Research On5-DOF Rotor System Based On Conical Bearingless Asynchronous Motors

Bearingless motors can provide the driving torque without friction, abrasion and lubrication,by comparison with the conventional motors suspended with magnetic bearings, bearinglessmotors have the merits of more sample structure, shorter rotor, higher rotational speed andoutput power.However, in order to actively control five-DOF of the rotors, the conventional bearinglessmotors would work with magnetic bearings. This approach increases power loss of entiresystem and the length of rotor, and the output power and critical rotational speed will be solimited.In order to avoid the inherent defects of the above rotor systems equipped with magneticbearings, and further make full use of bearingless motor, in this dissertation, a novel5-DOFactively controlled rotor system that combines two bearingless induction motors with conicalair-gap (5-DOF Rotor System Based on Conical Bearingless Asynchronous Motors，5-DOFCBAM) is presented. The5-DOF CBAM has obvious advantages of short rotor, highefficiency, simple structure and high reliability, and the rotor can be fully leviated and drivedwithout any magnetic bearing.Basing on the analysis of magnetic levitation forces of5-DOF CBAM, theelectro-mechanical structure and control system are fully and deeply researched in followingaspects.The mathematical models of levitation forces based of air-gap flux magnetic densities andflux linkages are deduced, and the effect of rotor current generated by levitation windings onlevitation forces and the ratio of levitation windings stator current and excitation currentvarying with frequency are analyzed, and the compensation method for the error of givenevitation force and actual evitation force is so proposed.A special differential control method of air-gap flux density amplitude (or air-gap fluxlinkage amplitude) generated by torque windings is proposed, to realize the linearization ofaxial controllable magnetic force. The interaction of radial displacement control subsystem,axial displacement control subsystem and speed control subsystem is analyzed, and thedecoupling control strategy is designed. In order to validate the models of magnetic levitationforces and control method, a simulation model is built, and the simulation results so obtainedare satisfactory.Combining the design methods of induction motors, conical induction motors andtraditional bearingless induction motors, a design flow of electro-mechanical structure for5-DOF CBAM is proposed, and the limiting condition and design method are given.Moreover, a prototype is designed according to the design goal of rating power, rotational speed and maximum levitation forces.The generalized coordinate system for rotor is established to describe the state of motion,with the state of inclined rotor considered. The5-DOF dynamics model of rigid rotor isdeduced by analyze the magnetic force and coupling characteristics in there dimension. Thestructures of centralized control system and distributed control system are designed andsimulated, and aiming at the situation of work at high speed, the structure of cross feedbackcontrol system using to compensate the effect of gyroscopic moment is designed.In order to cancel the displacement sensors, simplify the mechanical structure of system,and shorten the rotor, the effect of rotor displacements on self inductance and mutualinductance of torque windings and levitation windings is analyzed, on the basis of theequivalent model of two-phase windings. A rotor displacement self-sensing method in alldirection is further proposed for5-DOF CBAM. Filially, the effectiveness of this method isvalidated by simulation results.The circuits of the control system are designed, and the decoupling control program basedon double DSPs (Digital Singnal Processor) is compiled. The mechanical structure andwindings distribution are given, and the angles between the axes of2-pole and4-polewindings and horizontal axis are mensured when the prototype of5-DOF CBAM is placedhorizontally. The experimental platform for5-DOF CBAM is built, and the speed regulationusing DTC (Direct Torque Control) and static levitation are accomplished.