Author: Emad Roshandel
Roshandel, Emad, 2023 Electric Machines for Electric Vehicles Considering Efficiency Map, Flinders University, College of Science and Engineering
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The electric machines (EMs) utilized in the electric vehicles (EVs) and hybrid electric vehicles (HEVs) must operate efficiently over a wide range of torque and speed. Hence, the design procedure of such EMs is involved with the performance parameters estimation over various operating points. Efficiency maps (EffMs) project the maximum efficiency of the machine in the torque-speed envelope as well as the torque-speed capability of EMs. So, the EffMs can be used for prediction of the energy consumption of an EM during operation over a driving cycle.
The main objective of this thesis is the design of EMs for EVs and HEVs. The thesis starts with a short discussion on the advantageous of EVs and HEVs utilization. The different transmission system components in EVs and HEVs are introduced for better understanding of such vehicles. The different factors affecting the various components of the loss in the electric machines and transmission system of the EMs are described. The literature review highlights the role of EMs efficiency in the propulsion system of EVs and HEVs.
The EMs operating region and the control techniques for operation in different operating regions is introduced in the third chapter of the thesis. The procedure of the calculation of the EffM is explained. Different calculation methods are discussed, and their strengths and weaknesses are clarified. The experimental procedure of the calculation of EffM for EMs is described and the EffM is obtained experimentally for a sample induction machine (IM). The comparison of different EffM methods, in terms of accuracy and computation time, enables the designer to choose an optimum technique for a certain application.
A step-by-step design procedure based on the finite element analysis (FEA) is introduced for design of the 100kW IM and interior permanent magnet machine (IPMSM) for a HEV. The design procedure not only considers the electromagnetic validity of the designed machines, but also it covers the thermal aspects during the design. The importance of the optimal selection of the number of poles is studied. The effect of the V-shape angle on the torque ripple and cogging torque of IPMSMs is studied. The role of the number of rotor bars on the torque ripple of IMs is another aspect of the study. The advantageous and disadvantages of the IM and IPMSMs are highlighted based on a comparison of their cost, torque density, and power density.
The design of 100kW and 200kW axial flux permanent magnet machines (AFPMSMs) is the subject of study in the fifth chapter of the thesis. This chapter designs the AFPMSMs for a HEV with the series propulsion system. Initially a design procedure using 3-D FEA is described and applied to design a 100kW AFPMSM, with optimal operation in the constant torque (CT) region. It is shown that the double stator design in double sided axial flux machines can offer a better performance compared to the double rotor designs. The higher power density of the axial flux machines is highlighted by comparison of the performance parameters with existing axial and radial flux designs. The role of the slot numbers, number of pole effect, torque ripple, THD, slot numbers, solid loss, axial force, stator negative stiffness, rotor eddy current loss, demagnetization, and thermal constraints are discussed and analysed during the design. The kriging technique and multi-objective optimization are employed to find the 100kW design with the optimal performance in the CT region.
The fifth chapter is continued by the optimal AFPMSM designs with capability of the operation in the constant power region. The axial flux interior PMSM (AFIPMSM), fractional slots topologies, and number of poles are the subjects of the study for improvement of the AFPMSM performance in the field weakening region. It is shown that AFIMPSM suffers from a low power factor and low power density. The rotor eddy current losses are investigated in axial flux PM machines which results in the limitation of the utilization of fractional slot designs to achieve an acceptable field weakening performance. The change of the number of poles is investigated as another alternative for the performance improvement in FW. Finally, the optimal 200kW design with the slot per pole of one and capability of delivery of maximum power up to 4 times of rated speed is obtained through an optimization study. The results demonstrate that the increase of the PM and iron losses is an important limiting factor for using higher number of poles in electric machines especially in higher speeds.
The lack of availability of a fast and accurate model for performance prediction of the EMs is underlined in chapter six. The subdomain technique is used to develop an accurate model for prediction of the performance parameters of the IMs. The model is further improved by adding the saturation consideration capability using the subdomains magnetic vector potentials. This model is validated using the 2-D FEA, 3-D FEA, and experimental results in this chapter.
The fast speed and high accuracy of the developed analytical model enables to define an optimization problem over the driving cycle points. In chapter seven, an accurate lumped thermal model is proposed to predict the temperature of the IMs during operation in a driving cycle. An optimization design approach for design of IMs with and without consideration of overload condition is introduced. The optimal designs are compared with each other in terms of the performance parameters, power density, and energy consumption over driving cycle. The effect of the consideration of the overload operation on the weight and energy consumption of IMs is discussed.
A 4kW axial flux induction machine (AFIM) is designed in a same size as a commercial AFPMSM. The challenges for construction of the AFIM rotor and its manufacturing is discussed. The AFIM is constructed for experimental analysis. The effect of the airgap length on the axial force and stiffness is studied through the 3-D FEA and experimental results. It is shown that the axial stiffness in lower airgaps is high which is a limitation for designing the axial flux machines with small airgaps. The optimum airgap with the minimum stiffness is chosen to redesign the AFIM. The locked rotor test is performed experimentally to extract the equivalent circuit parameters of AFIM and validate the 3-D FEA results. The performance parameters of the proposed AFIM are compared with the commercial AFPMSM. The results show the similar power density of both design and less torque density of the AFIM design in the torque-speed envelope.
In summary, this thesis investigates different types of electric machines and their design procedure for EVs and HEVs. The main contributions of the thesis are (1) Detailed analysis of different physical phenomena on the loss of the electric machines; (2) Analysis and development of different methods for efficiency map calculation of electric machines; (3) A step-by-step design process for design of induction and permanent magnet radial flux machines; (4) Optimal design of axial flux permanent magnet machines with the capability of acceptable operation in the field weakening region; (5) Introduction of an analytical model for performance parameters prediction of induction machines; (6) Prediction of saturation level of induction machines using subdomain technique; (7) Optimal design of induction machine over driving cycle; (8) Design and construction of axial flux induction machine.
Keywords: Efficiency map, Electric machines, Electric vehicles, Hybrid Electric vehicles, Motor design, Optimization, Performance parameters estimation, Propulsion system, Thermal model.
Subject: Engineering thesis
Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Amin Mahmoudi