Three-phase induction motors (IMs) are electrical machines used on a large scale in industrial applications because they are versatile, robust and low maintenance devices. However, IMs are significantly affected when fed by unbalanced voltages. Prolonged operation under voltage unbalance (VU) conditions degrades performance and shortens machine life by producing imbalances in stator currents that abnormally raise the winding temperature. With the development of new technologies and researches on non-destructive techniques (NDT) for fault diagnosis in IMs, it shows relevant to obtain economically accessible, efficient and reliable sensors capable of acquiring signals that allow the identification of this type of failure. The objective of this work is to evaluate the application of low-cost piezoelectric sensors in the acquisition of acoustic emission (AE) signals and the identification of VU through the analysis of short-term Fourier transform (STFT) spectrogram. The piezoelectric sensor makes NDT feasible as it is an affordable and inexpensive component. In addition, STFT allows time-frequency analysis of acoustic emission signals. For this NDT, two sensors were coupled on both sides of the induction motor frame. The AE signals obtained during the IM operation were processed and the resulting spectrograms were analyzed to identify the different UV levels. After comparing the AE signals for faulty conditions with the signals for the IM operating at balanced voltages, it was possible to obtain the desired identification, which confirmed the successful application of low-cost piezoelectric sensors for VU condition detection in three-phase induction machines.