Dynamic performance limitations for MRAS based sensorless induction motor drives. I. Stability analysis for the closed loop drive

The paper deals with a speed sensorless direct rotor flux orientated vector controlled induction motor drive in which the speed and rotor flux angle are derived from a closed loop MRAS estimator. A small signal stability analysis is carried out on the combined estimatorcontroller, and allows for the stability effects of incorrect estimator parameters to be studied. The small signal stability analysis is sufficient to show that incorrect parameters give rise to oscillatory drive behaviour that worsens, even to the point of instability, as the closed loop speed bandwidth of the drive is increased. Experimental results are given to validate the analysis. It is concluded that speed and load torque rejection bandwidths comparable to sensored vector controllers cannot be obtained without online parameter tuning. A companion paper addresses this problem and experimentally verifies the transient oscillatory phenomena.