High-Speed Electric Machines and WBG Power Electronics Technology Trends and Design Techniques for Electric Vehicles

Abstract: The transition to electric road transport technologies is gaining momentum with record-high new electric vehicle registrations taking place in recent years. The market drivers for electric and hybrid electric vehicles (EVs and HEVs) are energy diversification, environmental concerns, and economic growth. The strict carbon emission standards set by the regulatory agencies fueled the growth of electric vehicles all over the world. According to a recent prediction, the total number of EVs will hit 125 million by the end of 2030. Moreover, the sales of EV in the USA has increased by 81% in 2018 compared to 2017. The main components of the electric propulsion system of
EVs are the electric machines and power electronics inverter. Due to the high density and high efficiency, Interior PM Synchronous Machines (IPMSM) is well adopted by the industry, although solutions with Induction Machines (IM) exists. The solution for the traction inverter is still silicon (Si) IGBT based; however, the Wide Bandgap (WBG) drives are emerging due to its high voltage, high frequency, high temperature, and low loss capabilities.
New EVs are being offered with improved performances and capabilities such as increased acceleration and extended range. These capability and performance enhancements are met with an increased demand in the propulsion system. Widespread research in the area of electric machines for traction applications is pushing the boundaries for maximum speed and power density with design innovations utilizing both conventional and emerging new materials. Electric machines with higher operating speeds are feasible using higher mechanical gear ratios. Consequently, higher torque-density and power density can be achieved. Recent evolution of Wide Bandgap (WBG) semiconductor-based drives with their capabilities of higher frequency and higher temperature operation is also a catalyst to increase the operating speed of traction machines. WBG drives improve efficiency, power density, and controllability on a system level. In the future, designs will need to evolve to meet new requirements such as moving to higher system voltages. DC-link voltage level up to 800-V standard is being considered to support charge rates up to 350kW. However, the interaction of high-speed electric machines with the WBG power electronics creates new sets of issues compared to the existing system. High torque-density and power-density machines for traction applications need special design methodology to achieve the electromagnetic, structural, and thermal performance requirements. Moreover, the price uncertainty of permanent magnets paves the path for non-rare earth electric machines where they have to offer specific advantages compared to the IPM machines in the traction applications. In the case of WBG drives, even though these improve the performance at the system level, the design of traction inverter has to address the voltage overshoot, EMI, and volume constraints.
This tutorial on high-speed electric machines and WBG power electronics is designed to cover the current trends, techniques, design methodologies, challenges, and design innovations of permanent magnet electric machines along with the design, challenges, and advantages of WBG power electronics for traction applications. This tutorial is organized into five parts: Part I gives a review of state-of-the-art, trends and design techniques of electric machines for current electric vehicles, and system architecture of electric vehicles; Part II presents machine sizing and specifications, and design methodology of high-speed electric machines; Part III covers emerging new materials, and design innovations for electric machines; Part IV presents alternative electric machine examples for traction applications; Part V shows the design, modeling, challenges, benefits, and examples of WBG traction inverter for electric machines, finally concluding with some thoughts on trends into the future for widespread industrial adoption.

Iqbal Husain is an IEEE Fellow, the Director of NSF FREEDM Systems Center, and the ABB Distinguished Professor at Electrical and Computer Engineering Department, North Carolina State University, Raleigh, NC. His research interests are in the areas of systems modeling and controls, distributed power generation, power conditioning circuits, solid state and hybrid circuit breakers, electrical drive systems, design of electric machines and actuators, electric automotive systems, and modeling of electric and hybrid vehicle systems. He has also developed graduate and undergraduate courses on electric and hybrid vehicles and published a textbook on the subject. In the FREEDM center, he is leading a multi‐university team on a concerted effort for power grid modernization utilizing wide band gap power electronics, solid state transformers, intelligent controls, and resilient power and energy management approaches.

Wensong Yu is the Thrust Leader for the Solid State Transformer at FREEDM System Center, the Department of Electrical and Computer Engineering at North Carolina State University, USA. Prior to joining the FREEDM as a Research Associate Professor, Dr. Yu worked as a postdoctoral researcher, research scientist and research assistant professor at the Bradley Department of Electrical and Computer Engineering at Virginia Tech, USA. His current research interests are medium-voltage solid-state transformer, advanced softswitching technique, wide bandgap device applications, ultra-high efficiency inverter, high-voltage power conversion and protection, WBG electric vehicle traction drive, distributed energy storage devices, and green energy grid infrastructure.

Md Sariful Islam is a PhD candidate in the Electrical and Computer Engineering department at North Carolina State University, Raleigh, NC, USA. His research interests include design, modeling and control of high-speed electric machines with mechanical stress, vibration and acoustic-noise analysis. He has extensive experience on designing electric machines for electric traction, aerospace, and automotive accessories while working at NC State and Halla Mechtaronics. He has previously offered electric machine design tutorial at the Altair conference.