Design and Control of a 100 kW SiC-Based Six-Phase Traction Inverter for Electric Vehicle Applications

Abstract

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.