Energy Efficiency Improvement Framework for Regenerative Braking System in Electric Vehicles

Electric vehicles (EVs) face challenges in enhancing regenerative braking (RB) efficiency, particularly at low speeds, where the traction motor’s back-electromotive force is insufficient for energy regeneration. Below a certain dynamic low-speed threshold, energy is extracted from the battery instead of being returned, exacerbating electrical losses in the drive. A model-based approach is proposed to analytically determine this dynamic low-speed threshold, alongside a loss minimization framework based on a variable flux approach to enhance energy recovery. The simulation results using a vector-controlled induction motor (IM) drive indicate a significant reduction in total system losses during braking in the high-speed, low-torque region. The loss reduction effectively lowers the low-speed threshold by 5%, depending on the specific driving conditions, for the considered target vehicle’s drive system. However, the optimal flux point varies depending on machine parameters. Hence, the effect of temperature-induced variations in stator and rotor resistances and magnetic saturation on loss minimization is also explored, showing that optimal flux point sensitivity is particularly impacted by resistance changes. The experimental validation on representative drive cycles corroborates the simulation results, showing a 13% reduction in system losses under the modified Indian driving cycle and 7% under the U.S. EPA highway cycle.