Abstract <p>Nowadays, one important scientific task consists in the development and improvement of methods for nondestructive testing of asynchronous electric motors during operation. One of the promising areas of diagnostics is the use of an external magnetic-field signal. However, its study in the run-down mode is not given due attention by the authors of scientific publications, despite a number of advantages of this signal. Therefore, the aim of this research is to confirm the possibility to record and study the signal of the radial component of the external magnetic field induction in the run-down mode of an asynchronous motor. The research is carried out using several test benches, real low-voltage and high-voltage asynchronous motors, and a simulation model of an asynchronous electric motor designed in the Ansys software package. To process the signals, namely, to construct the frequency-time spectra of the external magnetic field in the run-down mode, we have used a program based on the Short Time Fourier Transform using the Flat-top window as a window function to determine more accurately the amplitudes of harmonic components. During research carried out on test benches, it has been found that the external magnetic field of the motor in the run-down mode can exist due to the phenomenon of residual magnetization. The possibility of modelling asynchronous motors taking into account the residual magnetization has been proven and the reliability of the resulting model has been confirmed. The spectra of external magnetic-field signals on real motors and simulation models in the run-down mode are presented. The external magnetic-field signal in the run-down mode can be recorded both on simulation models and on real low- and high-voltage asynchronous electric motors. During further research, it is intended to analyze the possibility to identify faults in asynchronous motors using this signal.</p>