinate system,deduces the vector space decoupling transformation theory（VSD）,and applies this theory to the mathematical model of six-phase induction motor in the natural coordinate syst em to obtain the mathematical model in the stationary coordinate system.The theory decouples the six-dimensional space into three orthogonal subplanes,namelyα-βplane,x-y plane and o₁-o₂ plane,while the electromechanical energy conversion occurs only in theα-βplane.Then,the double dq mathematical model of six-phase motor is introduced,and the internal relationship between VSD model and double dq model is studied.Then the vector control method based on rotor field orientation is introduced and ver ified by simulation.Secondly,an overmodulation strategy,namely sector subdivision overmodulation strategy,is proposed,which can effectively reduce the total harmonic distortion（THD）of the output voltage.The pulse width modulation（PWM）technology of six-phase motor is deeply studied,and the space vector pulse width modulation（SVPWM）method of six-phase motor in linear modulation region and overmodulation region is studied,respectively.The switching sequences of twelve sectors strategy and twenty-four sectors strategy in the linear modulation region are analyzed,and it concludes that the twenty-four sectors strategy is better.The dowell time of voltage vector and linear modulation range of the twenty-four sector strategy are deduced.Since the impedance of the six-phase motor in the harmonic plane is very small,even if there is a low fifth,seventh and other low harmonic voltage,it will produce a large harmonic current.However,when the motor runs in the overmodulation region,the fifth,seventh and other low harmonics cannot be avoided.In order to reduce these harmonic voltages as much as possible,an overmodulation strategy based on twenty-four sectors,sector subdivision overmodulation strategy,is proposed.When the modulation index is 0.6 and 0.61,the phase voltage THD of the proposed sector subdivision strategy is reduced by 33.465%on average compared with the existing method,which effectively reduces the 5th harmonic content of the output voltage and improve the quality of the output current.Thirdly,two common mode voltage suppression strategies,namely common-mode voltage suppression（CMVS）and double zero-sequence injection common-mode voltage suppression（DZICMV）,are proposed.These two strategies can reduce the common-mode voltage amplitude by 66.67%,and have the advantages of small computation and convenient DSP implementation.Common-mode voltage is an inherent disadvantage of inverter based（VSI）drive motor system,which can cause bearing current and electromagnetic interference.Excessive bearing current will corrode the bearing,thereby reducing the life and reliability of the system.The common mode voltage generation mechanism is analyzed and the common mode circuit equivalent circuit of six phase motor is established.Based on this,two new common-mode voltage suppression strategies,namly CMVS and DZICMV,are proposed.Both of these strategies can reduce the peak value of total mode voltage and sub-common-mode voltage from±U_dc/2 to±U_dc/6,that is,66.67%.The modulation range of DZICMV can reach 1.15.Finally,the concept of"zero-common-mode voltage vector"is defined.Based on the existing literatures,two modulation strategies that can completely eliminate the total CMV are proposed,namely,the six-sector zero-common-mode voltage SVPWM strategy（SVPWM6）and the twelve-sector zero-common-mode voltage SVPWM strategy（SVPWM12）.The harmonic distortion factor（HDF）of the two strategies is analyzed and compared,and a harmonic flux optimization method is proposed.The modulation strategies of SVPWM6 and SVPWM12 are proposed by selecting the space vector that can make the total CMV be zero.By changing the order and number of zero vectors used,each strategy can be divided into several continuous and discontinuous schemes.The harmonic flux trajectories of each scheme on the fundamental plane and the harmonic plane are analyzed.The corresponding harmonic distortion factors are calculated,and the scheme with the lowest distortion factor(HDF_αβ)on the fundamental plane is obtained.In order to further reduce HDF_αβand improve the output current quality,the zero vector duty cycle of the scheme SVPWM6 is optimized.The HDF_αβof the optimized scheme is reduced by 21.37%on average.The optimized scheme is the best one among these schemes.