Spectral Analysis of Nonstationary Low-Frequency Magnetic-Field Emissions from Ship's Power Frequency Converters

Method of measurement and data analysis are considered for the magnetic-field emissions (MFEs) from the ship's electric motor drive with the frequency converter. Due to the nonstationary behavior of the sinusoidal components of the magnetic flux density (MFD), the time-frequency analysis based on the short-time Fourier transform (STFT) was performed. The selected results of the off-line analysis of the recorded signals of magnetic field of ship bow thruster drive with frequency converter are presented. The level of the magnetic field intensity was evaluated, in reference to the standards, which take into account the people protection against the excessive emission of the low-frequency magnetic field. In the special case of simultaneous exposure to multi-frequency MFEs, a summation formulae defined in International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines was applied.