Research On Design And Operation Control Of Five-phase Fault-tolerant Permanent Magnet Machine Used For Electric Vehicles

Due to the energy crisis and the environmental pollution, the all-electric vehicle is replacing the traditional internal combustion engine vehicle and becoming the future trend of the vehicle. The electric vehicle is an application of high reliability. In this application, fault-tolerance capacity, high power density and efficiency are required for the driving machine, and wide speed range, post-fault operation are demanded for the driving system. Post-fault operation is achieved in multi-phase permanent magnet synchronous machine(PMSM) by increasing the phase number. And multi-phase PMSM is the superior candidate for the driving machine in the electric vehicle application, because of its high power density, low torque ripple, and low-voltage high-power transmission.Electromagnetic and thermal isolations are necessary for Multi-phase fault-tolerance PMSM. Meanwhile, its efficiency and power density are required to be high. The control degrees of freedom of multi-phase fault-tolerance PMSM increase, and harmonic components influence the control of fundamental components. In post-fault operation, the distributions of phases are asymmetrical, and there are more disturbances. Thus improving the post-fault performance through the post-fault strategy design is the hot spot of the current research. In this paper, research on the design and operation control of the five-phase fault-tolerant PMSM are undertaken, which is applied in the direct-drive all-electric vehicle. The contributions of this paper mainly contain four parts:Firstly, the design of the five-phase fault-tolerant PMSM with Electromagnetic and thermal isolations is investigated. The principle and harmonic distributions of the magnetomotive force(MMF) synthesized by the armature of the machine is studied, based on which the impact of the slot-pole combination on the winding factor, the cogging torque and the core loss are analyzed. Then the selection principle of the slot-pole combination of the five-phase fault-tolerant PMSM with electromagnetic and thermal isolations is proposed. Considering the important influence of the inductance on the design of the machine and the driving system, the analytical calculation of the inductance is studied, the results of which are verified by the finite element simulation. The impact of the main structural parameters on the machine performance is analyzed and the optimization of the parameters using the finite element simulation is given. The performance of the designed machine is validated through the finite element simulation, and the simulation results indicate that the designed machine meets the requirements with good electromagnetic performance and fault-tolerance capability.Secondly, the principle and the digital implementation of PWM methods for the five-phase fault-tolerant PMSM are compared and analyzed under balanced and unbalanced load conditions. The principle and the implementation of CBPWM, 2V-SVPWM and 4V-SVPWM for the five-phase fault-tolerant PMSM are given. These methods cannot follow the reference voltage space-vector effectively under the unbalanced load condition, because they are unable to control the zero component of phase voltages independently. The NFV-SVPWM based on the five-phase six-leg inverter is presented to solve the problem. From the aspects of the voltage utilization ratio of DC link, the complexity of digital implementation and ability to deal with the unbalanced load condition, the given four PWM methods are compared, and the applicable situation of the PWM methods are presented. The simulation and experimental results of the presented PWM methods are compared, which show that only the proposed NFV-SVPWM is able to tackle with the unbalanced load problem.Thirdly, research on the vector control and the third-harmonic current injection of the five-phase fault-tolerant PMSM are undertaken. The mathematical model of the five-phase fault-tolerant PMSM is derived. By analyzing the influence of the third harmonic of the PM flux linkage on the machine control, the implementation and applicable situation of the single-plane and dual-plane vector control strategies are presented. Then the design of double-loop control system is introduced, which regulates both the current and the speed. When the PM flux linkage of the machine includes the third harmonic, it is able to enhance the output torque by injecting third-harmonic current into the phase current. Considering the effect of the third-harmonic current injection on the output torque, the loss, an optimal injection ratio of the third-harmonic current is proposed in the current amplitude constraint. The experimental platform is constructed to test the proposed control strategies. The experimental results agree with the analysis, and the effectiveness of the control strategies is validated.Lastly, the post-fault control strategies for the five-phase fault-tolerant PMSM under open-circuit fault are studied. The post-fault decoupling transformation matrix is constructed, by which the decoupled mathematical model of the five-phase PMSM in the synchronous rotating coordinate system is established. Then two post-fault control strategies are obtained based on the principle of minimum copper loss and equal amplitude of currents respectively by optimizing the reference of the z-axis current. For the five-phase fault-tolerant PMSM, the post-fault operation is significantly affected by the leakage inductance, the harmonic inductance and the third-harmonic of the magnetic flux linkage. Therefore, the post-fault control strategy is proposed, which is based on the feed-forward compensation to eliminate the influences of these three items. The presented post-fault control strategies are validated by the experiments. The experimental results show that the decoupled vector control of the machine under open-circuit fault can be achieved adopting the presented post-fault control strategies, and the post-fault control strategy based on the feed-forward compensation is able to reduce the q-axis current ripple efficiently, then achieves the purpose of reducing the torque ripple.