Adaptive sensorless control of induction motors for improved low-speed performance

A theoretical study of a method for improved stator flux oriented control of the induction motor is presented. To enable operation at very low rotor speeds, flux estimation through direct voltage integration is replaced by a full state observer. This alleviates problems such as integrator bias. The observer is made adaptive, giving also estimates of the rotor speed and stator resistance. This significantly improves the capability of low-speed operation. Although complete standstill with full torque is not achievable, simulations show stable operation at arbitrarily low rotor speeds, provided that the stator inductance is known with fairly high accuracy. The estimator uses information extracted from the fundamental frequency component; no injection is necessary.