WPT

Wireless power transfer is the delivery of electrical energy from a source to a load across a gap, without conductive wires connecting them. The energy crosses space as electromagnetic fields, induced currents, or radiated waves. WPT eliminates the need for plug connectors, slip rings, or trailing cables in applications where physical contact is impossible, hazardous, or inconvenient.

Three main methods dominate industrial and consumer WPT. Inductive coupling works over short distances (millimetres to centimetres) by placing a primary coil near a secondary coil; alternating current in the primary induces current in the secondary. Resonant inductive coupling extends the range to tens of centimetres by tuning both coils to the same frequency, allowing higher efficiency over slightly greater air gaps. Far-field radiation, using microwave or other radio frequencies, can work over metres but is less efficient and requires complex rectenna arrays to capture and convert the energy.

Practical limits and losses

Inductive WPT systems typically achieve 70 to 90 percent efficiency, with losses increasing sharply as gap distance grows. Misalignment, metal obstacles, and frequency shifts all degrade performance. Industrial applications include charging of autonomous vehicles, powering rotating equipment in clean rooms where sealed bearings are required, and energy delivery to sensors embedded in rotating shafts or sealed housings. Consumer electronics use inductive WPT in toothbrush chargers and phone charging pads, which operate at standardized frequencies such as 100 to 200 kHz.

The term WPT appeared in technical literature during the 1990s as practical systems moved beyond theory. The Qi standard, introduced in 2008, defined interoperable inductive charging for consumer devices and became the de facto baseline for modern WPT. Higher-power industrial variants operate at larger air gaps and demand custom coil design and matching networks to achieve acceptable efficiency.