Research On Radial Magnetic-field Modulated Brushless Double-rotor Machine For Hybrid Electric Vehicles

In hybrid drive technologies, the series-parallel one is the most advanced. At the international level, the research schemes on the hybrid drive technology now include two kinds, i.e., the mechanical scheme based on the planetary gear and the pure electrical scheme based on the compound-structure electrical machine(CSEM). The planetary gear, which is the key part of power distribution and continuously variable transmission in the mechanical scheme, is a precisely mechanical component. Hence there will always be some problems such as vibration, noise, abrasion and regular maintenance in the mechanical one. In contrast, the pure electrical scheme based on the CSEM not only has none of the above problems, but it also has advantages of simple and compact structure and easy control. So far, the research on the pure electrical scheme with brushes at the international level is becoming mature. The technology advancement and competitiveness of the pure electrical one have been verified by the current achievement. However, the research results also show that there are many problems to be solved, like the severe heating problem of the inner-rotor windings. To solve the above problems, a magnetic-field modulated brushless double-rotor machine(MFM-BDRM) is proposed in this paper. Cooperating with traditional machines, the MFM-BDRM can realize a novel pure electrical hybrid drive scheme. It also solves the problems in the early pure electrical ones, such as the severe heating problem of the inner-rotor windings, the poor reliability of brushes and slip rings and dynamic balance of the rotating windings easily to be broken. In this paper, the research work mainly focuses on the radial MFM-BDRM(RMFM-BDRM) as follows.Firstly, to simplify the analysis process of the complicated electromechanical energy conversion relation in the RMFM-BDRM, the diagrammatizing analysis method is proposed. The operating characteristic of RMFM-BDRM, such as speed, torque and so on, is investigated by this method. Then the comparative analysis of operating characteristic between the RMFM-BDRM and the mechanical planetary gear is further carried out, indicating that the RMFM-BDRM can functionally be equivalent to a mechanical planetary gear plus an electrical machine. According to differently connecting ways of the RMFM-BDRM rotors and the engine, two different kinds of hybrid drive systems are proposed. To evaluate their performances, the system efficiency of two schemes is further comparatively analyzed by the analytical method and the diagrammatizing one; and the suitable applications of two schemes are also determined, which can provide the theoretical basis for the selection of hybrid drive system. On that basis, combining the structure characteristic of hybrid system, the mathematical models of the RMFM-BDRM and its hybrid system are built, which can provide the theoretical basis for the accurate control of RMFM-BDRM.Secondly, to obtain high power density and good electromagnetic performance, the design principle and design method of the RMFM-BDRM in hybrid electric vehicle(HEV) applications are investigated. Proceeding from the engine characteristic, the design principle of the rated and feature parameters of RMFM-BDRM is proposed. It shows that the RMFM-BDRM can be designed by the maximum torque and the expected speed of the engine. Considering the requirement of the electromagnetic performance of electrical machine in HEV applications and the specificity of the RMFM-BDRM operating principle(the magnetic-field modulation principle), the design method of the RMFM-BDRM is investigated, like the optimal size ratio of magnetic block in the modulating ring and PMs, the optimal matching law of the number of magnetic blocks in the modulating ring rotor and pole-pair numbers of inner and outer magnetic field, the optimal arrangement of stator windings. Such the design method can provide the theoretical basis for obtaining the optimal scheme; and it also has great value for other electrical machines based on the magnetic-field modulated principle.Thirdly, to further improve the performance of RMFM-BDRM, the special electromagnetic problems of low power factor and high harmonic loss in the RMFM-BDRM are investigated. Due to the existence of two-layer air-gap structure and modulating ring rotor, the RMFM-BDRM appeares the problems of low power factor and large harmonic loss. In this paper, such the problems are named as the special electromagnetic problems of RMFM-BDRM. To improve the power factor of RMFM-BDRM, the influence factors of the power factor are firstly analyzed by the analytical method. Then some improvement methods of power factor are proposed from the aspect of the pole-pair combination ratio of stator and PM rotor, the design of slot shape, the inner power factor angle, etc. To reduce the harmonic loss of RMFM-BDRM, the loss distribution law of different parts is quantitatively analyzed by the analytical method and the finite-element method. Aiming at the problem of high stator loss, the influence of different materials on the stator loss is investigated. Aiming at the problem of high PM loss, a novel PM rotor is proposed and investigated about its loss, power factor and other electromagnetic performance. On that basis, combining the characteristics of hybrid drive working conditions, the calculation method of the RMFM-BDRM loss and efficiency is given; and the design flow of the RMFM-BDRM is also provided, with consideration of power factor and harmonic loss.Afterwards, to ensure RMFM-BDRM safe under the vehicle background, the problems of temperature rise and cooling are investigated. Aiming at the characteristic of the difficultly dissipating heat of two-layer air-gap structure and the high loss of stator and PMs in the RMFM-BDRM, the cooling systems of stator water cooling and axial wind cooling are proposed. Then the 2D equivalent thermal field model is built based on the characteristic of cooling system and the heat transfer of key regions. To evaluate the effectiveness of different cooling ways, the distribution law of temperature rise is investigated under different cooling conditions. On that basis, the effectiveness of different cooling ways is analyzed and evaluated. Aiming at the HEV background, the influence of different hybrid drive working conditions on the temperature rise of RMFM-BDRM is investigated. Finally, the matching law of the load working condition and cooling parameters is determined, which can provide the theoretical basis for the design of cooling system and its power source.Lastly, to validate the validity of theoretical analysis and comprehensively evaluate the performance of RMFM-BDRM, the a prototype of RMFM-BDRM is manufactured, and its performance is comprehensively tested and analyzed. Meanwhile, its functional validation under hybrid drive conditions is performed. To solve the contradictory problem of electromagnetic performance and mechanical strength of the modulating ring rotor, the electromagnetic performance and mechanical strength of modulating ring rotor with different structures and materials are analyzed; and the feasible modulating ring rotor is finally determined. To verify the validity of theoretical analysis and simulation calculation in the previous chapters, the performances of prototype is comprehensively tested and analyzed, such as electrical parameters, back electromotive force, torque-speed relation, loss, efficiency, power factor, temperature rise, and so on. The further improved suggestions are proposed based on the tested results. The simulation test of starting engine, generating, and speed adjustment conditions are performed for the RMFM-BDRM prototype in the HEV applications, which can provide the experimental basis for the further improvement and engineering application of the RMFM-BDRM.