The low-frequency magnetic field emissions on-board of the vessel

The data represents measurement results performed on board of the vessel. The acquired time-waveforms, corresponding to the instantaneous values of the magnetic field (MF) induction were acquired near the cable supplying the bow thruster motor. Data recording of the magnetic field density (MFD) emissions was carried out with a/d converters of the data acquisition board (DAQ), which was connected to the analog outputs of an isotropic field meter. A block diagram of the measuring system is presented in Figure. ESM-100 H/E field meter, which was applied in this system, enables the measurements of MFD in the measuring ranges from 1 nT to 20 mT. Four wide-band (5 Hz - 400 kHz) analog outputs of this meter afford possibilities for measuring the time waveforms of the magnetic fields, separately in the three orthogonal directions, in the output voltage range from 0 to 600 mV, with resolution 1 nT=30 mV. The multifunction DAQ board processes the analog signal with an 18-bit resolution and sampling rate up to 625 kS/s. The onboard lowpass filter with programmable cut-off frequency (up to 40 kHz) rejects high-frequency noise and prevents aliasing. Discrete signals are next analyzed (off-line) with the use of virtual instruments (VIs) in the LabVIEW environment. The digitized measuring signal is initially recorded by the VIs that perform data acquisition with repetition controlled programmatically. The sampling frequency is equal to 20 000 Hz and is inserted in the first row of the attached file.
The results of the preliminary measurements of MFD emissions, which are caused by running frequency converter confirmed that the MFD spectrum depends strongly on frequency. The magnetic field spectrum spreads out mainly in the low-frequency range below 2 kHz. The presented measurements and analyzing were performed within the frequency range from 5 Hz up to 2 kHz.