Fundamental Research On Permanent-Magnet Type Bearingless Motors

In virtue of the similarity of stator structure of motors and magnetic bearings, levitation windings can be wound together with conventional motor windings to produce radial electromagnetic forces. The new type magnetic suspension motors, namely bearingless motors, which combine the function of rotor support with motor rotation, inherit the advantages of no friction, no abrasion and no lubrication. Furthermore, high axial capacity factor and compactness, low power cost can be achieved in comparison with conventional magnetic bearing motors. With these advantages and other features, bearingless motors demonstrates the potential applications in space technology, machine tools, high vacuum technology, ultra-speed drives and so on.Compared with other type of bearingless motors, the permanent magnet type bearingless motor, due to its simple structure, absence of excitation windings, high power factor, high power density and high efficiency, can be widely used in the developing industry applications of flying wheel for energy storage, spindle machines, sealed pumps, centrifuge pumps, compressor, high speed micro-harddisk drive technology, etc.This dissertation focuses on the basic research including magnetic suspension theory, related mathematical models and control strategies of surface-mounted and consequent-pole permanent magnet type bearingless motors. Most of fulfilled studies are shown as follows:Firstly, a profound theoretical analysis on mechanism of the generation of magnetic levitation forces with rotor eccentricity for a surface-mounted bearingless permanent-magnet type motor is presented. The coupling effects on the control of torque and radial levitation force are especially discussed when considering the influence of Lorenz force and rotor eccentricity. Based on the detailed analysis of air-gap magnetic field under eccentricity condition, a feasible control strategy of levitation force and torque is developed and experimental results reveal the new control strategy effectively improves the static and dynamic performance of bearingless motors and vastly broadens their fields of application.Secondly, the amplitude and phase of the air-gap flux linkage in motor windings transferred by the torque subsystem are necessary for the radial levitation force control. The information transmissions between the two control subsystems make their control schemes restricted mutually. In allusion to the limitations in the previous strategy of rotor flux field-oriented vector control, a traditional method of voltage-current model is proposed to be applied to identify the required air-gap flux linkage of motor winding. The independent control of motor torque and levitation force can be realized to elimilate the coupling influence and accordingly improves the practicability of bearingless permanent-magnet synchronous motors in applications of super high-speed machines greatly.Thirdly, to verify the decoupling characteristics of torque and radial suspension force control without rotating angular position, a finite-element simulation analysis with electromagnetic softwares is used for a new-type consequent-pole permanent magnet bearingless motor. In succession, an in-depth simulation analysis on the comparison of torque and suspension force with the surface-mounted permanent-magnet bearingless motor is accomplished and simulation results indicate, with approximate torque output, suspension force of the consequent-pole permanent-magnet bearingless motor is much larger and thus make it avoid the compromise between radial suspension force and drive torque in conventional permanent-magnet type bearingless motors.Fourthly, due to the coupling effects of drive torque and radial suspension force in conventional permanent-magnet-type bearingless motors, a profound theoretical analysis on mechanism of magnetic levitation for a consequent-pole permanent magnet bearingless motor is presented and its mathematical model is also constructed according to the theory of air-gap magnetic permeance and magnetic circuit. On basis of levitation force and torque control strategy, an experimental platform of digital control system for real-time control is designed and realized on the single DSP (TMS320F2812) for this new permanent-magnet bearingless machine. Test results in a prototype motor suggest good performance of static and dynamic suspension and vadilitate the inherent decoupling characteristics between suspension and drive subsystems.Finally, with the reference of direct torque control of conventional permanent-magnet synchronous motor, a novel direct force control algorithm based on space vector pulse width modulation method is proposed to directly control magnetic levitation fore inside bearingless motors. A particular theoretical introduction on rudimental principles and basic derivations of direct force control is investigated and a real-time levitation force and torque control system is also devised. The results from simulation and experiments in a permanent-magnet type consequent-pole bearingless machine suggest successful suspension in steady state as well as transient state and confirm the validity and availability of this new control arithmetic. A new way of suspension force control is carved out for permanent-magnet type bearingless motors.