Research On Low - Speed Stable Operation Method Of Induction Motor Adaptive Observer

With the development of AC variable frequency speed regulation technology. AC frequency-variable drive drilling rig has become a widely used oil drilling equipment. Generally, the speed closed-loop control is necessary for high performance AC speed control. However, the coaxially mounted speed-sensor, not only increases the cost and reduces the system robustness, but also is resticted by many conditions. Therefore, the speed sensor-less control scheme is proposed and has become a research focus in the motor speed control field, among which the adaptive observer method has many advantages such as good comprehensive properties and wide applicability. Nevertheless, low-speed stability of adaptive observer needs further study. So this paper mainly studies the low-speed stable operation of induction motor vector control system based on adaptive observer, the main contents are as follows.Firstly, the parameter sensitivity of adaptive observer is analyzed. Due to the coupling effect of rotor flux observation and the speed identification, it is difficult to analyze the impact of motor parameters on rotor flux if the real speed is unknown. To simplify the analysis prcess and based on rotor flux orientation, the steady-state identified speed expression can be derived when the estimated value of the q-axis stator current is equal to its actual value. Then, the parameter sensitivity of rotor flux can be analyzed by applying the ratio vector function of the observed flux with motor actual flux when the steady-state identified speed is obtained. The three-dimension relation depicting the variation of speed error and rotor flux ratio vector function with motor speed and load are explored, where the effect of the main parameters of the motor is inspected on the rotor flux observer and speed identification when adaptive observer parameter is mismatched with the motor parameter. The simulation results show that the identified speed is sensitive to the stator and rotor resistance at overloaded condition and is sensitive to mutual inductance at low-speed region with light load. Additionally, the rotor flux is sensitive to the stator resistance and mutual inductance at low-speed region with light load, and it is insensitive to the rotor resistance.Secondly, the low-speed stability of adaptive observer is studied for induction motor. Only considering the speed identification, the instability phenomenon in the low-speed regenerating mode is analyzed by applying Popov’s hyperstability theory. Furthermore, stability analysis is simplified based on rotor flux orientation and the stability condition is achieved by using Routh-Hurwitz criterion. When adding the stator resistance estimation to the speed identification system, it becomes a multi-parameter identification system, and it is difficult to discuss the system stability because of the coupling effect of speed identification and stator resistance estimation. To analyze the complicated system, a2×2multiple-input-multiple-output closed-loop control system is obtained by taking speed error and stator resistance error as input and identification speed and estimated stator resistance as the output. If all zeros of the system characteristic equation lie on the left-hand side of s-plane. a stable feedback gain design criteria is obtained. Finally, some simulation and experiments were done based on MATLAB and TMS320F2812platform. The results show that the adaptive observer based on traditional poles assignment will cause an instability operation region. On the contrary, the estimation system based on the proposed speed identification method has the good performances both in convergence and stability at low-speed region, and improves the dynamic and static performances of vector control system.