Study On Magnetic Suspension Principles And Control Methods Of Bearingless Motors

Bearingless motors, which just emerged more than ten years ago, are a new type of electrical machines. Being very different from traditional motors, a bearingless motor not only generates electromagnetic torque, but also produces magnetic forces to suspend its rotor without using mechanical bearings. The magnetic suspension principles and control methods of bearingless motors are studied in this dissertation.Firstly, magnetic suspension principles are analyzed based on reviewing the development and research of bearingless motors. A model of magnetic suspension forces is built by using 2D field-circuit coupling and analytical methods. The model can reveal the magnetic suspension principle, and play an important role in design and control of a bearingless motor. Based on the analysis and comparison of different rotor structures, a novel bearingless motor with a hybrid rotor with permanent magnet and core is proposed. By means of the analysis of 2D and 3D magnetic field and calculation of electromagnetic force, a prototype of such kind of bearingless motor has been designed and built.Secondly, the decoupling control methods of magnetic suspension forces and rotor displacement are discussed. A new decoupling control strategy of magnetic suspension forces is put forward using the transformation of variables and reference frames for the proposed bearingless motor. The steady-state errors of current closed loop system with PI current regulator in stationary and synchronous frames are compared using complex vectors. Also, the corrector of rotor displacement control is studied by the root locus method.Thirdly, the measuring technique of rotor displacement is investigated. The performance, feature and application fields of non-contact displacement sensors used in bearingless motors are compared. The static and dynamic characteristics of the eddy current displacement sensor are studied experimentally. The errors in measuring system consisting of two sensors are examined, at the same time a method of eliminating the errors is introduced. And a self-detecting method of the rotorcenter is proposed.Finally, using DSP(TMS320C32) and CPLD(XC9572), a digital control system for the new type of bearingless motor has been designed and built. The experimental results show that the proposed decoupling control principle, method of magnetic suspension forces and the bearingless motor with a hybrid rotor are correct and feasible.