Stability analysis of permanent magnet synchronous machines under flux-weakening control with advanced control strategies

Flux-weakening (FW) control is essential for permanent magnet synchronous machine (PMSM) drives in high-speed applications, where the operating speed must extend beyond voltage and current limits. As the back-EMF approaches the DC-link voltage, the inverter operates near its voltage limit, and the reduction in voltage margin introduces strong nonlinearities in the current-regulation loop. This voltage-limited behaviour is the primary source of potential instability. Although various FW strategies, e.g. feedforward, feedback, and hybrid, have been developed based on field-oriented control, direct torque control, and model predictive control, their stability characteristics for different machine types and deep FW operation remain insufficiently understood.
This thesis establishes a unified modeling and control framework to address these issues and proposes advanced FW strategies for robust and efficient PMSM operation. A unified small-signal model is first developed for both surface-mounted PMSM and interior PMSM, enabling systematic stability analysis of representative voltage-feedback methods: d-axis-current-based and current-angle-based control. The study clarifies how saliency, controller structure, and operating point jointly influence loop gain and damping. An adaptive voltage feedback control method is then proposed to integrate the two feedback mechanisms through dynamically tuned weighting factors, ensuring consistent stability across the entire FW region. A model predictive flux control scheme with voltage feedback is further developed, introducing a low-pass-filtered voltage loop and a ripple-suppression cost function to improve transient stability. Finally, a frequency-domain framework based on finite impulse response modeling enables multi-objective optimization of MPC parameters, balancing stability, bandwidth, ripple, and computational cost.