Torque and Current control for EV Propulsion Motor Considering Cross Coupled Inductance

Abstract

In this thesis, the current and torque control method are proposed for electric vehicle (EV) propulsion motor considering the cross coupled inductance. The effects of cross coupled inductance are not small in high power density motors. It is observed here that the cross coupled inductance causes current and voltage overshoots and the torque error in the high-speed region. As the required voltage exceeds the voltage limit, the system becomes unstable. Further, it is shown that Lqd makes an uncanceled zero in the current transfer function, which invokes the overshoot. It can be compensated with an estimate Lqd. However, it is not easy to obtain correct estimates of Lqd in a wide current range. The accurate torque control is critical to the EV propulsion motor. Coasting is a fuel-saving driving mode that is usually active when the vehicle slows down. In an electric vehicle (EV), coasting can be achieved by letting the $q$-axis current to be set to zero. However, braking action often occurs at high speeds when the gas pedal is tapped off. That is, negative torque is produced while the $q$-axis current is equal to zero. Such a braking torque reduces the coasting distance. To prevent such undesirable braking and overshoot, an on-line overshoot suppression algorithm and angle compensation method for zero torque are proposed. The simulation and experimental results provide some evidences for the effectiveness of the proposed algorithms. Finally, it is shown experimentally that the proposed algorithm cured the vehicle hunting and coasting problem after tapping off the gas pedal in a high-speed operation.