Vehicle-mounted wireless power transfer: ensuring safety through magnetic field management

A comprehensive study has examined the magnetic field emissions (MFE) from vehicle-mounted wireless power transfer (WPT) systems, providing critical insights for ensuring user safety during electric vehicle charging. As wireless charging technology gains popularity for fleet vehicles and accessibility applications, understanding and controlling electromagnetic field exposure becomes increasingly important.

Researchers conducted extensive physical measurements around a vehicle equipped with an in-house designed WPT system, examining how various factors affect magnetic field emissions where users might be positioned during charging operations. The study specifically investigated:

- Alignment effects: How vehicle positioning relative to the charging pad impacts field strength

- Power transfer levels: Relationship between charging power and magnetic field intensity

- Measurement positioning: How probe height and distance affect readings

- Vehicle components: Contribution of high-frequency cables to overall field emissions

The study evaluated measurements against multiple international safety guidelines, including ICNIRP 1998, ICNIRP 2010, and ISO 14117. Results showed that during normal operation with good alignment at 10 kW power transfer, magnetic field emissions remained within all recommended exposure limits. Even during misalignment scenarios, the system only marginally exceeded the most stringent ICNIRP 1998 limits in small regions, while remaining well within the boundaries of other standards.

The research revealed several important considerations for wireless charging system designers:

1. Field distribution patterns: Highest field levels were measured close to the ground, opposite the air gap between vehicle and ground pads

2. Misalignment effects: Vehicle misalignment increased magnetic field emissions, though the system's DC-DC efficiency remained relatively stable

3. Power scaling relationship: At higher power transfer levels, magnetic field emissions increased at a lower rate than the corresponding increase in primary coil current

4. Cable shielding importance: Unshielded high-frequency cables within the vehicle significantly contributed to field emissions

The study also validated an approach combining physical measurements with finite element method (FEM) simulation. Using actual coil currents measured from the vehicle-installed WPT system, researchers created simulations that successfully predicted magnetic field trends, though they noted that accurate modeling requires inclusion of vehicle installation features. For physical measurements, the research highlighted the importance of taking readings at multiple points around the vehicle perimeter to capture the complete field profile, as localized components like unshielded cables can create significant variations in field strength.

This research provides valuable guidance for WPT system designers, installers, and regulatory bodies working to ensure the safe deployment of wireless charging technology. The findings suggest that with proper design considerations and installation practices, wireless charging systems can operate within established safety guidelines even at substantial power levels.

As wireless charging becomes more prevalent in electric vehicle applications, these insights will help ensure that the technology can be implemented without compromising user safety, while still delivering the convenience and accessibility benefits that make it an attractive alternative to traditional plug-in charging methods.