I knew if I just kept writing about electromagnetism, eventually I’d have enough foundation to talk about something really neat lots of people probably wonder about. I think my article on AC induction motors last week may have induced several readers to write me asking about wireless charging. I knew there were some cool topics related to induction but somehow wireless charging slipped my mind completely.
It’s true that wireless charging is often referred to as inductive charging because that is the principle behind operation. Several smartphone manufacturers over the past decade or so have offered phones that can be wirelessly charged. Palm’s Pre back in the late 2000s shipped with a heavy magnetic dock that the Pre would stick to while it charged without wires. I actually have a Pre2 and the wireless charging is very cool. It takes about the same amount of time to charge as with a cable. Apple still hasn’t released an iPhone capable of wireless charging…yet, but I suspect it’s on their radar.
Ok, on to the science behind wireless charging. First, the technical term for this type of power transmission is resonant inductive coupling. The operation is actually very simple. As always with anything where induction is involved, we need to have an oscillating power source like AC. The standard smartphone/small electronic device power chain is all DC, so somewhere in the charging process, some type of oscillating current will be present, though it need not necessarily be standard AC per se. In a basic inductive coupling setup, you’ll have two coils that will be tuned so that they oscillate (or resonate) at the same frequency. That’s where the resonant coupling plays in. The transmitter coil will be powered by the power source and a current will be induced in the receiving coil. This induced current can then go on to power whatever you’d like including charging a phone.
The current in the receiving coil is induced because of the changing magnetic field created by the changing current in the transmitter coil. When the transmitter coil is energized it creates a magnetic field around itself that looks like a donut. The receiving coil, when placed in close proximity to the transmitter coil, essentially shares in that magnetic field as though it were its own. That’s the coupling. It’s actually very, very simple. The only complicated parts are the drawbacks and limitations.
Why aren’t we transmitting power wirelessly all the time, everywhere? The magnetic field that is created in the transmitter coil is not very big and the most dense area of magnetic flux lines (needed to induce current) are located closest to the coil. So proximity of the receiver coil plays a huge part in both the amount of power that can be transmitted and the efficiency of doing so. In fact, the strength of the magnetic field decreases exponentially with distance (depending on the exact shape of the field). In the case of wireless charging, the coils are coupled by nothing but the air in between them. This works, but air doesn’t concentrate the magnetic flux in any way. It’s free to explode out unrestricted in its natural pattern. If you were to couple the coils using an iron core (for example) the magnetic flux would be concentrated in the core and could be transmitted (via the core) to the receiving coil at a much higher efficiency. Of course at that point, it’s no longer a wireless setup and actually resembles more of a transformer. It should be no surprise then that transformers are often extremely efficient at delivering power from their transmitter coil to their receiving coil.
Even given the drawbacks and limitations, resonant inductive coupling has a number of important advantages that allow it to excel in certain applications. Have you ever used an RFID chip card to enter a building? That process uses resonant inductive coupling. The RFID chip in the card has no power source. It’s idle circuitry. The base station on the door lock has a transmitter coil that couples to the receiving coil embedded in the card. This transmits enough power to energize the chip circuitry and transmit a radio signal back to the base station to unlock the door. That’s just a tiny, tiny amount of power used by the RFID chip. To power or charge a smartphone requires a lot more power and strong coupling. This can be achieved, but the advantages to doing so are minimal. You have the convenience of not having to plug the phone in, and there’s no cord or port to damage. I suspect many phone manufacturers therefore see it as a sort of novelty, though it’ll be interesting to see if Apple ever decides to incorporate it in any of their products.
Hopefully this article was informative on this very interesting subject. I don’t have any plans for next week’s article yet, but if you have any ideas, please write and let me know!