An Auxiliary Variable-Based MRAS Speed Observer for Stable Wide-Speed Operation in Sensorless Induction Motor Drives

For speed-sensorless induction motors (IMs) that require operation over a wide-speed range, the commonly used rotor flux-based model reference adaptive system (MRAS) cannot guarantee the speed estimation stability for the following two reasons: 1) the poor damping performance of current model (CM) observer resulting in the accuracy degradation and potential discretization instability in high-speed range and 2) the margin stability of the open-loop integrator in voltage model (VM) observer, resulting in the instability issue in low-speed range. To address these problems, this article proposes the corresponding solutions: 1) a coordinate transformation-based Euler method is studied to discretize CM observer leading to stability and accuracy improvement in high-speed range and 2) an auxiliary variable, representing the rotor flux error, is modeled in the VM, and a closed-loop auxiliary variable observer is designed to overcome the open-loop integrator problem. Furthermore, the stability of the overall MRAS is investigated in both continuous domain and discrete domain, verifying the stable wide-speed sensorless operation. Finally, the proposed auxiliary variable-based MRAS is tested on a 3.7-kW IM platform to prove its effectiveness.