Research On A Control Strategy Of Suppressing Torque Ripple Of Space Voltage Vector Variable Frequency Soft Starter

As the main starting equipment in the current industrial transmission,the asynchronous motor will generate a starting current 4~7 times higher than the rated current when the no-load starts directly.Such high starting current will have an impact on the motor,production equipment and the power grid.In the case of asynchronous motors,soft starters are required to reduce the motor starting current.At present,the most widely used thyristor voltage regulator soft starter has a large reduction in starting torque while it is stepping down,making it difficult to start heavy-duty equipment.With the support of the National Natural Science Foundation of China(51577110),aiming at the large torque ripple problem existing in the space voltage vector variable frequency(SVVF)soft starter,the control method of the existing asynchronous motor soft starter is further explored.The control principle and starting characteristics of SVVF soft starter and voltage regulator soft starter are analyzed.Find and design a scheme for the problem of large torque ripple existing in SVVF soft starter.The main work of this paper is summarized as follows:(1)According to the equivalent circuit of the asynchronous motor,the steady-state mathematical model of the asynchronous motor is established,and the causes of the starting current and starting torque of the motor are analyzed according to its steady-state model.The dynamic model of the asynchronous motor in the three-phase stationary coordinate system and the two-phase orthogonal coordinate system is built.According to the analysis of different conduction states in SVVF control,a dynamic mathematical model of SVVF soft starter and asynchronous motor is established.In the space voltage vector control process,the conduction of the vector has two states of two-phase conduction and three-phase conduction.In the ??0 coordinate system,the dynamic process of the two conduction states of the space voltage vector is analyzed,and two kinds of two processes are obtained.Mathematical analytical equations for statorcurrent,rotor current,control voltage,stator flux linkages and rotor flux linkages.(2)The control principle of space voltage vector based on power frequency grid power supply is analyzed,and the control method under frequency division mode is studied.According to the conduction characteristics of three opposite parallel thyristors in the main circuit of SVVF soft starter,the instantaneous functions of six basic space voltage vectors under power frequency grid are obtained.After calculation,a curved edge hexagonal flux linkage trajectory is obtained,which proves that the space voltage principle under the power frequency grid can be used to control the soft start of the asynchronous motor.According to the principle,the control method of the SVVF soft starter frequency division is studied.When the SVVF soft starter starts the motor,each frequency division control is the combination process of the voltage vector in different conduction states.And the method of using the space voltage vector of the voltage regulator soft starter is analyzed,and the difference of the space voltage vector between the SVVF soft starter and the voltage regulator soft starter during the starting process is obtained.(3)Combined with the steady-state mathematical model and dynamic mathematical model of the asynchronous motor,observe the operation status of the simulated waveforms of the electromagnetic torque,stator current and rotor speed of each sub-band in the SVVF soft starter,according to the SVVF soft starter and the voltage regulator soft starter.The control principle analyzes the cause of the large torque ripple of the current SVVF,and obtains the following two conclusions: The main cause of the torque ripple of the SVVF soft starter in each sub-band operation is the open zero voltage vector during the control voltage vector action.Turning on causes zero input response attenuation of the motor rotor current;its torque ripple during sub-band conversion is due to the overshoot of the sub-band transition and the correct selection of the frequency conversion point.(4)For the large torque ripple problem generated when the SVVF soft starter starts the motor,compared with the most widely used thyristor voltage regulator soft starter,the SVVF soft start is found based on the conduction analysis of its control voltage vector.The reason for the large torque ripple when starting themotor is as follows: one is the long-term effect of the open zero voltage vector during the two space voltage vector conduction process during the frequency division control;the other is the speed overshoot when the frequency band is switched.According to the two reasons of the large torque ripple generated when the SVVF soft starter starts the motor,the problem is studied from two aspects.The SVVF crossover switching scheme is proposed for the problem of frequency switching,and it is simulated and verified in MATLAB,and then the practicality of the scheme is verified on the soft start experimental platform.The experimental results show that the torque ripple in the sub-band switching has been greatly reduced after the frequency division switching scheme is applied.Then,for the torque ripple caused by the frequency band operation compared with the voltage regulator soft starter,the feasibility of torque ripple reduction is studied from three aspects: motor speed fluctuation,rotor current attenuation and open circuit zero voltage vector action.The results show that when the SVVF soft starter controls the motor to run in each sub-band,the torque ripple caused by the action of the open-circuit zero voltage vector is difficult to reduce.Based on the basic principle of hexagonal space voltage vector,this paper analyzes the control principle of SVVF soft starter under power frequency grid,analyzes the voltage regulator soft starter by using space voltage vector.Combined with the mathematical model of asynchronous motor and the two-phase and three-phase conduction model of SVVF soft starter,the existence of large torque ripple of SVVF soft starter is obtained.A control strategy for suppressing the torque ripple of SVVF soft starter is proposed.The torque ripple of the SVVF soft starter is reduced,which has great theoretical significance and application value.