Research On Multiphase Bearingless Motors

In early stage,the phase number of a motor was not more than three due to the limitation of power system.With the development of power electronics,it is possible to realize multiphase inverters,which remove the limits of phase number by power system.Multi-phase induction machine drives have attracted a considerable amount of interest due to its advantages over conventional three-phase drives,such as lower torque pulsation and better fault tolerance. Multiphase machines split the motor power across the higher phase number in order to realize a high power system by reducing the per-phase converter rating.The multiple control freedom of multiphase motor and its utilization are highlighted in this dissertation.The organization of this study is as follows:A deep insight into the multiphase motor performance can be gained by analysis of the air-gap MMF distribution.Formulas for the MMF in an n-phase motor that take into account current and space harmonics are derived.An analytical model of multiphase motor is built up.Based on it,the sinusoidal winding and square winding motors are discussed.MMF distribution and mathematical model are used to elaborate multiple control freedom of multiphase motor.A multiphase motor(n≥5 ) has multiple orthogonal d-q planes.Flux and torque control of a multiphase machine needs only one d-q plane,that is,one d-q plane can be used for torque control,and the others can be used in different manners.The applications of multiphase machines are analyzed from the point of view of multiple control freedoms.Flux density of teeth and back iron is analyzed.Rectangular flux distribution brings about improved iron utilization.The square flux distribution is obtained by injection of several current harmonics.In a 15 phase induction motor,how to gain the expected flux distribution is discussed when the motor is controlled by V/f scheme or Rotor-Flux-Orientation Control schemes.Compared to the machine with sinusoidal flux distribution,the machine with quasi-square flux distribution can output higher torque,and the reasons are given.Conventional bearingless machine has dual windings,levitation windings and torque windings.Bearingless motor with a single set of multiphase windings is proposed by introducing the multiple control freedom theory into bearingless motor principle.The two needed magnetic fields are generated by feeding two groups of currents to a single set of multiphase windings.The two groups of currents are mapped to different d-q planes.In other words,one d-q plane is used for motor drive,and the other is used for rotor levitation.Taking a six phase induction machines as example,the inductance matrices,considering air gap eccentricity,are calculated with the Modified Winding Function Method.The radial force analytical model is deduced by virtual displacement,and its results are validated by FEA.The mathematical model of the new bearingless machine is set up.A 6-phase induction bearingless motor prototype is developed.Simulation and experimental results verified the feasibility of this novel bearingless motor.One of the most important advantages of multiphase machines is that they can continue to operate after the loss of one(or more) phase(s).Under the faulted phase(s) conditions,based on the principle of an undisturbed rotating MMF,post-fault operating strategies of multiphase bearingless motors,sinusoidal-winding and square-winding motors are discussed respectively. The stator powerloss with different strategies is analyzed.