Flux-Weakening Control for Induction Motor in Voltage Extension Region: Torque Analysis and Dynamic Performance Improvement

Flux-weakening control for induction motor (IM) in voltage extension region (outside the inscribed circle but in the hexagon) is meaningful to yield a further maximum torque. However, as the voltage-limit trajectory migrates out of the inscribed circle, torque ripple becomes more severe. Meanwhile, the insufficient voltage margin results in the degradation of current dynamic performance in transition period (from base speed region to flux-weakening region), especially in harsh conditions, e.g., a step speed command. To address the problems above, this paper gives a quantitative analysis of the torque ripple and an explicit discussion on the current dynamic performance. A novel "Voltage Reference Adjustable" flux-weakening controller with "Self-Locking Limit Block" (SLLB) is proposed. There are two advantages. The first is the capability to operate in any voltage extension regions, offering a tradeoff between obtaining the maximum torque and suppressing the torque ripple. The second is an optimized voltage distribution, achieving a better track characteristic of d- and q-axis currents with the help of triggered SLLB in transition period. Experimental results on a commercial IM control system verify the validity of the proposed scheme.