Research On Optimal Control Of Discrete Variable Frequency Soft Starter

Due t.o its superior performance,asynchronous motor is widely used in industrial,transportation,national defense and other fields,and how to make it get good starting performance has increasingly become a special concern of people.Al.though the direct starting mode is simple wiring,easy to maintain,but the starting current is very large,easy to cause exces.sive current impact on the access to the same power grid of other power equipment,while the starting torque is reduced,not suitable for heavy-duty starting occasions;The traditional buck starting mode is slighitly better than the motor direct starting mode,However,the starting torque can not be increased while the starting current is reduced.The traditional soft starter reduces the starting current by changing the trigger angle,but also reduces the starting torque and limits its adaptation ran.ge.On the original basis of the traditional soft starter,the discrete frequency conversion soft start realizes the frequency contversion starting by changing the control mode on the original basis,while reducing the starting current,the starting torque is increased to meet the requirements of heavy load starting.In this paper,on the basis of the equivalent circuit of asynchronous motor,the two indexes that affeecting the starting characteristics of asynchronous motor are analyzed and studied,that is,the starting current multiples and starting torque multiples,and the conclusion is that only by reducing the starting current aind increasing the starting torque at the same time can the motor be fully loaded or even overloaded,and analyzed the speed and power factor characteristics of asynchronous motor in order to provide a theoretical basis for the optimal control scheme of the subsequent discrete variable frequency soft start,and on the above basis,makes a theoretical study on the discrete frequency conve'rsion soft starting frequency division algorithm,and finally determines the specific frequency band connection mode in the process of discrete frequency division soft start,the optimal phase combination under each frequency band and,the main switching modes between each frequency band.Selcondly,the control method of discrete frequency conversion soft start is determined,the two trigger modes of fixed angle trigger and equivalent sinusoidal trigger are analyzed in detail,the advantages and disadvantages are carefully compared,and the trigger control method which is most suitable for this paper is selected.The optimal control scheme is used,that is in the process of slope voltage stage implementing power factor angle closed-loop control,in the frequency switching stage consider speed to reach the rated speed and is positive integer times to reduce the electromagnetic oscillation caused by the failure of the trigger angle is uncompensated and instability of frequency band switching.Finally,through building the modeling and simulation of discrete frequency conversion soft start,the paper compares and analyzes at the same time the direct starting mode of asynchronous motor,the current,rotational speed and torque simulation waveform under the traditional soft starting mode of slope voltage,verifies the advantages of the discrete frequency conversion soft start,and constructs the experimental platform of its hardware and software,and the experimental results are consistent with the simulation.Thus,the superiority of discrete variable frequency soft start control technology in motor heavy-load and even full load is verified.