Nikola Tesla was right—you can transfer electrical power wirelessly. He demonstrated it in several ways in the late 19th century. Yet, as usual, he was ahead of his time. Up until recently, that work was generally lost, ignored, or dismissed. Now researchers and developers are finally finding ways to make wireless power transfer happen in a practical manner.
Wireless power transfer is used primarily to charge batteries in an incredible range of products. The earliest applications were plain inductive chargers in electric toothbrushes and shavers. Today, most wireless chargers are for smartphones, wearables, and even laptops. Other consumer targets include hearing aids and golf carts. One application that makes sense is wireless charging for hybrid electric vehicles (HEVs) and full electric vehicles (EVs). And a common industrial application is wireless charging of electric forklifts.
This article takes a look at the concepts behind the technology and provides an update on this movement today. Also, for a contrasting opinion on the wireless rage, see "Cordless Chargers: An Alternate View" at the end of the article.
The Theory of Wireless Power
There are two basic ways to transfer electrical energy without wires: near field and far field. The near-field method is basically just magnetic coupling. The operation is that of a transformer, where a transmitter (TX) coil is the primary winding and a receiver (RX) coil is the secondary winding.
The power transfer is wireless; no direct electrical connection exists between the transmitter and receiver. However, the key to making this work well is to keep the distance between TX and RX as short as possible, and to ensure that the two coils are optimally aligned. The amount of power transferred and the overall efficiency of the process depends on the amount of coupling between coils. Typical coupling is in the 0.3-0.6 range.
Far-field transmission is real radio rather than just magnetic-field induction. In far field, the TX antenna creates both electric and magnetic fields at a right angle to one another. At some distance from the antenna, usually several wavelengths, the fields break away and travel together through space to the RX antenna that captures the signal and generates a small useful voltage.
The problem with the far field is that the power level drops off at the square of the distance between TX and RX. To be usable, the far field or RF method must transmit higher power and keep the distances as short as is practical.