Research On Soft-starting For Three-phase Asynchronous Motor

When the motor starts it draws high inrush current.On one hand,it drops the grid voltage.On the other hand,it endangers the drive equipment and the motor itself.Thyristor soft starter is widely used,because it has the advantages of simple structure,low cost and flexible control.In this paper,the thyristor soft starter is taken as the research object.Application of fuzzy PID control in soft-starting and the way to reduce the oscillation in the starting process are studied.The work done and the results obtained of the paper are as follows:Firstly,the steady state mathematical model of asynchronous motor is studied to analyze the starting current and starting torque.Its dynamic model is analyzed to reveal the basic reason of current and torque oscillation.The topology of the main circuit and the mode of voltage regulation are selected.Secondly,Aiming at the problem of poor adaptability and low accuracy of three-phase asynchronous motor soft-start system based on PID control,fuzzy PID control which combines fuzzy control and PID control is put forward.Based on the analysis of working principle for PID constant current start,ramp voltage start,discrete frequency conversion starting and fuzzy PID constant current starting,the simulation model is built by MATLAB/Simulink.And the performance parameters such as starting current and starting torque of each starting mode are compared and analyzed.The fuzzy PID control is verified effectively and superiorly by the simulation results.The adaptability and control precision of the soft starter system are improved.The current and torque oscillation are reduced by reducing the input voltage at the beginning.However,there is still a large current and torque oscillation when the rotor speed is near the synchronous speed during soft starting.Thirdly,fuzzy PID control fails to resolve the problem of oscillation near the synchronous speed during soft starting.The effects of trigger angle,moment of inertia and load torque on the oscillation are studied.And the cause of the oscillation near the synchronous speed is analyzed.Thus,a new strategy based on turn-off angle control is put forward.Simulation results show that the way is effective.It reduces the oscillation near the synchronous speed greatly.Fourthly,the soft starting controller uses DSP(TMS320F28335)as major control chip.And It's hardware design,software design are finished.