PhD student Anuj Maheshwari with Advisor A. Banerjee

Figure 10. Torque-speed envelop of EV motors shrinks with decreasing battery SOC for drive trains directly connected to the battery.

The power flow architecture in electric vehicles must prioritize high efficiency and power density while consistently delivering the rated torque-speed envelope. In traditional setups, where the battery is directly connected to the traction inverter’s dc link, the torque-speed envelope diminishes as the battery discharges, as shown in Figure 10. To address this limitation, adding a dc-dc converter between the battery and the traction inverter can help maintain torque-speed capabilities. However, this approach requires the dc-dc converter to handle the full traction power, making the system bulkier and less efficient.  This paper introduces an alternative solution: leveraging the onboard charger to regulate the inverter dc-link during motoring.  The proposed architecture is shown in Figure 11 and utilizes a DPDT switch to transition between charging and motoring operation. The analysis demonstrates that even with the onboard charger rated at only 15% of the traction motor’s power, the system can maintain the rated torque-speed envelope down to a 20% battery state of charge (SOC).  Efficiency improvement of 2% is observed in analytical model across the torque-speed envelope utilizing the proposed architecture compared to a traditional dc-dc converter rated for the full power of traction inverter. Therefore, the proposed architecture not only removes the need of separated dc-dc converter increasing the power density of the drive train but also enhances its the efficiency.

Work is funded by the Grainger CEME

Figure 11. Proposed architecture using the onboard charger as a partial power processing converter to regulate the inverter dc-bus during motoring operation. 1-1′ connects to 2-2′ during motoring and 3-3′ during charging.