# Thermal-Mechanical Behaviour of Road-Embedded Wireless Charging Pads for EVs
###### Abstract  
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 \(\
icefrac{{1}}{{4}}\)-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 \(\
icefrac{{}^{\circ}}{C}\), the temperature in the middle of the copper litre wire coil was predicted to be 100 \(\
icefrac{{}^{\circ}}{C}\) 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 \(\
icefrac{{}^{\circ}}{C}\), the maximum tensile strain in the ferrites of the embedded pad was measured to be 62 microstrains (\(\mu\)\(\lambda\)), with a root-mean square error that was 18 \(\mu\)\(\lambda\) 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. inductive power transfer; finite element analysis; fibre Bragg gratings; multi-physics simulations; experimental methodology; thermal strain; temperature measurement; road-embedded wireless charging systems +
Footnote †: journal: Applied sciences  
research has been conducted to develop the electronics and magnetics of IPT systems for roadways applications for various power ratings and efficiencies [1; 2; 3; 4]. For IPT technology to be economically viable for EV charging, it is required that primary IPT pads have a robust mechanical design with comparable durability and longevity to that of traditional asphalt roads [5]. A critical mode of loading experienced by IPT pads is thermal loading. When they are energized with an alternating current, power loss is generated due to the electrical resistance of the litz wire and hysteresis in the magnetic cores. These power losses are converted to heat, which increases the operating temperature of the pad significantly. Temperatures of above 100 \({}^{\circ}\)C can be reached [6; 7]. The increase in temperature induces mechanical stresses in the pad, due to the different coefficients of thermal expansion (CTE) of the materials within [8]. During the development of these systems, an accurate measurement or numerical prediction of their response to thermal loading is essential to ensuring that they have the required durability and longevity. The measurement and prediction of the structural response of the pad's ferrite cores is particularly important, as they are a brittle ceramic with a very low failure strain (around 400 \(\mu\epsilon\)) and a high modulus (around 140 GPa). Ferrite also has a low thermal expansion compared to the surrounding polymeric components, which increases its thermally induced stresses.