Design and Multi-physical Fields Analysis of High Speed Permanent Magnet Machines

  • Yue Zhang

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

Due to the advantages of high power density, high efficiency and compact size, high speed permanent magnet machines (HSPMMs) have found wide application in industrial areas. Compared with a conventional speed permanent magnet machine, a HSPMM rotor can reach speeds of more than 10,000 rpm, which brings challenges with regard to electromagnetic, thermal and mechanical aspects of machine design. The higher power density also results in larger power loss per unit volume; due to the small machine size, machine thermal dissipation becomes difficult. Moreover, air frictional loss rises dramatically when the rotor is in high speed operation and this may also further increase rotor temperature. Therefore, research into HSPMM power losses and improving machine thermal dissipation capability is of significant interest. HSPMM mechanical issues also need to be considered to ensure safe and reliable machine operation. As rotor speeds rise, rotor strength becomes prominent and critical as the permanent magnets are vulnerable to the large centrifugal force. In addition, the machine rotor should also have enough rigidity and avoid operating at critical speeds. As such, this dissertation focuses on HSPMM design and research. Multi-physical fields analysis of a HSPMM is carried out to calculate machine power losses and temperature distribution, with factors influencing machine performance considered; HSPMM rotor mechanical research and analysis are also carried out and presented in this study.
Firstly, the HSPMM design methodology and process are illustrated with machine rotor parameters, PM material, pole numbers and rotor sleeve considered for a 150 kW, 17000 rpm HSPMM. Then, HSPMM performance for different machine stator structures and PM pole arc pole pitches is investigated using the Finite Element Method (FEM) for the machine operating at both no load and full load conditions; HSPMM electromagnetic performance and how it is impacted by machine parameters is also studied.
HSPMM power losses are comprehensively investigated in the following chapter. As machine core loss can be significantly increased with increasing machine frequency, it is critical to accurately estimate HSPMM iron loss. Based on the machine iron core magnetic field variation that is obtained by FEM analysis, machine steel iron core loss estimation for HSPMM is performed using an improved method with the influences of alternating and rotating magnetic fields, as well as harmonics effects, considered for high precision. Then the HSPMM air gap magnetic flux density distribution considering machine stator slotting effect is also analytically calculated with its effectiveness verified by FEM results.
Then rotor eddy current loss is studied by time-stepping FEM, while the effects of rotor sleeve dimensions and properties, copper shielding composite rotor structure, air gap length, as well as slot opening width are further researched in depth. A PM bevelling method is also proposed and investigated to reduce HSPMM rotor eddy current loss while having little effect on machine output torque. Then a fluid field analysis is carried out to study HSPMM rotor air frictional loss when the rotor is in high speed operation.
According to the characteristics of a machine axial forced air cooling system, the HSPMM temperature distribution is investigated by 3-D fluid–thermal coupling CFD modelling with the calculated power losses results. The machine thermal analysis theory and modelling method are also detailed and further explained. HSPMM thermal performance variation due to impacting factors of cooling air velocity, rotor eddy current loss and sleeve thermal conductivity are also comprehensively investigated and studied in this dissertation. The designed HSPMM is prototyped, and temperature experimental tests are also carried out to verify the effectiveness of the research and analysis for HSPMM.
Then, thick-walled cylinder theory is introduced to study rotor mechanical strength analytically, while it also verifies the FEM calculation results. Then based on FEM analysis, HSPMM rotor stress distribution is investigated with sleeve material effects on rotor strength discussed. In order to alleviate the rotor sleeve stress, three pole filler materials are comparatively studied, while the temperature impacts on rotor mechanical stress is further considered; sleeve thickness and the interference between PM and sleeve are investigated in an integrated fashion for HSPMM rotor strength analysis, with some conclusions also drawn for HSPMM rotor mechanical design. HSPMM rotor critical speeds are also calculated by the established 3D rotor dynamic analysis FEM model to ensure the rotor is operating in a desirable condition.
Date of AwardSep 2018
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorSeán McLoone (Supervisor)

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