Hi Nicol,
I've already made experiment on ACIM with sensored and sensorless control. I had same situation to you. My sensored control is successful. However, sensorless control didn't work at low speed. Although in high speed it worked well. First, I checked the motor parameters (Rs, Rr, Ls, Lr,...) matched with my hardware or not. Because, the IFOC (sensored control) is not sensitive with motor parameter but DFOC (sensorless control) is sensitive with motor parameter. After checking the parameter, I changed the PI gain in flux estimator. I reduced the Kp and Ki gain then the motor worked well at low speed (50 RPM).