Coasting control of EV motor considering cross coupling inductance

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 tipped 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, a positive q-axis current command is injected in practice. In this paper, a cross-coupling inductance, L
_qd
, is studied as a main cause for the braking torque. If it is present, torque can be developed by only the d-axis current. Further, it grows quickly as the speed increases. The problem can be solved by shifting the angle of the reference frame. The compensation angle is analytically derived using two different methods. The proposed compensation method was demonstrated effectively by simulation and experiments.