Multiphase Permanent Magnet Machines: Theory Analysis And Control

Thanks to the advantages of both multiphase machines and permanent magnet machines, multiphase permanent magnet machines (MPM) are well suitable for the high power electric machine drive applications. And the rapid development of interrelated science and technology brings out the expansion of the application fields of MPM. As a typical interdisciplinary product, MPM based adjusted speed drive system is becoming a new direction of the electric machine research and a focus of a substantial worldwide attention of the researchers. It is worth to investigate the fundamental theory and control strategies of MPM. In this thesis, the MPM with integral slots per pole per phase and full pitched windings is studied. The magnetic motive force (MMF) harmonics of the multiphase windings with various structures are analyzed. The influence of the different wave shapes of the back electromotive force (back-EMF) and the phase currents on the electromagnetic torque is studied and compared. An improved method to minimize the cogging torque is proposed. Precise mathematical modeling and simulation methods for MPM are studied. And the system solutions of the MPM adjusted speed drives as well as the novel control strategies are researched.Using winding function method, the analytical expressions of the MMF of multiphase winding are deduced, and the spectrums of MMF harmonics of the windings with different number of phases are figured. The quality factor and usability rate of the MMF harmonics are defined, and are regarded as the criterion for comparison. From the viewpoint of the MMF harmonics, under current harmonic injection mode, the electromagnetic torque characteristic of the nine-phase permanent magnet machine with symmetrical winding connection is optimal, which coincides with the conclusion drawn by analyzing with the back-EMF and the phase currents.An improved pole arc combination method for reducing the cogging torque of the permanent magnet machines is presented. The rotor is consisted with several segments along the axial direction with different pole arc widths. With this method, the rotor structure can keep symmetrical, and it is easy to process. In addition, the sensitivity of the processing error is receded.The phase-variable modeling method based on finite element analysis is adopted to the MPM. An improved modeling method based on circuit components is proposed, which has the advantages of both the S-function based model and the circuit component based model. The simulation efficiency is greatly improved and the simulation accuracy is comparable with the finite element analysis. According to the characteristic of the nine-phase permanent magnet machine, the windings are divided into three units, and the mutual inductances between the winding units are neglected when designing the control method, then each unit can be regarded as a non-sinusoidal three-phase permanent magnet machine. Based on the instantaneous power theory, a novel optimal reference current control method is proposed. For given electromagnetic torque, the optimal reference current can be calculated directly provided that the data of the winding inductances and no-load flux is obtained already. With the optimal reference current, the permanent magnet machine can get maximal torque per ampere without torque ripple. The effectiveness of the proposed method is confirmed by finite element analysis. And this method is suitable for any kinds of three-phase permanent magnet machines.In the conventional vector control based on Park transformation, the d-axis is fixed on the direct-axis of the rotor, and the decoupling control can be realized by maintain the d-axis current equal to zero. However, the reluctance torque can not be used and only the fundamental harmonic of the current can be tracked. A novel vector control strategy based on the generalized synchronous rotating frame (named D-Q frame) is presented. The Q-axis is oriented on the back-EMF of electromechanical energy conversion, which is the same with the d-axis only in a non-salient sinusoidal permanent magnet machine. The maximal torque per ampere vector control can be implemented by holding the D-axis current be zero. Simulation results show that the dynamic and steady state performance of the proposed vector control for non-sinusoidal permanent magnet machines are similar with the conventional vector control for sinusoidal permanent magnet machines. The proposed vector control strategy is then used in the control system of the nine-phase permanent magnet machine, and the simulation results validate its feasibility.