Research And Development Of The Main Theory Of The Bearingless Induction Motor

The interest in ultra-speed motors and generators in industry applications is increasing recently. Bearingless motors, which combine magnetic levitation bearings with the drive winding, have the advantages of no abrasion, no lubrication and no particle generation caused by bearings. Furthermore, in comparison with conventional magnetic bearing motors, the axial length of the rotor is decreased, which results in increased frequency of the mechanical natural vibration. Furthermore, the efficiency can be increased because the magnetic flux of the motor can be partly used for levitation. This dissertation focuses on the main theory of bearingless induction motors including mathematical models, flux field oriented vector controllers, independent control of levitation subsystem, magnetic flux detecting with search coils, the influence and compensation of levitation controlled coupling, and design of digital control system. Successful levitation operations are realized based on diverse control strategies. In this dissertation, the mathematical models of the bearingless induction motor with rotor eccentricity are analyzed. Inductance matrices of squirrel cage induction motors are derived in detail. The equations of axial torque and radial forces are derived from the stored magnetic energy. In order to realize the decoupling control of electromagnetic torque and radial levitation force of bearingless induction motors, the air-gap flux-oriented controller for the drive control system is utilized. However, in the control algorithm based on air-gap flux orientation of drive winding, there are many limitations such as great amount control computation, inherent pull-out torque and difficulty in realizing the adaptive control. In that case, a nonlinear control algorithm based on rotor flux orientation of motor winding is proposed. With this control method, the speed regulation performance and flexibility in usage can be improved. The stable levitation can also be assured because the needed information about the air-gap magnetic flux of drive winding in the radial levitation controller can be obtained by system identification. The independent control between the levitation and drive subsystem is an effective way for bearingless induction motors in ultra-speed operation. The levitation subsystem is controlled independently from drive control subsystem based on magnetic flux detecting with search coils or voltage-model method identified. Furthermore, three methods of detecting the flux densities of both the drive winding and levitation winding with search coils are analyzed in detail: evenly distributed position method, typical position method and analytic method. The experimental results verify the validity of the proposed methods. The effect of delay between the actual radial air-gap flux and command air-gap flux is analyzed. One of the methods to compensate the phase delay, which is caused by rotor current of the levitation winding or digital filter, is to adjust the parameters of the PID controllers or direct compensator. The air-gap flux feedback tracker of the levitation winding is proposed to eliminate coupling of levitation control caused by neglecting rotor current of the levitation winding. A digital control system for real-time control is designed on double-DSP (TMS320LF240A). Moreover, the main design schemes of the software and hardware are introduced in detail. Test results in a prototype motor validate the performance efficiency of the proposed controller in steady state, as well as transient state.