This paper presents the modeling and the broken rotor bar fault diagnostics by time–fre-quency analysis of the motor current under an extended startup transient time. The transient cur-rent-based nonstationary signal is retrieved and investigated for its time–frequency response to seg-regate the rotor faults and spatial harmonics. For studying the effect of reduced voltage on various parameters and the theoretical definition of the fault patterns, the winding function analysis (WFA)-based model is presented first. Moreover, an algorithm to improve the spectrum legibility is pro-posed. It is shown that by efficient utilization of the attenuation filter and consideration of the area containing the maximum power spectral density, the diagnostic algorithm gives promising results. The results are based on the machine’s analytical model and the measurements taken from the la-boratory setup.