Speed sensorless AC drive fed by three-level inverter with full-dimensional spiral vector control for improved low-speed performance

In low speed control of a speed-sensorless induction motor drive, machine stator resistance variation is commonly attributed to the deterioration in motor flux and speed estimation. Further analysis shows that the estimated value of field orientation angle is corrupted by accumulative errors from the integration of motor terminal variables which have low signal/noise ratio. An error loop-effect speeds up the degradation process. Control information starvation takes place in conventional sensorless control systems which control the machine spiral vectors in reduced dimension in this situation. A novel control scheme is developed to improve the control of motor current, torque and speed. The scheme employs full dimensional vector control approach and is less sensitive to the combined effect of the error sources and loop effect in the frequency range of interest. Preliminary experimental tests demonstrate promising results which are achievable even below 0.5 Hz