==== Type of coils in an IPT system ====

The type of coils used in an IPT system can influence the performance of an IPT system. It is therefore important to consider different key aspects when designing a coil for the IPT system. The mutual inductance, self-inductance, coupling coefficient, and misalignment variation are among the key factors that are affected by the coil geometry designs. This also includes the number of turns in the coil, the number of turns and the pitch. Another important factor is the type of wire the coil is made of. Copper is commonly selected because of its better conductivity and lower cost compared to other materials. Using copper reduces the voltage required to produce a sufficient magnetic field, which helps minimize the heat generated by the coil. However, using a solid conductor introduces challenges such as the skin effect and proximity effect. These can reduce the efficiency of the system. As mentioned earlier, when a high-frequency voltage is applied to the coil, a high-frequency current flows through it. This will generate a time-varying magnetic field and can lead to the induction of eddy currents, which further affect efficiency and increase power loss.

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Circular coils offer a simple, symmetric design and are commonly used in low-power applications such as Qi phone chargers. They achieve strong coupling under perfect alignment and have efficient copper use, but they are sensitive to lateral misalignment and offer limited coverage.
Rectangular coils are particularly suitable for dynamic systems. The elongated shape provides better tolerance along one axis. They can be tiled and customized to match vehicle shapes but suffer from field non-uniformity and reduced mutual inductance in corners.
Double-D coils (DD) are widely used in the research for wireless EV chargers. Their two opposing D-shaped windings form magnetic poles that generate a uniform and strong magnetic field, offering excellent tolerance to lateral misalignment. Compared to circular or rectangular coils, DD coils reduce leakage and provide better EMI control but are slightly heavier and require more complex winding.

Double-D Quadrature (DDQ) coils are based on the DD concept but a second set of coils at 90° is added. This enhances tolerance in both axes. This results in a very uniform magnetic field over a larger area and higher efficiency under misalignment. These are mainly used in high-performance or research-based systems but at the cost of increased complexity, weight and expense.
Bipolar coils are typically figure-8 shaped and produce concentrated flux between two poles. They are highly efficient when aligned and provide strong coupling across gaps. However, their performance drops sharply with perpendicular misalignment. They also tend to be heavier and more complex due to required ferrite cores.