| Asynchronous motor driving a pulping equipment of papermaking industry is large power and large starting current,which will lead to equipment damaging and grid voltage fluctuating in a wide range.If the motor was disorderly halted,it will need a large electromagnetic torque to start the pulping equipment with load again.However,existent soft starting methods for asynchronous motor have many disadvantages such as small starting torque,large torque ripple or no easy bypass switching.Aiming at the problems,under the help of the national natural science foundation project of China(51577110),the paper studied the method of variable frequency soft starting control for asynchronous motor,the theory of small switching capacitor frequency converter and its bypass switching method based on space voltage vector theory.The contributions of the study were shown as follows:(1)Study on the cause and improvement of torque ripple of an asynchronous motor controlled by discrete variable frequencyAiming at the characteristic that asynchronous motor needed large electromagnetic torque to start a pulping equipment,the paper proposed a novel high starting torque DVF control method for an asynchronous motor.The method was based on a hexagonal voltage vector locus and contributed to remove the negative electromagnetic torque ripple of an asynchronous motor controlled by the traditional discrete variable frequency(DVF)control method.The DVF control method based on a hexagonal voltage vector locus was studied from three parts.Firstly,the theory of space voltage vector was used to analyze the voltage waveform of the traditional DVF control.When two phase thyristor circuits were triggered at the same time,stator current could be generated.According to the relationship of stator flux and stator voltage vector,torque ripple cause of an asynchronous motor controlled by the traditional DVF method was revealed.The reason is that there were many voltage vectors rotating in the reversed direction of the stator flux linkage.Next,after these reversed voltage vectors were removed,effective voltage vectors would work in order of a hexagon.So space vector DVF control method based on a hexagonal voltage vector locus was proposed.Finally,when the stator current was cut off,the rule of rotor current changing was analyzed and the influence on stator flux damping was also estimated.Study results of this part show that the voltage vector rotating in the reversed direction of the stator flux locus is the main cause generating negative electromagnetic torque and negative electromagnetic torque of an asynchronous motor can be removed by using the novel method.(2)Study on the method of discrete variable frequency soft starting for an asynchronous motor based on space voltage vectorOne novel DVF soft starting method was proposed.The variation of single stator voltage amplitude would influence the direction of the compound voltage vector.So the paper studied the stator flux changing rule of an asynchronous motor controlled by the hexagonal voltage vector method and suggested a novel DVF soft starting method based on stator flux locus control.The novel DVF soft starting method was developed by three steps.Firstly,space voltage vector's impact on asynchronous motor stator flux was analyzed on condition that two of three-phase thyristor circuits were conductive.In order to create a circular flux locus,working method of the voltage space vector at each discrete frequency was concluded.Next,the dynamic model of an asynchronous motor was built when two-phase thyristor circuits were conductive.In virtue of the model,it was not hard to deduce stator flux equations and stator flux decrement when three-phase thyristor circuits were all turn-off.Finally,the novel DVF soft starting control strategy for an asynchronous motor was proposed.In this way,output voltage frequency would change in the sequence of the frequency f/7,f/4,f/3 and grid fundamental frequency.Study results of this part demonstrate that an asynchronous motor can be stepwise started by the novel method.In comparison with the traditional DVF soft starting control method,starting current of an asynchronous motor can be decreased by 20 percent and torque ripple can be markedly reduced.(3)Study on the theory of small switching capacitor frequency converter and the mechanism of asynchronous motor energy feedbackWhen an asynchronous motor was controlled by the novel DVF control method,the speed of a pulping equipment couldn't be smoothly regulated and the current of an asynchronous motor was discontinuous.On the other hand,the traditional AC-DC-AC frequency converter could be used to smoothly start the pulping equipment,but the converter couldn't be directly bypass switched.Aiming at the disadvantages,one novel small switching capacitor frequency converter and its control strategy was proposed.The study of small switching capacitor frequency converter was composed of four parts.Firstly,the function of large electrolytic capacitor in parallel with the DC-link of AC-DC-AC frequency converter was explored.The electrolytic capacitor increased the electromagnetic inertia of the converter DC-link and obstructed the input voltage and the output voltage of the converter,which was the basic reason that the traditional frequency converter couldn't be directly bypass switched.Secondly,the characteristic of energy feedback was analyzed when the traditional converter feeding an asynchronous motor was controlled by different space voltage vectors.The energy feedback model of the asynchronous motor in steady state was also built.Thirdly,one novel circuit structure of a small switching capacitor frequency converter was presented and parameter calculating method for the capacitor of the converter was presented,which was accorded to that discharging voltage wave of the capacitor was approximately same as the six-pulse voltage wave.Finally,voltage transfer ratio along with output voltage harmonic characteristic of the novel converter was studied and one control method of maximum voltage transfer ratio was proposed.Study results of this part show that feedback energy of an asynchronous motor fed by the small switching capacitor frequency converter is very small.The energy is less than the consumed energy of the motor.The capacitor of the novel converter is a quarter of the capacitor of the traditional converter and voltage transfer ratio is approximately equal to the value of the traditional frequency converter.Output voltage harmonic of the novel converter is near to the value of the traditional converter.(4)Study on the methods of continuous variable frequency soft starting and bypass switching based on hexagonal voltage vector locusThe power of an asynchronous motor driving a pulping equipment is large and the switching loss of the small capacitor frequency converter feeding the motor is also large.In order to reduce the switching loss and switching stress,the switching frequency of the small capacitor frequency converter should be low.So the paper proposed the methods of an asynchronous motor soft starting and bypass switching based on a hexagonal voltage vector locus.The methods were developed by three steps.Firstly,the paper studied the theories of 120 degrees square wave inverting and 180 degrees square wave inverting and proposed a continuous variable frequency soft starting method based on a hexagonal voltage vector locus.The method could reduce the switching frequency and switching loss of the small capacitor frequency converter.Then,the paper studied the dynamic circuit structure of the converter.According to the equivalent instantaneous circuit of the small capacitor frequency converter,one important rule was revealed that two-phase circuits of the converter were directly connected in time of 3.3 millisecond.So two-phase circuits direct bypass switching strategy was suggested.Finally,the switching strategy was detailly described when the converter was controlled by the methods of 120 degrees square wave inverting and 180 degrees square wave inverting.The simulation results for two methods were further compared.Study results of this part show that DC-link bus voltage of the converter is six-pulse voltage when asynchronous motor is in steady state and two-phase circuits are connected directly in time of 3.3 millisecond when small capacitor frequency converter is controlled by the method based on the hexagonal voltage vector locus.The bypass switching performance based on 180 degrees square wave inverting control is superior to the performance based on 120 degrees square wave inverting control.(5)Study on the energy saving method of pulping equipment with light load based on frequency variable soft starting controlThe asynchronous motor driving pulping equipment consumed large amount of energy.So the paper studied the energy saving theory of the transporting pulp pump and the fiberizer,which were all important equipment in papermaking industry.The study was composed of three parts.Firstly,the energy saving theory of transporting pulp pump with light load based on speed control was investigated and energy saving model was also built.Then,the energy saving effect based on variable frequency control was contrasted with the effect based on adjusting voltage control.Finally,control theory and energy consumption model of the fiberizer were studied in the paper.One energy saving method of the fiberizer with light load based constant torque frequency control was also suggested.The energy saving method considered motor loss.Study results of this part show that when pump speed is reduced by percent eleven point five,power consumption of an asynchronous motor driving the pump is reduced by percent thirty for variable frequency soft starting method,Which is greater than the value of percent twenty for the traditional voltage regulation soft starting method And speed adjustable range for variable frequency soft starting method is wider than the traditional method.Rotor copper loss along with stator copper loss and stator iron loss was all decreased when speed of the fiberizer with light load was reduced.Comprehensive energy saving effect for variable frequency soft starting method is remarkable.In conclusion,main innovative content of the paper included five parts.Firstly,the paper discovered the basic cause of negative electromagnetic torque for thyristor DVF soft starting method and proposed a novel DVF control method based on hexagonal voltage vector locus.The method is effective to eliminate the negative electromagnetic torque of an asynchronous motor controlled by the traditional DVF method.Secondly,in this way,one novel DVF soft starting method of the frequency by f/7,f/4,f/3 and basic frequency based on flux locus control was presented.This novel method removed the reverse voltage vector.Thirdly,the paper proposed one circuit structure of small capacitor frequency converter and the method of two-phase direct bypass switching based on hexagonal voltage vector locus.Forth,energy feedback model of asynchronous motor fed by a frequency converter was built and capacitor parameter calculating method was also suggested.Fifth,the models of variable frequency energy saving of pump and fiberizer with light load were built.The loss of asynchronous motor was included in the model. |
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