Performance Optimization Research Of Induction Motor Variable Frequency Drive System With Open-Loop Speed Control

With the development of power electronic technology, the AC (alternating current) speed regulation technology has been widely used. Particularly variable-frequency regulating speed of AC induction motor based on V/F control has a broad application on a low requirement for dynamic performance, such as air blower, water pump. However, there are still many deficiencies and problems on the low-speed performance, including weak starting performance, poor low frequency loading ability, and degradation of system performance at no-load or light load. Aiming at afore-mentioned key issues, a deep study has been carried out in this paper.In order to eliminate the impacts of dead-time effect to output currents and voltages, dead-time compensation method is put forward on the basis of pulse width. Active component isd and reactive component isq can be obtained by coordinate revolution transformation on the basis of stator voltage vector orientation. Then the low pass filter and the inverse transformation are used to get the three-phase currents in ABC coordinate systems. Finally, voltage space vector needed to be compensated can be judged according to polarities of three-phase currents, moreover the values of compare registers in DSP are changed. The experiment results show that the current waveforms are improved in low frequency.In order to improve the starting performance and low frequency loading ability, low frequency voltage compensation is proposed based on the stator currents decoupled control. In this method, voltage compensation amount is calculated according to currents producing torque, besides the resistance drop on the stator can been compensated. It is proved by experiment that the starting performance has been noticeably improved in low frequency case.To get high system stability, the reasons for V/F control system oscillation in the induced motor are analyzed in this paper, and an oscillation suppression scheme is adopted which is based on the stator currents decoupled control. By theoretical analysis and experimental data, it can be concluded that the oscillation will be suppressed effectively if we can keep the current constant. The experimental researches indicate that this method is effective.The basic principles of the weak magnetism speed governing system of induction motor are introduced in this paper, in order to realize the weakening magnet field control in higher than basic frequency. Considering that the V/F control can is no longer applicable beyond rated frequency, the six-pulse control is applied. In this study, most of the experiments are made on the100kW AC frequency speed-control experimentation system, whereas some of them are accomplished on2.2kW experimentation system. From the experiments, we can draw that the low speed performances of the induction motor speed-adjustment system based on the V/F control are greatly improved.In addition, to monitor the system operation easily, man and computer interaction system with keyboard and liquid crystal display has been designed. The operating condition can be changed online, such as starting, stopping, frequency, the rise time of speed, falling time of speed, forward moving, backward moving, system reset. At the same time, the running condition is displayed including frequency, sampling period, rising and falling time as well as false signal. Much convenience is provided for system debugging and operating.