Research On Voltage Extension And Current Dynamic Performance Optimization Of Induction Motor In Field-weakening Region

Induction motor(IM)high speed field-weakening(FW)control is one of the core technologies in computer numerical control(CNC)machine tools,electric vehicles,electric locomotives,all-electric aircraft,and other fields of life and production.There is still a big gap between our country and developed countries in IM frequency conversion driving products.Under the background of industrial transformation and energy reform,the high speed FW control technology of IM begins to develop towards high-end and intelligent.Therefore,the research and development of high performance IM FW drive algorithm has important strategic significance and market value.This paper improves the performance of IM FW control system from the following four aspects:(1)Tradeoff between overmodulated torque improvement and harmonic suppression at maximum torque conditions;(2)Overmodulated harmonic suppression under non-maximum torque conditions;(3)Suppression of current fluctuation in FW transition region;(4)Optimization of current dynamic performance in FW region.The specific contents of the paper are as follows:The overmodulation strategy can expand the maximum output voltage of the inverter in FW region.The maximum output torque can be increased by the extended voltage in FW region.However,overmodulation can cause torque harmonics while increasing the torque.The increase of fundamental torque and torque harmonic suppression cannot be balanced.Thus,this pa per studies the tradeoff method between the fundamental torque increase and the torque harmonic suppression.First,based on superposition theorem,the fundamental and harmonic components of voltage,current,and torque under overmodulation are quantitativ ely analyzed in multi-coordinate system.Second,the overmodulation tradeoff fuction in maximum torque condition is proposed.The inscribed circle voltage and six-step voltage of inverter are used as reference.The fundamental torque increasing characteristic and torque harmonic suppression characteristic of the overmodulated intermediate state are comprehensively evaluated.According to the maximum value of the overmodulation tradeoff fuction,the overmodulated intermediate state which can best balance the torque improvement and harmonic suppression is found.Finally,the overmodulated intermediate voltage curve is applied to increase the maximum torque output in FW region and avoid overmodulated harmonics.Theoretical analysis and comparative experimental results show that the proposed scheme can make use of the low harmonic growth rate range of overmodulation,and then,the torque harmonics generated corresponding to unit fundamental torque increase are suppressed.Further,this paper analyzes the operating point in FW region under the condition of non-maximum torque demand.The analysis shows that due to the voltage closed-loop control structure,the conventional FW overmodulation operation can only realize the constant voltage reference.As a result,the overmodulation voltage is still used in the condition of low torque demand in which overmodulation is not necessary,resulting in torque fluctuation.Focusing on this problem,this paper proposes the torque adaptive overmodulation strategy and designs a cascade voltage control structure.The proposed structure introduces the freedom of voltage amplitude control to the system.Overmodulation is used to increase the maximum output torque only in high torque demand conditions.In non-maximum torque conditions,the cascaded control structure can reduce the overmodulation depth according to the torque current component,and then,the torque harmonics can be suppressed.Theoretical analysis and comparative experiments show that the proposed scheme changes the current vector trajectory as the load increases.On this basis,the torque harmonics in low torque demand condition and medium torque demand condition are completely and partially eliminated,respectively.Moreover,the proposed method does not affect the maximum torque improvement effect of the conventional overmodulation strategy in high torque demand conditions such as step acceleration and overload.Third,the unsmooth transition from the base-speed region to the FW region is studied in this paper.The output voltage of current regulator is decomposed into voltage base component and voltage growth component.On this basis,this paper analyzed the influence of voltage boundary limitation on the variation trend of voltage vector and current vector in the process of FW transition.The analysis shows that the sudden drop of voltage growth component to zero is the root cause of the uncontrollable drop of current in transition region.To solve this problem,the arc voltage transition trajectory is designed to connect the based-speed region and FW region.The transition trajectory is tangent to both the linear voltage growth trajectory in based-speed region and the voltage-limited circular trajectory in FW region.The transition trajectory can avoid sudden change of voltage vector growth and eliminate current fluctuation in the transition process.Theoretical analysis and comparative experiment show that the proposed method can effectively eliminate the current,torque,and speed fluctuations in the transition region.Overshoot of the output voltage of the current regulator is also suppressed due to the reduction of the uncontrolled current drop.Finally,the dynamic control of current under zero voltage margin in FW region is studied.Since the maximum torque trajectory in FW region is derived based on the steady-state voltage equation,the voltage margin required for dynamic current control is not considered in FW control.In this paper,the dynamic characteristics are analyzed when the operating point moves along the maximum torque trajectory.Due to too many unknowns in the transient voltage equation,the current change rate cannot be analyzed through the original transient voltage equation.Thus,this paper obtains the approximate solution of current change rate b y differentiating the steady-state voltage equation.The analysis shows that when the current vector moves along the maximum torque trajectory,the voltage margin required for field current dynamic control is more than 20 times that required for torque current.Based on this conclusion,the voltage boundary limiting strategy is optimized in this paper.The distribution of the output voltage of the current regulator is optimized and a part of the q-axis voltage is converted to the d-axis.The effectiveness of the proposed scheme is verified by comparing experimental results,and the current dynamics in FW region is optimized.