Vector control optimization of bearingless induction motor considering skin effect of cage rotor

Abstract: The bearingless induction motor (BIM) is a new type motor without contact and friction and has the function of rotation and suspension. The rotor parameters are important factors that affect the accuracy of the field orientation control strategy of BIM, however, they are easily affected by the skin effect during the process of starting and high speed running, which result in the changes of rotor resistance and leakage inductance. The variations of the rotor parameters will cause the deviation of field orientation and reduce the motor control performance. Aiming at these problems, a parameter identification method based on finite element calculation and least square method was proposed in this paper. Firstly, based on the analysis of suspension force generation principle of BIM, the 2 D finite element model of the motor was constructed and values of the rotor resistance and leakage inductance at different frequencies were calculated by the finite element simulation. Then, by reasoning on the mathematical model, the influence of skin effect on the rotor field orientation and suspension control of BIM was analyzed in detail. The results showed that the variations of rotor parameters caused by the skin effect would affect rotor flux field orientation and made the actual suspension force deviate from the given value. Next, the least square method was used to realize fast identification of the rotor parameters, and the function relationship between rotor parameters and current frequency was obtained. Based on this, a parameters identification model was presented and a BIM field-oriented control system with considering rotor skin effect was proposed. The real-time value of the rotor resistance was obtained through the identification module to correct the phase angle deviation of rotor flux linkage so as to completely decouple torque component and exciting component. Finally, in order to verify the accuracy and effectiveness of the proposed control strategy based on curve fitting identification by the least square method, the simulation of BIM vector control system was established by MATLAB Simulink toolbox. Simulation results showed the motor took about 0.12 s arriving at the given speed after considering the skin effect, the response speed increased by 20% and the overshoot of speed response was very small. The maximum displacement in the x direction was reduced from about 12 μm to 8 μm and the maximum displacement in the y direction was reduced from about 24 μm to 18 μm after considering the skin effect. That means the motor could enter stable suspension state quickly and had a good suspension performance. To further validate the feasibility and effectiveness of the proposed control method, a BIM prototype was taken as experimental object, and a digital control system based on DSP TMS320F2812 was constructed. In the process of experiment, the rotor speed was measured by photoelectric encoder so as to calculate the slip frequency. The displacement of the BIM was measured by the eddy current displacement sensor to determine the required suspension force. The experimental results showed the speed rise time reduced from 0.2 s to 0.15 s with considering the skin effect, the response speed increased by 25%. This indicated the motor had a good start performance. And the peak to peak values of the rotor radial displacement decreased from 40 μm to 20 μm, reduced by about 50%. The proposed control strategy of the BIM can effectively improve speed response and reduce the radial displacement, and achieve good torque and suspension performance. This study can provide a reference for the performance optimization of BIM control system.