Stability and performance analysis of permanent magnet motors operating in flux-weakening region

Permanent magnet (PM) motors for traction drive applications generally require wide-speed range operation, where flux-weakening control is employed once the inverter voltage limit is reached. Constrained by the inverter voltage limit (voltage circle), the current control loop may become open loop due to the lack of voltage margin and the current can not be effectively controlled with respect to the reference. In addition, the control of d-axis and q-axis currents will affect each other although the cross-coupling compensation is applied. This paper presented an analytical model to describe the above issues and attempts to predict the current control loop performance in the flux-weakening region. By analyzing eigenvalues of the state matrix of the linearized current-loop model, the d-axis and q-axis current control are found to be not independent, while the system may remain stable. The closed-loop transfer function is also derived, which shows lower bandwidth than the designed value in the frequency domain. The analysis is further carried out with various motor speeds, control bandwidth as well as the load torque. Time-domain simulation results verify the analytical model and prediction given in the paper.