Speed sensorless AC drive fed by a 3-level inverter with improved low-speed torque and speed control

In a speed-sensorless induction motor drive, the accuracy of motor flux and speed estimation deteriorates at low speed which is commonly imputed to stator resistance variation. Further analysis shows that the estimated value of field orientation angle is corrupted by accumulative errors from integration of the motor variables with low signal/noise ratio at low frequency region. This in turn introduces errors in variable transformation between reference frames and in motor torque generation, which further degrades control. An error loop-effect speeds up the degradation process. A novel control scheme is developed to improve the control of motor current, torque and speed at low frequency. The scheme is less sensitive to the combined effect of the error sources and the loop effect in the frequency range of interest. The control scheme is implemented in a DSP based fully digital control system of a three-level IGBT PWM inverter drive. Preliminary experimental tests demonstrate promising results which are achievable even below 0.5 Hz