The paper discusses the problem of rotor flux position estimation for the induction motor (IM) drive and presents a full-order nonlinear observer that is developed based on the motor's stationary reference frame model. In the method, the IM speed is treated as a slowly time varying parameter and nonlinear control methods are used to design the observer. In the initial development, the speed of the IM is assumed as known and the paper shows how to design the gains of the observer for convergence - some of the resulting gains are time varying. Later, the speed signal is replaced by a speed estimate in an attempt to obtain a speed sensorless observer. The resulting observer is analyzed and it is shown that, despite the lack of asymptotic stability, it allows to estimate the rotor flux position of the IM drive with good accuracy. The theoretical developments are validated with simulations and experimental tests.