Study On The Electric Machine And Its Drive System Of Hybrid Excavator

Traditional excavators use engines to drive pumps directly. Due to the special working conditions and the variational load of traditional excavators, the operating points of the engine in the power-train system vary in a great range. Therefor, the efficiency of the traditional excavator power-train system is low. To overcome the disadvantages of the traditional excavator power-train system, an electric machine is employed to cooperate with the engine and drive the load. However, because of the special structure and working conditions of the hybrid hydraulic excavator (HHE), the electric machine and its drive system should be reconsidered and redesigned.In this paper, practical working conditions and system performance requirements of the HHE are taken into consideration to study on the electric machine and its drive system. Firstly, the research status of the electric machine and its application to the hybrid vehicle are reviewed. Then, the parameters optimization design is carried out for the HHE power-train system, especially for the electric machine and its drive system. Thirdly, according to the designed parameters of the electric machine and the performance requirement of the HHE, an electric machine with high efficiency、fast dynamic response and low torque ripple is designed and fabricated. To improve the electric machine design accuracy and efficiency, a design which optimizes the stator and rotor stepwise based the cooperation of an analytical parametric model and finite element method is applied. Fourthly, the controller for the electric machine is analyzed and designed to improve its dynamic response and resist the fluctuation of the power supply voltage. Finally, combining the electric machine and its drive system, an experimental platform is built. To verify the effectiveness of above research results, a control strategy for the HHE power-train system is proposed and some experimental research are conducted to study on the dynamic performance and the energy conservation property of the HHE power-train system. The results show that the designed system and the control strategy can effectively improve the engine fuel efficiency and satisfy the performance requirements of the traditional excavator.The dissertation is organized as follows.In Chapter 1, the significance of the study on the electric machine and its drive system of excavators under the background of energy crisis and environmental pollution is discussed. The structures of hybrid power-train systems are mentioned briefly. In the introduction of the state-of-art of the electric machine in hybrid vehicle, the working condition of vehicles is analyzed. For the special working conditions of construction machinery, the research status of the electric machine in hybrid contstruction machine, especially in the HHE is illustrated. The permanent magnet synchronous machines (PMSMs) are reviewed in terms of design and control. Finally, the main research contents of the dissertation are presented.In Chapter 2, the power-train system of hybrid excavators is researched in whole. A 20-ton hybraulic excavator is tested and its load profile is obtained to analyze the working conditions of hybraulic excavator power-train systems. The evaluation indexs of the power-train system are summarized as energy conservation and operability. Based on the load profile, the model-based method is used to discuss the design of components key parameters in the system and the mathematical models of the main components are built in MATLAB. The dynamic programming (DP) which takes the minimum energy consumption as objective function and the super capacitor state of charge (SOC) as boundary conditions is applied to control the power-train system. The genetic algorithm is used to optimization design the conponents parameters.In Chapter 3, as a key component in the hybrid power-train system, the structure and parameters of the electric machine is optimized and designed. The performance requirements of the electric machine are analyzed. A papameterized design approach, which combines analytical and the 2-D finite-element method (FEM), is applied to the electric machine to improve the design efficiency and accuracy. The analytical model is employed to optimize the electric machine efficiency and obtain the stator dimensions as well as flux density distributions. The rotor is designed with FEM to satisfy the flux requirements obtained in the stator design. The rotor configuration of the PMSM employs an interior magnet structure, thus resulting in some inverse saliency, which allows for much higher values in the magnetic flux density. To reduce the rotor leakage, a disconnected-type silicon steel block structure is adopted. In order to improve the air gap flux density distribution, the trapezoid PM and centrifugal rotor structure are applied to the PMSM. Demagnetization and armature reactions are also taken into consideration and calculated by FEM. A prototype of the designed electric machine has been fabricated and tested on an experimental platform. The analytical design results are validated by measurements.In Chapter 4, according to the performance requirements of the HHE, a high performance vector controller of electric machine is designed to improve dynamic performance and resist the fluctuation of the power supply voltage. Simulation models are built in MATLAB Simulink to verify the effectiveness of the high performance vector controller system. An experimental platform is constructed to study the dynamic performance of systems with vector control.In Chapter 5, the direct torque controller is studied to further improve the dynamic performance of the electric machine. To resist the fluctuation of the power supply voltage, a high performance direct torque controller is proposed. To verify the effectiveness of the high performance direct torque controller applied to electric machine, simulations are carried out in MATLAB Simulink. Experiments on dynamic performance are conducted to analyze the influence from electric machine with direct torque controller on hybrid power-train system.In Chapter 6, the researches on the controller of HHE electric machine drive system are carried out. According to the requirements of HHE, in order to ensure the stable operation of components in system and realized maximum energy conservation, a rule based control strategy which maintains the operating point of the engine in high efficiency area and restricts the super capacitor SOC as well is adopted for the HHE power-train system. Combining the desiged electric machine and controller in chapter 3,4 and 5, an experimental platform is built. The load profile obtained from a 20-ton hybraulic excavator is used to load for the hydraulic pump. Experimants on the dynamic performance and the energy conservation property of the HHE power-train system are carried out. The results show that the designed system and the control strategy can effectively improve the engine fuel efficiency and satisfy the performance requirements of the traditional excavator.In Chapter 7, the main conclusions and achievements are summarized and the further research work is put forward.