Thermal-Mechanical Behaviour of Road-Embedded Wireless Charging Pads for EVs

Road-embedded inductive power transfer (IPT) systems have the potential to accelerate the electrification of the transportation sector. For these systems to be economically viable, however, they need to have a similar durability and lifespan to those of asphalt roads. One area that has lacked investigation is thermally induced stresses in a primary IPT pad, which are caused by the increase in the temperature of the pad when it is energized and the differing thermal expansion of the materials within. This paper presents an experimental and a finite element-based methodology for investigating the thermal-mechanical behaviour of a 1/4-scale double-D pad, which was energized while suspended in air, as well as energized when embedded in pavement. A focus was placed on the measurement and prediction of strains in the magnetic ferrite cores because of their brittleness. Ferrite strains were measured using a combination of resistive strain gauges and non-metallic fibre Bragg grating (FBG) sensors. Coupled electromagnetic-thermal-structural simulations were conducted to predict temperature and strains in the system, with temperature-dependent properties obtained through physical testing. At an ambient temperature of 50 degrees C, the temperature in the middle of the copper litz wire coil was predicted to be 100 degrees C in both the suspended and embedded case. There was an excellent correlation with the experimental results, with a difference of less than 10% for most temperature measurements. When energized, the pad was predicted to experience an upward bow due to its temperature rise, resulting in bending strains in the ferrite cores. At an ambient temperature of 50 degrees C, the maximum tensile strain in the ferrites of the embedded pad was measured to be 62 microstrains (mu epsilon), with a root-mean square error that was 18 mu epsilon across three sensors. The experimental and validated numerical methodology can be applied to full-scale operational IPT pads to analyse and improve their thermal-mechanical performance.