This thesis investigates the feasibility of using Silicon Carbide (SiC)-based multiphase inverters (MPIs) for transportation electrification applications. The research begins with a comprehensive review on the state-of-the-art of MPIs, focusing on voltage source inverters (VSIs) and nine-switch inverters (NSIs), with five-, six-, and nine-phase configurations. The quantitative and qualitative analyses demonstrate that the six-phase VSI is the most promising topology, offering reduced DC-capacitor requirements, lower cabling cost, and higher fault tolerance capability while maintaining the same efficiency and power device count of a three-phase VSI. The feasibility of the SiC-based six-phase inverter is further investigated at the vehicle level, where a vehicle model is developed to study the energy consumption under different drive cycles. The resulting indicate an 8% improvement in vehicle mileage and fuel economy of the SiC-based six-phase inverter compared to its Si-based counterpart. This thesis also examines the current and voltage stresses on the DC-bus capacitor in two-level six-phase VSIs. The study considers two configurations of load/winding spatial distribution: symmetric and asymmetric. Consequently, analytical formulas for the DC-bus capacitor current and voltage ripples are derived. Furthermore, simple capacitor sizing rules in six-phase VSIs with different load configurations are provided. The accuracy of the derived formulas is verified by simulation and experimental testing, and their boundary conditions are identified. Six-phase VSI supplying symmetric loads was found to yield the smallest capacitor size. Based on the foregoing technology review and analyses, a holistic design methodology for a 100 kW SiC-based six-phase traction inverter for an electric vehicle application is presented. The proposed methodology considers the device power level, where discrete SiC MOSFETs are utilized, and the DC-capacitor sizing, where a multi-objective optimization algorithm is proposed to find the most suitable capacitor bank. Mechanical and thermal design constraints are also explored to deliver a compact housing with an integrated coolant channel. The resultant inverter design from the proposed electrical-thermal-mechanical design methodology is prototyped and experimentally tested, demonstrating a 7% reduction in DC-capacitor volume and 21% reduction in cabling cost when compared to conventional three-phase inverters of the same volt-ampere rating. The peak power density of the prototype inverter is 70 kW/L, demonstrating a compact design. Besides, the proposed design is benchmarked against commercial six-phase inverter models, whereby the competitiveness of the proposed design is highlighted. Finally, the unique control aspects of six-phase electric motor drives are investigated to identify suitable controls strategies for various operating conditions. The study places special emphasis on high-speed operation and evaluates several overmodulation techniques. An adaptive flux-weakening control algorithm is also proposed for the six-phase motor drive, which significantly improves the DC-bus voltage utilization of the inverter when used in conjunction with overmodulation. Overall, this thesis provides a comprehensive study of SiC-based six-phase traction inverters and proposes a holistic design methodology that considers electrical, thermal, and mechanical aspects. The results demonstrate the feasibility and advantages of SiC-based six-phase traction inverters for electric vehicle applications. / Thesis / Doctor of Philosophy (PhD) / Electric cars are continuously challenged to meet regulatory mandates that become stricter by the day. This is driven by the need for a clean, reliable, affordable, and sustainable transportation system. In this research, a novel, more reliable, and cost-effective power control unit (PCU) is proposed. The PCU manages the power flow regulation between the battery and the motor(s). The proposed PCU employs the same number of devices as a traditional counterpart, yet in a more modular architecture that doubles the safety factor compared to the standard design. In fault scenarios where the traditional PCU would fail, the proposed PCU would continue operating at half power, allowing the driver and passengers to reach a safe destination before the car is repaired. Extensive analyses were undertaken to identify an optimal design in terms of performance, size, and cost. Then, an engineering prototype is constructed and tested on an electric drivetrain testbed. Finally, the prototype is benchmarked against commercial competitors in the market to establish its economical feasibility.

This thesis proposes novel methods and comprehensive analysis for power loss calculation, DC-link current and voltage ripple estimation, and bus bar design in two-level three-phase voltage source inverters (VSIs). A novel method of MOSFET voltage rise- and fall-time estimations for the switching power loss calculation is developed. The estimation accuracy is significantly improved by the proposed method. In order to provide a reference for thermal management design, inverter power loss analysis is presented. Using the parameters obtained from the semiconductor device datasheets and inverter operating conditions, power loss calculations of three types of devices, namely IGBT, MOSFET, and diode, are discussed. The conduction power loss calculations for these three devices are straightforward; and, the switching power loss of IGBTs and diodes can be obtained from the energy losses given by datasheets. However, many MOSFET datasheets do not provide the switching energy losses directly. Therefore, to acquire MOSFET switching energy losses, switching transient times must be estimated as accurately as possible. The impacts of inverter anti-parallel diode reverse recovery on the DC-link current and voltage ripples are investigated. According to the analysis, the impact of diode reverse recovery on the voltage ripple is negligible, while the RMS value of current ripple is influenced by both diode reverse recovery and inverter switching frequency. A novel method is developed to calculate the ripple current RMS value and the estimation accuracy is significantly improved. Depending on the calculated current and voltage ripples, DC-link capacitor selection is introduced. Generally speaking, failures in the DC-link capacitors take place more frequently than the failures in other parts of the inverter system, and plenty of research has been focusing on minimizing the required DC-link capacitance. As a result, the accurate estimations of DC-link current and voltage ripples are vital in the optimization methods. In addition, with the accurate estimations, the over-design in the DC-link capacitance could be reduced. Finally, the design of a practical bus bar is presented. The DC current distribution is aff ected by the numbers and locations of the DC input tabs, while the AC current distribution is influenced by the numbers and locations of the installation holes for DC-link capacitors and semiconductor devices. Furthermore, parasitic parameters of the bus bar, especially the stray inductance and voltage spikes caused by this inductance during switching turn-o transients, are also discussed from the angle of the design rules and correlation between the parameters and bus bar geometry structure. In the end, a bus bar is designed with balanced current distribution and low stray inductance. / Thesis / Doctor of Philosophy (PhD)