The paper discusses the problem of sensorless rotor flux angle/rotor position estimation for the permanent magnet synchronous motor (PMSM) and for the induction motor (IM) and presents a family of sensorless observer designs that use a speed estimate. In sensorless AC drive control, it is typical to measure the motor's voltages and currents and to estimate the other quantities of interest: speed, fluxes (EMFs) and rotor position. The simultaneous estimation of these quantities is possible, however, the methods available are rather complicated and the accuracy is often questionable, especially under parameter variations. The paper proposes a sequential approach which is simpler: first, estimate the drive's speed; then, use this speed estimate along with the measurements to estimate the states of the motor model and to obtain the field orientation angle. A family of observers for the IM and the PMSM is presented - these are constructed using their models in the stationary reference frame. The observers are developed assuming that the speed estimate obtained is different from the real speed; it is shown that despite this inaccuracy, with special gain designs, the correct field orientation angle is obtained. The observers are developed using Sliding Mode and/or Lyapunov methods. They can be directly applied in sensorless field-oriented drives that do not require the magnitude of the flux.