Research On Fault-tolerant Control Of Five-phase Induction Motor With Non-sinusoidal Power Supply

Multiphase machine has been used widely in electric ship propulsion,air-craft drives,locomotive traction,and high-power industrial applications.As number of phases increase,the required stator phase current amplitude can be reduced without changing phase voltages and output power.Moreover,increasing the number of phases improves the reliability when faults occur.Another distinguished advantage is that it can give additional degrees of freedom to fulfill high torque density and high power density using harmonic current injection.This paper is based on the research of the five-phase induction motor(IM)drive system,relatively in-depth focusing on the non-sinusoidal power supply and fault tolerant control.The main contents of the article are shown as follows:Firstly,the history of development is overviewed,and the state of art about fault-tolerant algorithms and non-sinusoidal power supply technique for multiphase IM is introduced in this article.Then the multiphase machine's MMF is calculated according to the winding function,moreover,the combination mode of winding structure and hannonic currents is presented according to the time harmonic and spatial harmonic.Meanwhile,by means of multiphase transformation matrix,the mathematic model of multiphase IM in arbitrary speed coordinate system is established.Secondly,take the traditional non-sinusoidal power supply technique for example,the torque gain is computed based on the mathematic model of five-phase IM and the reason of high power density is presented with harmonic current injection.As the traditional controller only suits for the motor operating with no load,three constraints considering flux linkage amplitude,phase angle between rotor flux and air-gap flux and slip in both subspaces are proposed in order to generate a rectangular air-gap flux.The no-linear relationship between current commands in both subspaces is presented based on amplitude constraint and angle constraint so as to realize the real-time adjustment of the third hannonic air-gap flux under various loads.In this way,the rectangular air-gap flux is generated with no load and heavy load.As a result,the torque density and power density are increased with the proposed controller.Thirdly,a steady model for five-phase IM under open circuit fault condition with and without harmonic current injection is established.The average torque,torque ripple,total losses and efficiency are given in terms of symmetrical co,mponent(SC)theory so as to lay the foundation for evaluation and desigin of fault tolerant strategies.By SC transformation,the fundamental currents and third harmonic currents are transformed into four SCs,respectively.According to the formula of ripple torque under fundamental supply and third harmonic supply,there arc two parts:one is the function of the first SC and the fourth SC,and the other is the function of second SC and third SC.Because the coefficient of the first part is three times larger than the second part,a concept that the first part of ripple torque is reduced as far as possible is proposed in order to minimize the torque ripple in the post-fault operation.Based on the concept,the fundamental currents and third harmonic currents are optimized with the prerequisite that the invariance of MMF.In the case of fundamental supply,only the first SC and the second SC or the first SC and the third SC are reserved.In this way,the torque ripple will be reduced to zero.By comparing the average torque and efficiency,the first method is choosed in fault-tolerant control.Considering the third harmonic current injection,only the second SC and the third SC of third harmonic currents are reserved.Combined with the fundamental currents commands.the torque ripple is minimized under third harmonic supply.Moreover,in order to demonstrate the superiority of the proposed strategy,the existed minimum torque strategy and maximum torque strategy are introduced for comparison.Fourthly,with the increase of the phase number and the number of fault phases,the complexity of calculation of fault-tolerant currents commands increase.In order to simplify the calculation,a current optimization technique based on genetic algorithm is presented.Optimization objectives include maximizing the magnitude of positive SC.,minimizing the magnitude of negative SC and coupled SC in fundamental and the third harmonic subspace provided that magnitude of all phase currents equal to l p.u,inverse transformation constraint is satisfied and the invariance of MMF.As a result.the SC distribution of fundamental currents is the same with maximum torque strategy.Moreover,the amplitudes of coupled SCs for the third harmonic currents are reduced in a large extent.At the same time,the torque ripple,average torque and efficiency are improved under third harmonic supply compared with maximum torque strategy.