A new overmodulation strategy for traction drive

This paper proposes a new overmodulation strategy to give a better voltage utilization by tracking the voltage vector along the hexagon sides of traction motor drives. This strategy enables the inverter to control both the magnitude and angle of current. Therefore, vector control using this strategy can lead to better output torque dynamics compared to the conventional slip frequency control with six-step voltage, which is widely used in traction drives. In this strategy, the d-axis output voltage of a current controller to control the flux is conserved and the q-axis output voltage to control the torque is controlled to place the voltage vector on the hexagon boundary in case of overmodulation. The limited q-axis voltage is used for anti-windup of q-axis current controller. This paper also presents a new field weakening scheme which incorporates the proposed overmodulation strategy. In this scheme, the flux level is selected by both required current limit and the available maximum voltage along hexagon sides. The validity of the proposed overall scheme is confirmed by computer simulation and experiments with a prototype setup.