In this paper, an analysis of techniques of digital signal processing used for estimating the motor slip and rotational speed of an induction motor operating in stationary or nonstationary condition by using rotor slot harmonics observed in the stator current waveform is presented. The novelty of this paper is related with three topics. First, zoom short time Chirp-Z transform was used to find the fundamental supply frequency intending to improve accuracy without increasing computational complexity. Second, it presents a tool that can be used to identify whether a motor is capable or not of producing principal slot harmonics. Finally, an algorithm was developed to find an optimal window length using rotational speed maximum relative deviation. The use of this approach was analyzed when the motor was powered by a frequency converter driving a variable load and operating in strongly nonstationary conditions. The correctness of the method has been confirmed by experimental test results. Finally, the proposed method was implemented in a conventional sucker-rod pumping unit (25D-67-36-American Petroleum Institute), to estimate the rotational speed and slip when the motor is powered by a frequency converter.