A high-frequency pulsating dc link for electric vehicle drives with reduced losses

Abstract

This paper proposes a motor drive suitable for electric vehicles (EVs) with notably reduced losses as compared to conventional drives. The proposed drive feeds the dc-link with a dynamically reconfigurable battery formed by cascaded half-bridges containing battery units. The reconfigurable battery concurrently forms a modular multilevel dc-link able to synthesize variable dc-link voltages, which can adjust the dc link voltage to run a motor to operate at the optimum load point or even to contribute to modulation and greatly reduce the switching loss of the main inverter—in this case a standard two-level three-phase inverter. Up to 2/3 switching actions are avoided in the main inverter, which can run at maximum duty cycle and fundamental-frequency commutation. The saved switching effort in the main inverter is shifted to the modular multilevel dc link, but with much less loss due to the fractionized switching voltage and the use of field-effect transistors (FETs). At high drive speeds, the converter halves the total loss compared to a standard inverter only; at lower speeds and thus smaller modulation indices, the advantage is even more pronounced because of the dynamically lowered dc-link voltage. Other benefits include alleviated insulation stress for motor windings and direct battery balancing. The proposed motor drive is verified on a down-scaled setup with eight modules constituting the dc link.