Design And Simulation Of Discrete Frequency Soft Starter Based On Fuzzy Variable Firing Angle

Because it is difficult to fulfill medium-sized induction motor heavy-dutystarting, and there is no effective solution to solve this problem. Discrete frequencytechnology can fulfill low current, high torque starting. But when the motor usesopen-loop discrete frequency soft starting technology, the motor will have suchproblems as speed down, torque fluctuations etc. So the research of new discretefrequency control strategy which can not only achieve high torque, low currentstarting, but also can eliminate torque, speed fluctuations at switching instant, willhave great significance.This paper determined the phase sequence combination which had maximumpositive sequence component by analyzing the phase sequence after frequencydividing. After that this paper proposed a thyristor trigger pulse scheme which can beused in the open-loop discrete frequency control method based on the analysis oftraditional discrete frequency technology. It can set switching phases of differentfrequency bands specifically. This paper ultimately determined the optimal switchingphase of different frequency bands by simulation comparative analysis of thesituation about torque and speed fluctuations when different bands used differentswitching frequency. Simulation results show that when the optimal switching phaseapplied to the open-loop discrete frequency control, the speed drop and torquefluctuation can reduce greatly.Secondly, in order to further reduce the starting current of induction motor, andeliminate the speed drop and torque shock, this paper used the discrete frequencytechnology to propose a high torque soft starting control strategy which used fuzzycontrol algorithm to calculate the firing angle of each band in order to achieve theeffect of discrete frequency dividing. This paper derived the formula of the requiredfiring angle of different bands based on the analysis of the equivalent sine trigger,and get the real-time changing firing angle of each band changing as the input variables by correcting the relationship between linguistic variables and outputvariables through using a correction factor, and achieved discrete frequency softstarting control strategy of the fuzzy variable firing angle.At last, in order to verify the effectiveness and superiority of the fuzzy variablefiring angle control strategy, this paper analyzed and designed the hardware of themotor soft starter based on C8051F310. This paper can obtain such conclusions asfollows: When using fuzzy variable trigger angle discrete optimal control, the motorhas such significant advantages as motor torque increased3times of the rate valuethan the traditional way, and its current dropped4times of the rate value than directstarting, and no torque fluctuations, no speed dropping, etc. Test results show that thevoltage waveform is consistent with the simulation voltage waveform in cycle andstructure, and the motor can achieve full load fast starting, has a smooth start-upprocess, and speed and torque ripple does not appear at switching time. Then itproved that this control strategy could be better applied to the field of overload softstarting of the motor. This strategy can obtain good starting characteristics, and hasbroad application prospects.