Field-oriented control of a speed-sensorless induction motor for the complete speed range using a nonlinear observer

An observer for field-oriented control for the speed-sensorless induction motor is proposed that works over the complete speed range. The speed is reconstructed with a nonlinear estimator based on the stator model and the stator current derivatives, and the rotor flux is calculated with an open-loop rotor model. Both reconstruction methods are combined in an asymptotic observer, for which convergence is proved analytically. In motor operation, the scheme is stable and fully converging for any speed, torque and excitation frequency including zero, which is an unobservable operation point. Restrictions appear in generator operation - the regenerative power is limited, and below nominal slip speed, torque is limited. Experiments are presented which confirm these results and highlight low- and zero-speed performance. The scheme is stable at low and zero speed up to half rated load, even under strong resistance errors.