With the increasing concern of energy crisis and environmental pollution, electric vehicles (EVs) have been paid much attention as the effective solution. As the motor drive system is the key part of the EVs, its reliable operation directly relates to the personal safety. Due to the advantages of high torque density, high efficiency and wide speed range, the interior permanent magnet (IPM) motors with rotor PM have been widely used in EVs. Therefore, enhancing the fault-tolerant capacity of IPM motors with rotor PM has been the research focus of automotive area.
Based on the technology of brushless harmonic excitation, a new five-phase brushless fault-tolerant hybrid-excitation (BFTHE) motor is proposed in this thesis. To improve the reliability and fault-tolerant capacity of motor drive system, multi-phase and single-layer concentrated winding are introduced in the design of V-type IPM motor. Then, the fault-tolerant capacity can be enhanced by the phase separation of the phase windings. In addition, by the effective regulation of air-gap flux, the wide speed regulation of the driving motor for EVs can be realized. This motor can obtain the following performance: high efficiency, high power density, high fault-tolerance, good flux-adjusting capability, as well as keeping a balance between simple structure and no sliding contacts, which makes this five-phase BFTHE motor a promising candidate for the application of EVs.
Apart from the motor ontology, the controller and sensor are also the important part of motor drive system. The fault of each part will influence the normal operation of the whole system. Also, the position sensor in the motor drive system is a part which is prone to fault. At the same time, due to the installation of mechanical sensor, the complexity of EVs will be increased and the reliability and robustness of the vehicle system will be reduced. Therefore, apart from the investigation of motor’s reliability from the perspective of motor design, the fault-tolerant control strategies of motor and position sensor have been studied in this thesis. To enhance the reliability of motor drive system for EVs, the drive motor, the controller and the position sensor are taken into full account in this thesis. The organization of this thesis is as follows:
Based on the structure and operating principle of the five-phase BFTHE motor, the motor design is studied. The power size equation of the five-phase BFTHE motor is derived. A prototype with 2 kW is preliminarily designed, including the design of motor’s fault-tolerance, structure and size of permanent magnet, and the system of harmonic excitation. Also, the asymmetric airgap and the unequal tooth width are applied to improve the back-EMFs and the output torque. Furthermore, the electromagnetic performances of the five-phase BFTHE motor are analyzed based on the finite element method, including magnetic field distribution, radial airgap flux density, cogging torque and output torque. And the performances of flux-regulation, efficiency and fault-tolerance are emphatically analyzed. Then an experimental prototype of the five-phase BFTHE motor is manufactured. And the test experiments of the back-EMFs, cogging torque, flux-regulation capacity, efficiency and fault-tolerance are carried out.
The direct torque control (DTC) of the five-phase BFTHE motor is described. First, the mathematic model of the five-phase BFTHE motor is established based on its structural characteristics and working principle. When the five-phase BFTHE motor runs below the based frequency, the SVPWM based DTC (SVM-DTC) strategy is employed to achieve good dynamic and static performances and strong robustness. When the five-phase BFTHE motor runs above the based frequency, the flux-weakening DTC scheme based on harmonic current injection is investigated to control the excitation current for realizing the effective regulation of air-gap flux. Both the simulation and experimental results prove the validity of the proposed SVM-DTC strategy and the flux-weakening DTC scheme based on harmonic current injection.
The fault-tolerant control of the five-phase BFTHE motor is studied when one phase winding is in open-circuit. When the open-circuit fault happens, an analogous three-phase SVPWM fault-tolerant control scheme is proposed by the utilization of the reconfiguration of voltage vector. Due to the division of six sectors and the reconfiguration of six equal nonzero voltage vectors, the proposed fault-tolerant control strategy is quickly computed and easily realized. Also, the problems of large torque ripples and large switching loss of inverter for the conventional fault-tolerant control strategy can be overcomed. Besides, the average value of torque keeps not dropping, while the torque ripple can be suppressed.
The sensorless control of the five-phase BFTHE motor is investigated. For the medium/high speed operation, the sensorless control strategy based on wide-speed strong-robustness sliding mode observer (SMO) is proposed. In this algorithm, the sigmoid function is used to replace the switch function to reduce the system chattering; the back-EMF is obtained by using the back-EMF observer to solve the phase delay problem caused by the low-pass filter in the conventional SMO; the adaptive algorithm of the back-EMF is applied to improve the estimation accuracy under low speed. Furthermore, the disturbance sensor and the active state feedback gain are employed to improve the robustness to motor parameters, fault and load disturbance for the SMO system. Moreover, to overcome the problem of the wide-speed strong-robustness SMO in zero/low speed, the sensorless control method based on pulsating torque injection (PTI) is studied. Additionally, the hybrid method based on wide-speed strong-robustness SMO and PTI is researched to achieve self-sensing position estimation under full-speed region including the zero speed.
Finally, based on the real-time dSPACE, the digital drive hardware circuit and control experimental platform of the motor drive system are built, which lay a foundation for verifying the effectiveness of the proposed DTC scheme, fault-tolerant control method and sensorless control strategy.