Research On Harmonics Of Multi-phase Induction Motors

The theme of this dissertation focuses on eliminating the harmonics in multi-phase induction motor fed by voltage source inverter (VSI). On the theme, the research is carried out systematically by combining theoretical analysis, numerical simulation as well as laboratory experiments. The contents are summarized as follows.First of all, principles of winding's configurations of multi-phase induction motors are investigated according to the precise definition of the multi-phase order. And the harmonics of magneto-motive-force (MMF) produced by typical multi-phase windings are analyzed. The process of estimating harmonics is simplified based on the principle of space vector. For those machines that are constructed by multiple three-phase windings, the space vector voltages are selected for analyzing magnetic harmonics. By composing the space vector voltages according to the rule of volt-second, the process of harmonic analysis is further simplified.Several types of multi-phase reference transformation are discussed. Their principles of decomposition are investigated mathematically and physically. And their charateristics and application scopes are analyzed. Thus the theory of multi-phase reference transformation for harmonics research is formed. Based on the theory, models of multi-phase induction motors are set up. The models can be extensively used for analyzing harmonic, steady and dynamitic problems.Harmonic problems of multi-phase motor can be classified as substantial harmonic currents in stator and the pulsing electromagnetic torque. Substantial harmonic currents, which have drawn much attention, are phenomenon specially occurring in multiphase induction motor. The pulsing torque, which has been reduced obviously by multi-phase scheme, should be further reduced to meet more strict requirements. In this dissertation, the causes of these harmonics are explained and their influences on motors are discussed. The main methods for harmonics elimination presented previously are summarized according to their relations and features. This summarization is helpful to propose more perfect schemes.To eliminate the substantial harmonic currents in stators, a novel reactance filter based on the previous ideas is proposed in this dissertation. The construction of the filter is resembled to the stator of multi-phase motor in the system. When the distribution of multi-winding being properly adjusted, the filter is able to eliminate the substantial harmonic currents selectively but free to the fundamental currents. The research results show that the scheme of the novel filter has obvious advantages of minimizing the weight and the space.To eliminate the pulsing torques of a multi-phase motor, selected harmonic elimination (SHE) PWM is introduced. The rules of switch angles for regulating the output voltage of inverter are investigated. The strategy to realize SHEPWM by combining lookup table and interpolation is discussed. An improved method is proposed to avoid the complicated calculation of switch angles in traditional way. According to the method, when controlling a motor of dual three-phase spatially shifted by 30°, the switch angles of SHEPWM can be acquired from the harmonics existing in the difference between the practical system and a supposed system of fourfold three-phase spatially shifted by 15°.Finally, based on extensive numerical simulation, laboratory prototype of dual three-phase induction motor and the proposed filter were set up. The experimental results show the validity of the schemes for harmonic elimination proposed by the dissertation.