Types of Wireless Power Transfer Systems

There are two main types of wireless power transfer systems: inductive and resonantly-coupled systems. Inductive systems use magnetically coupled coils, while resonantly-coupled systems utilize electromagnetic fields.

Inductive wireless power transfer systems consist of two or more coils of wire that are placed in close proximity to each other. An alternating current supplied to the primary coil generates a time-varying magnetic field, which induces an alternating current in the secondary coil. This concept is similar to a transformer, but with physically separated primary and secondary coils. The efficiency of an inductively-coupled system is maximized when the distance between the coils is minimized, as a larger separation reduces power transfer.

Increasing the separation between the coils leads to circulating currents between the power source and the primary coil’s leakage inductance, resulting in reduced efficiency. While magnetic fields of the inductively-coupled system are not blocked by non-magnetic metals, such as aluminum and copper, they can induce currents in these metals, lowering overall transfer efficiency.

Inductively-coupled wireless power transfer systems are commonly used at lower power levels where a low separation distance is easily maintained. The Hughes MagneCharge system, which operated at 6.6 kW, achieved high efficiency. However, inductively-coupled systems require physical interaction to initiate charging, as the primary coil needs to be inserted into a mating slot on the vehicle.

Resonantly-coupled systems offer a more convenient charging experience. They can tolerate larger air gaps between the transmitter and receiver and do not require precise positional alignment. This allows the use of fixed coils on the ground and on the vehicle. Resonantly-coupled systems accommodate air gaps over 100 times larger than inductive systems, making them more flexible in terms of coil placement and alignment.