A study of the magnetic field emissions from a vehicle-mounted wireless power transfer system for safe operation when charging EV batteries

Wireless Power Transfer (WPT) is an alternative method of Electric Vehicle (EV) battery charging, particularly for fleet vehicles and people with mobility issues. The safe operation of WPT systems should therefore be of interest and importance to system designers, installers, and end-users. One aspect of safe operation is the potential exposure to high-power electromagnetic fields. There are international guidelines with recommended exposure limits that system designers can design and test to. Simulations can be used to predict magnetic field levels, but these should be developed in conjunction with physical measurements to improve the accuracy of such simulations. 1Several factors can influence the WPT generated electromagnetic field, in regions where end users could be located during charging operation. These factors were studied for an in-house designed WPT system retrofitted to an electric vehicle. The magnetic field was physically measured around the vehicle for different operating conditions (alignment, power transfer level and probe position) to assess performance against recommended exposure levels, observe any trends in measurements and study the impact of the probe position. Coil currents were measured and used within an initial simulation to predict magnetic field for comparison to physical values. The initial simulation predicted the trend of the magnetic field with reasonable accuracy. Where there was a difference in magnitude, the physical measurements highlighted that a High Frequency (HF cable) used within the vehicle assembly (not included in initial simulation) contributed to the magnetic field intensity. Overall, magnetic fields were within permitted exposure limits at 10 kW power and good alignment, and with misaligned coils, the system showed only minor exceedance of the most stringent limits, and DC-DC system efficiency was only slightly reduced. • Magnetic Field Emissions (MFE) physically measured for a vehicle mounted WPT system • Trends from simulated results compared favourably with physical measurements • Factors relating to the test method for physical MFE measurements were studied • In-house system studied was within ICNIRP 2010 limits even with misalignment • Although misalignment found to increase MFE, charging efficiency remained high