Study on Regenerative Energy Recovery of Electric Vehicle Through Voltage Control Using Switched Capacitor

When an electric vehicle (EV) is driving at a very low or high speed, there are cases where energy recovery is not possible because the back electromotive force generated in the motor is too small or large. In this paper, we investigate the application of the regenerative electromagnetic retarder's energy recovery method to the permanent magnet synchronous motor (PMSM) of an EV. When the EV was driven downhill or coasts, L-C resonance via switched capacitor was used to control the voltage generated by the PMSM as a voltage that can recharge the battery. During regeneration, the voltage generated in the PMSM varied along with the degree of resonance through switching the duty of the proposed circuit. Through simulation, DC-Link voltage according to switching duty ratio and motor speed is plotted in a 3D map. Based on this, voltage control algorithm using PI and reverse PI duty control has been proposed. To prove the proposed algorithm, the DC-link stage of the inverter is simply equivalent to the R-C model. And, if the PMSM operates at a constant speed, the generated voltage is controlled so as to charge the battery. In addition, the discharging and charging energies of the battery are simulated when the EV with certain load torque is driving uphill and downhill. Considering the dynamics of the car, the energy recovery rate was simulated when the EV is driving uphill and downhill in a 50:50 ratio at the same slope and distance. The simulations were conducted using MATLAB Simulink, and the experiments were carried out using a motor-generator set and a small electric truck equipped with a 13 kW PMSM, respectively.