A Study Of The High-voltage Asynchronous Motor Soft Start Based On The On-off Transformer

The direct-on-line start of high power induction motor can generate great impact current, which will damage the motor itself and contaminate the power supply system. Therefore, reduced-voltage motor starter is usually employed to suppress this current impact. However, this traditional approach is very crude and its voltage regulation is discontinous, thus resulting in secondary current impact during the start process. Electronic soft starter is a kind of novel motor control device, which can implement the soft start, soft stop and energy-saving of the asynchronous motor. Comparing with the traditional motor starter, it has outstanding advantages such as stepless regulation of the motor's start, avoid the inrush of the torque and the current, and reliable protections, convenient operation and repetitive starting. While the domestic research on the field is still in its infant stage. The high-voltage asynchronous motor soft starter based on the on-off transformer is researched in the paper. The main content include:1. The mathematics model of the on-off transformerThe mathematics model of the n-phase on-off transformer is introduced. The mathematics model expression of the 3-phase on-off transformer is gotten and it can be look as a varying reactance. The equivalent circuit model provides a very simple method for analyzing and designing the high-voltage soft starter. The simulation is carried on using the producing equivalent model. The effective tool is offered for designing the high-voltage motor soft starter and advancing the successful ratio of soft start.2. The operating phase analysis of the high-voltage motor soft starterThe starting mathematical model of the motor in terms of ABC/dq axes quantities is deduced, which greatly simplifies the motor mathematical. All operating model of the soft start is analyzed. The motor mathematical model of every operation phase is gotten.3. Oscillation phenomena and settlement method of soft start under light-loadThe motor power factor and the reason of the orque and current oscillation is researched during the soft-starting of three-phase induction motor under Ioght-Load. a closed-loop power factor control method for the soft-starting system is proposed, the power factor angle is treated as a feedback variable. The freewheeling angle of the motor is determined by its power factor, in order to regulate the stator voltage to its reference, the adjustment of the firing angle must follow the change of the power factor angle. Otherwise, the oscillation of the motor torque and current will arise.4. The control strategy research of the soft starterTo minish the torque pulsation during motor starting, the torque performance of different switching points in the first cycle, when the voltage is applied to the motor, is studied thoroughly. The torque pulsation is well eliminated when the properly switching instant is selected. The optimum switching instant has nothing to do with the load when the motor is operating. Since the measurement of current is easy, the control strategy with the constant current is presented. Combining these two methods, the inrush current is limited and torque pulsation is eliminated during starting the motor.The paper aims at the condition that AC motor is a controlled object of non-linear and strong-coupled and it is difficult to obtain a satisfactory result by means of the common control. So the paper provides a high-voltage AC motor soft starter's designing method based on the adjusting fuzzy control rules. The controller is to perform the self-adjust for the adjusting factorβ, and through the genetic algorithm, a parameter optimization program is developed. According to the current error△I and the changing error△I_c, the adjusting factorβis readjusted via the fuzzy inference adjust to ensure that the soft start has the best dynamic and stable performance.5. Design of the high-voltage soft starterThe novel high-voltage soft starter adopts the PIC16F877 single chip. The high performance control system of the novel soft starter for a three-phase high voltage induction motor is developed. The system hardware configuration, flow chart of the software and related experimental results are also given and the experimental results indicate that the proposed system has a perfect performance.