Analysis of spectral signatures of stator currents on a three-phase induction motor operating in non stationary mode for rotational speed and slip detection using rotor slot harmonics

This paper investigates some techniques of digital signal processing, using the rotor slot harmonics present in the stator currents waveforms, to estimate the motor slip and the rotational speed when operating in stationary or in nonstationary mode. This research also highlights the importance of identifying whether a motor is capable or not of producing rotor slots harmonics, and shows how it is possible to do this using simple tables. An algorithm to estimate the slip and speed variation was developed. Finally, it has been done some analysis about issues such as: sample rate, window length and the use of the chirp z-Transform with the short time Fourier transform to improve the tests results. It has also been analysed the use of this methodology when the motor is operating powered by a frequency converter and variable load. Furthermore, the efficiency of the method has been confirmed by experimental results.