Next generation chevy volt electric machines; design, optimization and control for performance and rare-earth mitigation

Abstract

This paper presents the design, performance and control details of traction electric machines for GM's second generation Extended Range Electric Vehicle (EREV). Chevy Volt was the first personal vehicle in the industry with EREV power flow configuration which is carried over to the second generation. Since its introduction in 2011 Chevy Volts have been driven over half a billion miles, 67% of which in EV mode. The second generation of Volt brings a significant mass reduction and increased performance, EV driving range and fuel economy while simultaneously reducing rare earth content in its traction electric motors. The electric propulsion system is built on two electric machines; both PMAC topology. While hybrid-electric vehicles are gaining in popularity in hopes of addressing cleaner, energy sustainable technology in transportation, materials sustainability and rare earth dependence mitigation has not been the first priority in the hybrids available on the market today. However, design robustness to material cost volatility is crucial in automotive industry success and therefore designing electric propulsion to minimize or eliminate rare earth usage plays a major role in HEVs success. The objective of this paper is to present the newly redesigned electric traction machines for added performance while simultaneously reducing the rare earth and heavy rare earth content by over 80% and 50% respectively and in turn the cost of the system and yielding all around “cleaner” and more sustainable vehicle. A tall order by any measure; so various technologies were utilized to achieve this goal. The paper discusses grain boundary dysprosium diffusion process in permanent magnets as means to rare earth reduction in PMAC machines and design challenges surrounding such material use. We also discuss innovative PMAC topologies employing ferrite magnets to completely eliminate rare earth usage while maintaining the electric drive unit performance. The design of electric machines is presented in detail along with performance measurement results as well as thermal and NVH aspects. It is absolutely crucial that high performance electric machines are coupled with high performance control algorithms to enable maximum system efficiency and performance. Specifically, key challenges toward that goal are inverter voltage utilization, for maximum power capability and switching loss minimization. In order to address those, six-step mode of inverter control is a must. We focus on a specific challenge associated with this operation mode to keep the closed-loop current control regulation in the full six-step mode while losing a degree of freedom in the controls scheme. We present a novel PMSM control algorithm with a closed-loop current control regulation that can be used in both the SVPWM and full six-step mode.